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I have a confusion about the way the LSTM networks work when forecasting with an horizon that is not finite but I'm rather searching for a prediction in whatever time in future. In physical terms I would call it the evolution of the system.
Suppose I have a time series $y(t)$ (output) I want to forecast, and some external inputs $u_1(t), u_2(t),\cdots u_N(t)$ on which the series $y(t)$ depends.
It's common to use the lagged value of the output $y(t)$ as input for the network, such that I schematically have something like (let's consider for simplicity just lag 1 for the output and no lag for the external input):
[y(t-1), u_1(t), u_2(t),\cdots u_N(t)] \to y(t)
In this way of thinking the network, when one wants to do recursive forecast it is forced to use the predicted value at the previous step as input for the next step. In this way we have an effect of propagation of error that makes the long term forecast badly behaving.
Now, my confusion is, I'm thinking as a RNN as a kind of an (simple version) implementation of a state space model where I have the inputs, my output and one or more state variable responsible for the memory of the system. These variables are hidden and not observed.
So now the question, if there is this kind of variable taking already into account previous states of the system why would I need to use the lagged output value as input of my network/model ?
Getting rid of this does my long term forecast would be better, since I'm not expecting anymore the propagation of the error of the forecasted output. (I guess there will be anyway an error in the internal state propagating)
Thanks !
Please see DeepAR - a LSTM forecaster more than one step into the future.
The main contributions of the paper are twofold: (1) we propose an RNN
architecture for probabilistic forecasting, incorporating a negative
Binomial likelihood for count data as well as special treatment for
the case when the magnitudes of the time series vary widely; (2) we
demonstrate empirically on several real-world data sets that this
model produces accurate probabilistic forecasts across a range of
input characteristics, thus showing that modern deep learning-based
approaches can effective address the probabilistic forecasting
problem, which is in contrast to common belief in the field and the
mixed results
In this paper, they forecast multiple steps into the future, to negate exactly what you state here which is the error propagation.
Skipping several steps allows to get more accurate predictions, further into the future.
One more thing done in this paper is predicting percentiles, and interpolating, rather than predicting the value directly. This adds stability, and an error assessment.
Disclaimer - I read an older version of this paper.
Learner might be in training stage, where it update Q-table for bunch of epoch.
In this stage, Q-table would be updated with gamma(discount rate), learning rate(alpha), and action would be chosen by random action rate.
After some epoch, when reward is getting stable, let me call this "training is done". Then do I have to ignore these parameters(gamma, learning rate, etc) after that?
I mean, in training stage, I got an action from Q-table like this:
if rand_float < rar:
action = rand.randint(0, num_actions - 1)
else:
action = np.argmax(Q[s_prime_as_index])
But after training stage, Do I have to remove rar, which means I have to get an action from Q-table like this?
action = np.argmax(self.Q[s_prime])
Once the value function has converged (values stop changing), you no longer need to run Q-value updates. This means gamma and alpha are no longer relevant, because they only effect updates.
The epsilon parameter is part of the exploration policy (e-greedy) and helps ensure that the agent visits all states infinitely many times in the limit. This is an important factor in ensuring that the agent's value function eventually converges to the correct value. Once we've deemed the value function converged however, there's no need to continue randomly taking actions that our value function doesn't believe to be best; we believe that the value function is optimal, so we extract the optimal policy by greedily choosing what it says is the best action in every state. We can just set epsilon to 0.
Although the answer provided by #Nick Walker is correct, here it's some additional information.
What you are talking about is closely related with the concept technically known as "exploration-exploitation trade-off". From Sutton & Barto book:
The agent has to exploit what it already knows in order to obtain
reward, but it also has to explore in order to make better action
selections in the future. The dilemma is that neither exploration nor
exploitation can be pursued exclusively without failing at the task.
The agent must try a variety of actions and progressively favor those
that appear to be best.
One way to implement the exploration-exploitation trade-off is using epsilon-greedy exploration, that is what you are using in your code sample. So, at the end, once the agent has converged to the optimal policy, the agent must select only those that exploite the current knowledge, i.e., you can forget the rand_float < rar part. Ideally you should decrease the epsilon parameters (rar in your case) with the number of episodes (or steps).
On the other hand, regarding the learning rate, it worths noting that theoretically this parameter should follow the Robbins-Monro conditions:
This means that the learning rate should decrease asymptotically. So, again, once the algorithm has converged you can (or better, you should) safely ignore the learning rate parameter.
In practice, sometimes you can simply maintain a fixed epsilon and alpha parameters until your algorithm converges and then put them as 0 (i.e., ignore them).
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Everyone is aware of Dijkstra's Letters to the editor: go to statement considered harmful (also here .html transcript and here .pdf) and there has been a formidable push since that time to eschew the goto statement whenever possible. While it's possible to use goto to produce unmaintainable, sprawling code, it nevertheless remains in modern programming languages. Even the advanced continuation control structure in Scheme can be described as a sophisticated goto.
What circumstances warrant the use of goto? When is it best to avoid?
As a follow-up question: C provides a pair of functions, setjmp() and longjmp(), that provide the ability to goto not just within the current stack frame but within any of the calling frames. Should these be considered as dangerous as goto? More dangerous?
Dijkstra himself regretted that title, for which he was not responsible. At the end of EWD1308 (also here .pdf) he wrote:
Finally a short story for the record.
In 1968, the Communications of the ACM
published a text of mine under the
title "The goto statement considered
harmful", which in later years would
be most frequently referenced,
regrettably, however, often by authors
who had seen no more of it than its
title, which became a cornerstone of
my fame by becoming a template: we
would see all sorts of articles under
the title "X considered harmful" for
almost any X, including one titled
"Dijkstra considered harmful". But
what had happened? I had submitted a
paper under the title "A case against
the goto statement", which, in order
to speed up its publication, the
editor had changed into a "letter to
the Editor", and in the process he had
given it a new title of his own
invention! The editor was Niklaus
Wirth.
A well thought out classic paper about this topic, to be matched to that of Dijkstra, is Structured Programming with go to Statements, by Donald E. Knuth. Reading both helps to reestablish context and a non-dogmatic understanding of the subject. In this paper, Dijkstra's opinion on this case is reported and is even more strong:
Donald E. Knuth: I believe that by presenting such a
view I am not in fact disagreeing
sharply with Dijkstra's ideas, since
he recently wrote the following:
"Please don't fall into the trap of
believing that I am terribly
dogmatical about [the go to
statement]. I have the uncomfortable
feeling that others are making a
religion out of it, as if the
conceptual problems of programming
could be solved by a single trick, by
a simple form of coding discipline!"
A coworker of mine said the only reason to use a GOTO is if you programmed yourself so far into a corner that it is the only way out. In other words, proper design ahead of time and you won't need to use a GOTO later.
I thought this comic illustrates that beautifully "I could restructure the program's flow, or use one little 'GOTO' instead." A GOTO is a weak way out when you have weak design. Velociraptors prey on the weak.
The following statements are generalizations; while it is always possible to plead exception, it usually (in my experience and humble opinion) isn't worth the risks.
Unconstrained use of memory addresses (either GOTO or raw pointers) provides too many opportunities to make easily avoidable mistakes.
The more ways there are to arrive at a particular "location" in the code, the less confident one can be about what the state of the system is at that point. (See below.)
Structured programming IMHO is less about "avoiding GOTOs" and more about making the structure of the code match the structure of the data. For example, a repeating data structure (e.g. array, sequential file, etc.) is naturally processed by a repeated unit of code. Having built-in structures (e.g. while, for, until, for-each, etc.) allows the programmer to avoid the tedium of repeating the same cliched code patterns.
Even if GOTO is low-level implementation detail (not always the case!) it's below the level that the programmer should be thinking. How many programmers balance their personal checkbooks in raw binary? How many programmers worry about which sector on the disk contains a particular record, instead of just providing a key to a database engine (and how many ways could things go wrong if we really wrote all of our programs in terms of physical disk sectors)?
Footnotes to the above:
Regarding point 2, consider the following code:
a = b + 1
/* do something with a */
At the "do something" point in the code, we can state with high confidence that a is greater than b. (Yes, I'm ignoring the possibility of untrapped integer overflow. Let's not bog down a simple example.)
On the other hand, if the code had read this way:
...
goto 10
...
a = b + 1
10: /* do something with a */
...
goto 10
...
The multiplicity of ways to get to label 10 means that we have to work much harder to be confident about the relationships between a and b at that point. (In fact, in the general case it's undecideable!)
Regarding point 4, the whole notion of "going someplace" in the code is just a metaphor. Nothing is really "going" anywhere inside the CPU except electrons and photons (for the waste heat). Sometimes we give up a metaphor for another, more useful, one. I recall encountering (a few decades ago!) a language where
if (some condition) {
action-1
} else {
action-2
}
was implemented on a virtual machine by compiling action-1 and action-2 as out-of-line parameterless routines, then using a single two-argument VM opcode which used the boolean value of the condition to invoke one or the other. The concept was simply "choose what to invoke now" rather than "go here or go there". Again, just a change of metaphor.
Sometimes it is valid to use GOTO as an alternative to exception handling within a single function:
if (f() == false) goto err_cleanup;
if (g() == false) goto err_cleanup;
if (h() == false) goto err_cleanup;
return;
err_cleanup:
...
COM code seems to fall into this pattern fairly often.
I can only recall using a goto once. I had a series of five nested counted loops and I needed to be able to break out of the entire structure from the inside early based on certain conditions:
for{
for{
for{
for{
for{
if(stuff){
GOTO ENDOFLOOPS;
}
}
}
}
}
}
ENDOFLOOPS:
I could just have easily declared a boolean break variable and used it as part of the conditional for each loop, but in this instance I decided a GOTO was just as practical and just as readable.
No velociraptors attacked me.
Goto is extremely low on my list of things to include in a program just for the sake of it. That doesn't mean it's unacceptable.
Goto can be nice for state machines. A switch statement in a loop is (in order of typical importance): (a) not actually representative of the control flow, (b) ugly, (c) potentially inefficient depending on language and compiler. So you end up writing one function per state, and doing things like "return NEXT_STATE;" which even look like goto.
Granted, it is difficult to code state machines in a way which make them easy to understand. However, none of that difficulty is to do with using goto, and none of it can be reduced by using alternative control structures. Unless your language has a 'state machine' construct. Mine doesn't.
On those rare occasions when your algorithm really is most comprehensible in terms of a path through a sequence of nodes (states) connected by a limited set of permissible transitions (gotos), rather than by any more specific control flow (loops, conditionals, whatnot), then that should be explicit in the code. And you ought to draw a pretty diagram.
setjmp/longjmp can be nice for implementing exceptions or exception-like behaviour. While not universally praised, exceptions are generally considered a "valid" control structure.
setjmp/longjmp are 'more dangerous' than goto in the sense that they're harder to use correctly, never mind comprehensibly.
There never has been, nor will there
ever be, any language in which it is
the least bit difficult to write bad
code. -- Donald Knuth.
Taking goto out of C would not make it any easier to write good code in C. In fact, it would rather miss the point that C is supposed to be capable of acting as a glorified assembler language.
Next it'll be "pointers considered harmful", then "duck typing considered harmful". Then who will be left to defend you when they come to take away your unsafe programming construct? Eh?
We already had this discussion and I stand by my point.
Furthermore, I'm fed up with people describing higher-level language structures as “goto in disguise” because they clearly haven't got the point at all. For example:
Even the advanced continuation control structure in Scheme can be described as a sophisticated goto.
That is complete nonsense. Every control structure can be implemented in terms of goto but this observation is utterly trivial and useless. goto isn't considered harmful because of its positive effects but because of its negative consequences and these have been eliminated by structured programming.
Similarly, saying “GOTO is a tool, and as all tools, it can be used and abused” is completely off the mark. No modern construction worker would use a rock and claim it “is a tool.” Rocks have been replaced by hammers. goto has been replaced by control structures. If the construction worker were stranded in the wild without a hammer, of course he would use a rock instead. If a programmer has to use an inferior programming language that doesn't have feature X, well, of course she may have to use goto instead. But if she uses it anywhere else instead of the appropriate language feature she clearly hasn't understood the language properly and uses it wrongly. It's really as simple as that.
In Linux: Using goto In Kernel Code on Kernel Trap, there's a discussion with Linus Torvalds and a "new guy" about the use of GOTOs in Linux code. There are some very good points there and Linus dressed in that usual arrogance :)
Some passages:
Linus: "No, you've been brainwashed by
CS people who thought that Niklaus
Wirth actually knew what he was
talking about. He didn't. He doesn't
have a frigging clue."
-
Linus: "I think goto's are fine, and
they are often more readable than
large amounts of indentation."
-
Linus: "Of course, in stupid languages
like Pascal, where labels cannot be
descriptive, goto's can be bad."
In C, goto only works within the scope of the current function, which tends to localise any potential bugs. setjmp and longjmp are far more dangerous, being non-local, complicated and implementation-dependent. In practice however, they're too obscure and uncommon to cause many problems.
I believe that the danger of goto in C is greatly exaggerated. Remember that the original goto arguments took place back in the days of languages like old-fashioned BASIC, where beginners would write spaghetti code like this:
3420 IF A > 2 THEN GOTO 1430
Here Linus describes an appropriate use of goto: http://www.kernel.org/doc/Documentation/CodingStyle (chapter 7).
Today, it's hard to see the big deal about the GOTO statement because the "structured programming" people mostly won the debate and today's languages have sufficient control flow structures to avoid GOTO.
Count the number of gotos in a modern C program. Now add the number of break, continue, and return statements. Furthermore, add the number of times you use if, else, while, switch or case. That's about how many GOTOs your program would have had if you were writing in FORTRAN or BASIC in 1968 when Dijkstra wrote his letter.
Programming languages at the time were lacking in control flow. For example, in the original Dartmouth BASIC:
IF statements had no ELSE. If you wanted one, you had to write:
100 IF NOT condition THEN GOTO 200
...stuff to do if condition is true...
190 GOTO 300
200 REM else
...stuff to do if condition is false...
300 REM end if
Even if your IF statement didn't need an ELSE, it was still limited to a single line, which usually consisted of a GOTO.
There was no DO...LOOP statement. For non-FOR loops, you had to end the loop with an explicit GOTO or IF...GOTO back to the beginning.
There was no SELECT CASE. You had to use ON...GOTO.
So, you ended up with a lot of GOTOs in your program. And you couldn't depend on the restriction of GOTOs to within a single subroutine (because GOSUB...RETURN was such a weak concept of subroutines), so these GOTOs could go anywhere. Obviously, this made control flow hard to follow.
This is where the anti-GOTO movement came from.
Go To can provide a sort of stand-in for "real" exception handling in certain cases. Consider:
ptr = malloc(size);
if (!ptr) goto label_fail;
bytes_in = read(f_in,ptr,size);
if (bytes_in=<0) goto label_fail;
bytes_out = write(f_out,ptr,bytes_in);
if (bytes_out != bytes_in) goto label_fail;
Obviously this code was simplified to take up less space, so don't get too hung up on the details. But consider an alternative I've seen all too many times in production code by coders going to absurd lengths to avoid using goto:
success=false;
do {
ptr = malloc(size);
if (!ptr) break;
bytes_in = read(f_in,ptr,size);
if (count=<0) break;
bytes_out = write(f_out,ptr,bytes_in);
if (bytes_out != bytes_in) break;
success = true;
} while (false);
Now functionally this code does the exact same thing. In fact, the code generated by the compiler is nearly identical. However, in the programmer's zeal to appease Nogoto (the dreaded god of academic rebuke), this programmer has completely broken the underlying idiom that the while loop represents, and did a real number on the readability of the code. This is not better.
So, the moral of the story is, if you find yourself resorting to something really stupid in order to avoid using goto, then don't.
Donald E. Knuth answered this question in the book "Literate Programming", 1992 CSLI. On p. 17 there is an essay "Structured Programming with goto Statements" (PDF). I think the article might have been published in other books as well.
The article describes Dijkstra's suggestion and describes the circumstances where this is valid. But he also gives a number of counter examples (problems and algorithms) which cannot be easily reproduced using structured loops only.
The article contains a complete description of the problem, the history, examples and counter examples.
Goto considered helpful.
I started programming in 1975. To 1970s-era programmers, the words "goto considered harmful" said more or less that new programming languages with modern control structures were worth trying. We did try the new languages. We quickly converted. We never went back.
We never went back, but, if you are younger, then you have never been there in the first place.
Now, a background in ancient programming languages may not be very useful except as an indicator of the programmer's age. Notwithstanding, younger programmers lack this background, so they no longer understand the message the slogan "goto considered harmful" conveyed to its intended audience at the time it was introduced.
Slogans one does not understand are not very illuminating. It is probably best to forget such slogans. Such slogans do not help.
This particular slogan however, "Goto considered harmful," has taken on an undead life of its own.
Can goto not be abused? Answer: sure, but so what? Practically every programming element can be abused. The humble bool for example is abused more often than some of us would like to believe.
By contrast, I cannot remember meeting a single, actual instance of goto abuse since 1990.
The biggest problem with goto is probably not technical but social. Programmers who do not know very much sometimes seem to feel that deprecating goto makes them sound smart. You might have to satisfy such programmers from time to time. Such is life.
The worst thing about goto today is that it is not used enough.
Attracted by Jay Ballou adding an answer, I'll add my £0.02. If Bruno Ranschaert had not already done so, I'd have mentioned Knuth's "Structured Programming with GOTO Statements" article.
One thing that I've not seen discussed is the sort of code that, while not exactly common, was taught in Fortran text books. Things like the extended range of a DO loop and open-coded subroutines (remember, this would be Fortran II, or Fortran IV, or Fortran 66 - not Fortran 77 or 90). There's at least a chance that the syntactic details are inexact, but the concepts should be accurate enough. The snippets in each case are inside a single function.
Note that the excellent but dated (and out of print) book 'The Elements of Programming Style, 2nd Edn' by Kernighan & Plauger includes some real-life examples of abuse of GOTO from programming text books of its era (late-70s). The material below is not from that book, however.
Extended range for a DO loop
do 10 i = 1,30
...blah...
...blah...
if (k.gt.4) goto 37
91 ...blah...
...blah...
10 continue
...blah...
return
37 ...some computation...
goto 91
One reason for such nonsense was the good old-fashioned punch-card. You might notice that the labels (nicely out of sequence because that was canonical style!) are in column 1 (actually, they had to be in columns 1-5) and the code is in columns 7-72 (column 6 was the continuation marker column). Columns 73-80 would be given a sequence number, and there were machines that would sort punch card decks into sequence number order. If you had your program on sequenced cards and needed to add a few cards (lines) into the middle of a loop, you'd have to repunch everything after those extra lines. However, if you replaced one card with the GOTO stuff, you could avoid resequencing all the cards - you just tucked the new cards at the end of the routine with new sequence numbers. Consider it to be the first attempt at 'green computing' - a saving of punch cards (or, more specifically, a saving of retyping labour - and a saving of consequential rekeying errors).
Oh, you might also note that I'm cheating and not shouting - Fortran IV was written in all upper-case normally.
Open-coded subroutine
...blah...
i = 1
goto 76
123 ...blah...
...blah...
i = 2
goto 76
79 ...blah...
...blah...
goto 54
...blah...
12 continue
return
76 ...calculate something...
...blah...
goto (123, 79) i
54 ...more calculation...
goto 12
The GOTO between labels 76 and 54 is a version of computed goto. If the variable i has the value 1, goto the first label in the list (123); if it has the value 2, goto the second, and so on. The fragment from 76 to the computed goto is the open-coded subroutine. It was a piece of code executed rather like a subroutine, but written out in the body of a function. (Fortran also had statement functions - which were embedded functions that fitted on a single line.)
There were worse constructs than the computed goto - you could assign labels to variables and then use an assigned goto. Googling assigned goto tells me it was deleted from Fortran 95. Chalk one up for the structured programming revolution which could fairly be said to have started in public with Dijkstra's "GOTO Considered Harmful" letter or article.
Without some knowledge of the sorts of things that were done in Fortran (and in other languages, most of which have rightly fallen by the wayside), it is hard for us newcomers to understand the scope of the problem which Dijkstra was dealing with. Heck, I didn't start programming until ten years after that letter was published (but I did have the misfortune to program in Fortran IV for a while).
There is no such things as GOTO considered harmful.
GOTO is a tool, and as all tools, it can be used and abused.
There are, however, many tools in the programming world that have a tendency to be abused more than being used, and GOTO is one of them. the WITH statement of Delphi is another.
Personally I don't use either in typical code, but I've had the odd usage of both GOTO and WITH that were warranted, and an alternative solution would've contained more code.
The best solution would be for the compiler to just warn you that the keyword was tainted, and you'd have to stuff a couple of pragma directives around the statement to get rid of the warnings.
It's like telling your kids to not run with scissors. Scissors are not bad, but some usage of them are perhaps not the best way to keep your health.
Since I began doing a few things in the linux kernel, gotos don't bother me so much as they once did. At first I was sort of horrified to see they (kernel guys) added gotos into my code. I've since become accustomed to the use of gotos, in some limited contexts, and will now occasionally use them myself. Typically, it's a goto that jumps to the end of a function to do some kind of cleanup and bail out, rather than duplicating that same cleanup and bailout in several places in the function. And typically, it's not something large enough to hand off to another function -- e.g. freeing some locally (k)malloc'ed variables is a typical case.
I've written code that used setjmp/longjmp only once. It was in a MIDI drum sequencer program. Playback happened in a separate process from all user interaction, and the playback process used shared memory with the UI process to get the limited info it needed to do the playback. When the user wanted to stop playback, the playback process just did a longjmp "back to the beginning" to start over, rather than some complicated unwinding of wherever it happened to be executing when the user wanted it to stop. It worked great, was simple, and I never had any problems or bugs related to it in that instance.
setjmp/longjmp have their place -- but that place is one you'll not likely visit but once in a very long while.
Edit: I just looked at the code. It was actually siglongjmp() that I used, not longjmp (not that it's a big deal, but I had forgotten that siglongjmp even existed.)
It never was, as long as you were able to think for yourself.
Because goto can be used for confusing metaprogramming
Goto is both a high-level and a low-level control expression, and as a result it just doesn't have a appropriate design pattern suitable for most problems.
It's low-level in the sense that a goto is a primitive operation that implements something higher like while or foreach or something.
It's high-level in the sense that when used in certain ways it takes code that executes in a clear sequence, in an uninterrupted fashion, except for structured loops, and it changes it into pieces of logic that are, with enough gotos, a grab-bag of logic being dynamically reassembled.
So, there is a prosaic and an evil side to goto.
The prosaic side is that an upward pointing goto can implement a perfectly reasonable loop and a downward-pointing goto can do a perfectly reasonable break or return. Of course, an actual while, break, or return would be a lot more readable, as the poor human wouldn't have to simulate the effect of the goto in order to get the big picture. So, a bad idea in general.
The evil side involves a routine not using goto for while, break, or return, but using it for what's called spaghetti logic. In this case the goto-happy developer is constructing pieces of code out of a maze of goto's, and the only way to understand it is to simulate it mentally as a whole, a terribly tiring task when there are many goto's. I mean, imagine the trouble of evaluating code where the else is not precisely an inverse of the if, where nested ifs might allow in some things that were rejected by the outer if, etc, etc.
Finally, to really cover the subject, we should note that essentially all early languages except Algol initially made only single statements subject to their versions of if-then-else. So, the only way to do a conditional block was to goto around it using an inverse conditional. Insane, I know, but I've read some old specs. Remember that the first computers were programmed in binary machine code so I suppose any kind of an HLL was a lifesaver; I guess they weren't too picky about exactly what HLL features they got.
Having said all that I used to stick one goto into every program I wrote "just to annoy the purists".
If you're writing a VM in C, it turns out that using (gcc's) computed gotos like this:
char run(char *pc) {
void *opcodes[3] = {&&op_inc, &&op_lda_direct, &&op_hlt};
#define NEXT_INSTR(stride) goto *(opcodes[*(pc += stride)])
NEXT_INSTR(0);
op_inc:
++acc;
NEXT_INSTR(1);
op_lda_direct:
acc = ram[++pc];
NEXT_INSTR(1);
op_hlt:
return acc;
}
works much faster than the conventional switch inside a loop.
Denying the use of the GOTO statement to programmers is like telling a carpenter not to use a hammer as it Might damage the wall while he is hammering in a nail. A real programmer Knows How and When to use a GOTO. I’ve followed behind some of these so-called ‘Structured Programs’ I’ve see such Horrid code just to avoid using a GOTO, that I could shoot the programmer. Ok, In defense of the other side, I’ve seen some real spaghetti code too and again, those programmers should be shot too.
Here is just one small example of code I’ve found.
YORN = ''
LOOP
UNTIL YORN = 'Y' OR YORN = 'N' DO
CRT 'Is this correct? (Y/N) : ':
INPUT YORN
REPEAT
IF YORN = 'N' THEN
CRT 'Aborted!'
STOP
END
-----------------------OR----------------------
10: CRT 'Is this Correct (Y)es/(N)o ':
INPUT YORN
IF YORN='N' THEN
CRT 'Aborted!'
STOP
ENDIF
IF YORN<>'Y' THEN GOTO 10
"In this link http://kerneltrap.org/node/553/2131"
Ironically, eliminating the goto introduced a bug: the spinlock call was omitted.
The original paper should be thought of as "Unconditional GOTO Considered Harmful". It was in particular advocating a form of programming based on conditional (if) and iterative (while) constructs, rather than the test-and-jump common to early code. goto is still useful in some languages or circumstances, where no appropriate control structure exists.
About the only place I agree Goto could be used is when you need to deal with errors, and each particular point an error occurs requires special handling.
For instance, if you're grabbing resources and using semaphores or mutexes, you have to grab them in order and you should always release them in the opposite manner.
Some code requires a very odd pattern of grabbing these resources, and you can't just write an easily maintained and understood control structure to correctly handle both the grabbing and releasing of these resources to avoid deadlock.
It's always possible to do it right without goto, but in this case and a few others Goto is actually the better solution primarily for readability and maintainability.
-Adam
One modern GOTO usage is by the C# compiler to create state machines for enumerators defined by yield return.
GOTO is something that should be used by compilers and not programmers.
Until C and C++ (amongst other culprits) have labelled breaks and continues, goto will continue to have a role.
If GOTO itself were evil, compilers would be evil, because they generate JMPs. If jumping into a block of code, especially following a pointer, were inherently evil, the RETurn instruction would be evil. Rather, the evil is in the potential for abuse.
At times I have had to write apps that had to keep track of a number of objects where each object had to follow an intricate sequence of states in response to events, but the whole thing was definitely single-thread. A typical sequence of states, if represented in pseudo-code would be:
request something
wait for it to be done
while some condition
request something
wait for it
if one response
while another condition
request something
wait for it
do something
endwhile
request one more thing
wait for it
else if some other response
... some other similar sequence ...
... etc, etc.
endwhile
I'm sure this is not new, but the way I handled it in C(++) was to define some macros:
#define WAIT(n) do{state=(n); enque(this); return; L##n:;}while(0)
#define DONE state = -1
#define DISPATCH0 if state < 0) return;
#define DISPATCH1 if(state==1) goto L1; DISPATCH0
#define DISPATCH2 if(state==2) goto L2; DISPATCH1
#define DISPATCH3 if(state==3) goto L3; DISPATCH2
#define DISPATCH4 if(state==4) goto L4; DISPATCH3
... as needed ...
Then (assuming state is initially 0) the structured state machine above turns into the structured code:
{
DISPATCH4; // or as high a number as needed
request something;
WAIT(1); // each WAIT has a different number
while (some condition){
request something;
WAIT(2);
if (one response){
while (another condition){
request something;
WAIT(3);
do something;
}
request one more thing;
WAIT(4);
}
else if (some other response){
... some other similar sequence ...
}
... etc, etc.
}
DONE;
}
With a variation on this, there can be CALL and RETURN, so some state machines can act like subroutines of other state machines.
Is it unusual? Yes. Does it take some learning on the part of the maintainer? Yes. Does that learning pay off? I think so. Could it be done without GOTOs that jump into blocks? Nope.
I actually found myself forced to use a goto, because I literally couldn't think of a better (faster) way to write this code:
I had a complex object, and I needed to do some operation on it. If the object was in one state, then I could do a quick version of the operation, otherwise I had to do a slow version of the operation. The thing was that in some cases, in the middle of the slow operation, it was possible to realise that this could have been done with the fast operation.
SomeObject someObject;
if (someObject.IsComplex()) // this test is trivial
{
// begin slow calculations here
if (result of calculations)
{
// just discovered that I could use the fast calculation !
goto Fast_Calculations;
}
// do the rest of the slow calculations here
return;
}
if (someObject.IsmediumComplex()) // this test is slightly less trivial
{
Fast_Calculations:
// Do fast calculations
return;
}
// object is simple, no calculations needed.
This was in a speed critical piece of realtime UI code, so I honestly think that a GOTO was justified here.
Hugo
One thing I've not seen from any of the answers here is that a 'goto' solution is often more efficient than one of the structured programming solutions often mentioned.
Consider the many-nested-loops case, where using 'goto' instead of a bunch of if(breakVariable) sections is obviously more efficient. The solution "Put your loops in a function and use return" is often totally unreasonable. In the likely case that the loops are using local variables, you now have to pass them all through function parameters, potentially handling loads of extra headaches that arise from that.
Now consider the cleanup case, which I've used myself quite often, and is so common as to have presumably been responsible for the try{} catch {} structure not available in many languages. The number of checks and extra variables that are required to accomplish the same thing are far worse than the one or two instructions to make the jump, and again, the additional function solution is not a solution at all. You can't tell me that's more manageable or more readable.
Now code space, stack usage, and execution time may not matter enough in many situations to many programmers, but when you're in an embedded environment with only 2KB of code space to work with, 50 bytes of extra instructions to avoid one clearly defined 'goto' is just laughable, and this is not as rare a situation as many high-level programmers believe.
The statement that 'goto is harmful' was very helpful in moving towards structured programming, even if it was always an over-generalization. At this point, we've all heard it enough to be wary of using it (as we should). When it's obviously the right tool for the job, we don't need to be scared of it.
I avoid it since a coworker/manager will undoubtedly question its use either in a code review or when they stumble across it. While I think it has uses (the error handling case for example) - you'll run afoul of some other developer who will have some type of problem with it.
It’s not worth it.
Almost all situations where a goto can be used, you can do the same using other constructs. Goto is used by the compiler anyway.
I personally never use it explicitly, don't ever need to.
You can use it for breaking from a deeply nested loop, but most of the time your code can be refactored to be cleaner without deeply nested loops.
I just wrote a simple Unix command line utility that could be implemented a lot more efficiently. I can measure its performance by just running it on a number of inputs and measuring the time it takes. This will produce a set of pairs of numbers, s t, where s is the input size and t the processing time. In order to determine the performance characteristics of my utility, I need to fit a function through these data points. I can do this manually, but I prefer to be lazy and let a utility do it for me.
Does such a utility exist?
Its input is a sequence of pairs of numbers.
Its output is a formula that expresses how the second number depends as a function on the first, plus an error measure.
One step of the way is to have a utility that does this just for polynomials.
This has been discussed here but it didn't produce a ready-to-use solution.
The next step is to extend the utility to try non-polynomial terms: negative-degree polynomials (as in y = 1/x) and logarithmic terms (as in y = x log x) will need to be tried as well. One idea to cope with the non-polynomial terms is to just surround the polynomial fitting with x and y scale transformations. I don't know whether that will do. This question is related but not exactly the same.
As I said, I'm lazy: I'm not looking for ideas on how to to write this myself, I'm looking for a reliable result of a project that has already done it for me. Any suggestions?
I believe that SAS has this, RS/1 has this, I think that Mathematica has this, Execel and most spreadsheets have a primitive form of this and usually there are add-ons available for more advanced forms. There are lots of Lab analysis and Statistical analysis tools that have stuff like this.
RE., Command Line Tools:
SAS, RS/1 and Minitab were all command line tools 20 years ago when I used them. I bet at least one of them still has this capability.
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Everyone is aware of Dijkstra's Letters to the editor: go to statement considered harmful (also here .html transcript and here .pdf) and there has been a formidable push since that time to eschew the goto statement whenever possible. While it's possible to use goto to produce unmaintainable, sprawling code, it nevertheless remains in modern programming languages. Even the advanced continuation control structure in Scheme can be described as a sophisticated goto.
What circumstances warrant the use of goto? When is it best to avoid?
As a follow-up question: C provides a pair of functions, setjmp() and longjmp(), that provide the ability to goto not just within the current stack frame but within any of the calling frames. Should these be considered as dangerous as goto? More dangerous?
Dijkstra himself regretted that title, for which he was not responsible. At the end of EWD1308 (also here .pdf) he wrote:
Finally a short story for the record.
In 1968, the Communications of the ACM
published a text of mine under the
title "The goto statement considered
harmful", which in later years would
be most frequently referenced,
regrettably, however, often by authors
who had seen no more of it than its
title, which became a cornerstone of
my fame by becoming a template: we
would see all sorts of articles under
the title "X considered harmful" for
almost any X, including one titled
"Dijkstra considered harmful". But
what had happened? I had submitted a
paper under the title "A case against
the goto statement", which, in order
to speed up its publication, the
editor had changed into a "letter to
the Editor", and in the process he had
given it a new title of his own
invention! The editor was Niklaus
Wirth.
A well thought out classic paper about this topic, to be matched to that of Dijkstra, is Structured Programming with go to Statements, by Donald E. Knuth. Reading both helps to reestablish context and a non-dogmatic understanding of the subject. In this paper, Dijkstra's opinion on this case is reported and is even more strong:
Donald E. Knuth: I believe that by presenting such a
view I am not in fact disagreeing
sharply with Dijkstra's ideas, since
he recently wrote the following:
"Please don't fall into the trap of
believing that I am terribly
dogmatical about [the go to
statement]. I have the uncomfortable
feeling that others are making a
religion out of it, as if the
conceptual problems of programming
could be solved by a single trick, by
a simple form of coding discipline!"
A coworker of mine said the only reason to use a GOTO is if you programmed yourself so far into a corner that it is the only way out. In other words, proper design ahead of time and you won't need to use a GOTO later.
I thought this comic illustrates that beautifully "I could restructure the program's flow, or use one little 'GOTO' instead." A GOTO is a weak way out when you have weak design. Velociraptors prey on the weak.
The following statements are generalizations; while it is always possible to plead exception, it usually (in my experience and humble opinion) isn't worth the risks.
Unconstrained use of memory addresses (either GOTO or raw pointers) provides too many opportunities to make easily avoidable mistakes.
The more ways there are to arrive at a particular "location" in the code, the less confident one can be about what the state of the system is at that point. (See below.)
Structured programming IMHO is less about "avoiding GOTOs" and more about making the structure of the code match the structure of the data. For example, a repeating data structure (e.g. array, sequential file, etc.) is naturally processed by a repeated unit of code. Having built-in structures (e.g. while, for, until, for-each, etc.) allows the programmer to avoid the tedium of repeating the same cliched code patterns.
Even if GOTO is low-level implementation detail (not always the case!) it's below the level that the programmer should be thinking. How many programmers balance their personal checkbooks in raw binary? How many programmers worry about which sector on the disk contains a particular record, instead of just providing a key to a database engine (and how many ways could things go wrong if we really wrote all of our programs in terms of physical disk sectors)?
Footnotes to the above:
Regarding point 2, consider the following code:
a = b + 1
/* do something with a */
At the "do something" point in the code, we can state with high confidence that a is greater than b. (Yes, I'm ignoring the possibility of untrapped integer overflow. Let's not bog down a simple example.)
On the other hand, if the code had read this way:
...
goto 10
...
a = b + 1
10: /* do something with a */
...
goto 10
...
The multiplicity of ways to get to label 10 means that we have to work much harder to be confident about the relationships between a and b at that point. (In fact, in the general case it's undecideable!)
Regarding point 4, the whole notion of "going someplace" in the code is just a metaphor. Nothing is really "going" anywhere inside the CPU except electrons and photons (for the waste heat). Sometimes we give up a metaphor for another, more useful, one. I recall encountering (a few decades ago!) a language where
if (some condition) {
action-1
} else {
action-2
}
was implemented on a virtual machine by compiling action-1 and action-2 as out-of-line parameterless routines, then using a single two-argument VM opcode which used the boolean value of the condition to invoke one or the other. The concept was simply "choose what to invoke now" rather than "go here or go there". Again, just a change of metaphor.
Sometimes it is valid to use GOTO as an alternative to exception handling within a single function:
if (f() == false) goto err_cleanup;
if (g() == false) goto err_cleanup;
if (h() == false) goto err_cleanup;
return;
err_cleanup:
...
COM code seems to fall into this pattern fairly often.
I can only recall using a goto once. I had a series of five nested counted loops and I needed to be able to break out of the entire structure from the inside early based on certain conditions:
for{
for{
for{
for{
for{
if(stuff){
GOTO ENDOFLOOPS;
}
}
}
}
}
}
ENDOFLOOPS:
I could just have easily declared a boolean break variable and used it as part of the conditional for each loop, but in this instance I decided a GOTO was just as practical and just as readable.
No velociraptors attacked me.
Goto is extremely low on my list of things to include in a program just for the sake of it. That doesn't mean it's unacceptable.
Goto can be nice for state machines. A switch statement in a loop is (in order of typical importance): (a) not actually representative of the control flow, (b) ugly, (c) potentially inefficient depending on language and compiler. So you end up writing one function per state, and doing things like "return NEXT_STATE;" which even look like goto.
Granted, it is difficult to code state machines in a way which make them easy to understand. However, none of that difficulty is to do with using goto, and none of it can be reduced by using alternative control structures. Unless your language has a 'state machine' construct. Mine doesn't.
On those rare occasions when your algorithm really is most comprehensible in terms of a path through a sequence of nodes (states) connected by a limited set of permissible transitions (gotos), rather than by any more specific control flow (loops, conditionals, whatnot), then that should be explicit in the code. And you ought to draw a pretty diagram.
setjmp/longjmp can be nice for implementing exceptions or exception-like behaviour. While not universally praised, exceptions are generally considered a "valid" control structure.
setjmp/longjmp are 'more dangerous' than goto in the sense that they're harder to use correctly, never mind comprehensibly.
There never has been, nor will there
ever be, any language in which it is
the least bit difficult to write bad
code. -- Donald Knuth.
Taking goto out of C would not make it any easier to write good code in C. In fact, it would rather miss the point that C is supposed to be capable of acting as a glorified assembler language.
Next it'll be "pointers considered harmful", then "duck typing considered harmful". Then who will be left to defend you when they come to take away your unsafe programming construct? Eh?
We already had this discussion and I stand by my point.
Furthermore, I'm fed up with people describing higher-level language structures as “goto in disguise” because they clearly haven't got the point at all. For example:
Even the advanced continuation control structure in Scheme can be described as a sophisticated goto.
That is complete nonsense. Every control structure can be implemented in terms of goto but this observation is utterly trivial and useless. goto isn't considered harmful because of its positive effects but because of its negative consequences and these have been eliminated by structured programming.
Similarly, saying “GOTO is a tool, and as all tools, it can be used and abused” is completely off the mark. No modern construction worker would use a rock and claim it “is a tool.” Rocks have been replaced by hammers. goto has been replaced by control structures. If the construction worker were stranded in the wild without a hammer, of course he would use a rock instead. If a programmer has to use an inferior programming language that doesn't have feature X, well, of course she may have to use goto instead. But if she uses it anywhere else instead of the appropriate language feature she clearly hasn't understood the language properly and uses it wrongly. It's really as simple as that.
In Linux: Using goto In Kernel Code on Kernel Trap, there's a discussion with Linus Torvalds and a "new guy" about the use of GOTOs in Linux code. There are some very good points there and Linus dressed in that usual arrogance :)
Some passages:
Linus: "No, you've been brainwashed by
CS people who thought that Niklaus
Wirth actually knew what he was
talking about. He didn't. He doesn't
have a frigging clue."
-
Linus: "I think goto's are fine, and
they are often more readable than
large amounts of indentation."
-
Linus: "Of course, in stupid languages
like Pascal, where labels cannot be
descriptive, goto's can be bad."
In C, goto only works within the scope of the current function, which tends to localise any potential bugs. setjmp and longjmp are far more dangerous, being non-local, complicated and implementation-dependent. In practice however, they're too obscure and uncommon to cause many problems.
I believe that the danger of goto in C is greatly exaggerated. Remember that the original goto arguments took place back in the days of languages like old-fashioned BASIC, where beginners would write spaghetti code like this:
3420 IF A > 2 THEN GOTO 1430
Here Linus describes an appropriate use of goto: http://www.kernel.org/doc/Documentation/CodingStyle (chapter 7).
Today, it's hard to see the big deal about the GOTO statement because the "structured programming" people mostly won the debate and today's languages have sufficient control flow structures to avoid GOTO.
Count the number of gotos in a modern C program. Now add the number of break, continue, and return statements. Furthermore, add the number of times you use if, else, while, switch or case. That's about how many GOTOs your program would have had if you were writing in FORTRAN or BASIC in 1968 when Dijkstra wrote his letter.
Programming languages at the time were lacking in control flow. For example, in the original Dartmouth BASIC:
IF statements had no ELSE. If you wanted one, you had to write:
100 IF NOT condition THEN GOTO 200
...stuff to do if condition is true...
190 GOTO 300
200 REM else
...stuff to do if condition is false...
300 REM end if
Even if your IF statement didn't need an ELSE, it was still limited to a single line, which usually consisted of a GOTO.
There was no DO...LOOP statement. For non-FOR loops, you had to end the loop with an explicit GOTO or IF...GOTO back to the beginning.
There was no SELECT CASE. You had to use ON...GOTO.
So, you ended up with a lot of GOTOs in your program. And you couldn't depend on the restriction of GOTOs to within a single subroutine (because GOSUB...RETURN was such a weak concept of subroutines), so these GOTOs could go anywhere. Obviously, this made control flow hard to follow.
This is where the anti-GOTO movement came from.
Go To can provide a sort of stand-in for "real" exception handling in certain cases. Consider:
ptr = malloc(size);
if (!ptr) goto label_fail;
bytes_in = read(f_in,ptr,size);
if (bytes_in=<0) goto label_fail;
bytes_out = write(f_out,ptr,bytes_in);
if (bytes_out != bytes_in) goto label_fail;
Obviously this code was simplified to take up less space, so don't get too hung up on the details. But consider an alternative I've seen all too many times in production code by coders going to absurd lengths to avoid using goto:
success=false;
do {
ptr = malloc(size);
if (!ptr) break;
bytes_in = read(f_in,ptr,size);
if (count=<0) break;
bytes_out = write(f_out,ptr,bytes_in);
if (bytes_out != bytes_in) break;
success = true;
} while (false);
Now functionally this code does the exact same thing. In fact, the code generated by the compiler is nearly identical. However, in the programmer's zeal to appease Nogoto (the dreaded god of academic rebuke), this programmer has completely broken the underlying idiom that the while loop represents, and did a real number on the readability of the code. This is not better.
So, the moral of the story is, if you find yourself resorting to something really stupid in order to avoid using goto, then don't.
Donald E. Knuth answered this question in the book "Literate Programming", 1992 CSLI. On p. 17 there is an essay "Structured Programming with goto Statements" (PDF). I think the article might have been published in other books as well.
The article describes Dijkstra's suggestion and describes the circumstances where this is valid. But he also gives a number of counter examples (problems and algorithms) which cannot be easily reproduced using structured loops only.
The article contains a complete description of the problem, the history, examples and counter examples.
Goto considered helpful.
I started programming in 1975. To 1970s-era programmers, the words "goto considered harmful" said more or less that new programming languages with modern control structures were worth trying. We did try the new languages. We quickly converted. We never went back.
We never went back, but, if you are younger, then you have never been there in the first place.
Now, a background in ancient programming languages may not be very useful except as an indicator of the programmer's age. Notwithstanding, younger programmers lack this background, so they no longer understand the message the slogan "goto considered harmful" conveyed to its intended audience at the time it was introduced.
Slogans one does not understand are not very illuminating. It is probably best to forget such slogans. Such slogans do not help.
This particular slogan however, "Goto considered harmful," has taken on an undead life of its own.
Can goto not be abused? Answer: sure, but so what? Practically every programming element can be abused. The humble bool for example is abused more often than some of us would like to believe.
By contrast, I cannot remember meeting a single, actual instance of goto abuse since 1990.
The biggest problem with goto is probably not technical but social. Programmers who do not know very much sometimes seem to feel that deprecating goto makes them sound smart. You might have to satisfy such programmers from time to time. Such is life.
The worst thing about goto today is that it is not used enough.
Attracted by Jay Ballou adding an answer, I'll add my £0.02. If Bruno Ranschaert had not already done so, I'd have mentioned Knuth's "Structured Programming with GOTO Statements" article.
One thing that I've not seen discussed is the sort of code that, while not exactly common, was taught in Fortran text books. Things like the extended range of a DO loop and open-coded subroutines (remember, this would be Fortran II, or Fortran IV, or Fortran 66 - not Fortran 77 or 90). There's at least a chance that the syntactic details are inexact, but the concepts should be accurate enough. The snippets in each case are inside a single function.
Note that the excellent but dated (and out of print) book 'The Elements of Programming Style, 2nd Edn' by Kernighan & Plauger includes some real-life examples of abuse of GOTO from programming text books of its era (late-70s). The material below is not from that book, however.
Extended range for a DO loop
do 10 i = 1,30
...blah...
...blah...
if (k.gt.4) goto 37
91 ...blah...
...blah...
10 continue
...blah...
return
37 ...some computation...
goto 91
One reason for such nonsense was the good old-fashioned punch-card. You might notice that the labels (nicely out of sequence because that was canonical style!) are in column 1 (actually, they had to be in columns 1-5) and the code is in columns 7-72 (column 6 was the continuation marker column). Columns 73-80 would be given a sequence number, and there were machines that would sort punch card decks into sequence number order. If you had your program on sequenced cards and needed to add a few cards (lines) into the middle of a loop, you'd have to repunch everything after those extra lines. However, if you replaced one card with the GOTO stuff, you could avoid resequencing all the cards - you just tucked the new cards at the end of the routine with new sequence numbers. Consider it to be the first attempt at 'green computing' - a saving of punch cards (or, more specifically, a saving of retyping labour - and a saving of consequential rekeying errors).
Oh, you might also note that I'm cheating and not shouting - Fortran IV was written in all upper-case normally.
Open-coded subroutine
...blah...
i = 1
goto 76
123 ...blah...
...blah...
i = 2
goto 76
79 ...blah...
...blah...
goto 54
...blah...
12 continue
return
76 ...calculate something...
...blah...
goto (123, 79) i
54 ...more calculation...
goto 12
The GOTO between labels 76 and 54 is a version of computed goto. If the variable i has the value 1, goto the first label in the list (123); if it has the value 2, goto the second, and so on. The fragment from 76 to the computed goto is the open-coded subroutine. It was a piece of code executed rather like a subroutine, but written out in the body of a function. (Fortran also had statement functions - which were embedded functions that fitted on a single line.)
There were worse constructs than the computed goto - you could assign labels to variables and then use an assigned goto. Googling assigned goto tells me it was deleted from Fortran 95. Chalk one up for the structured programming revolution which could fairly be said to have started in public with Dijkstra's "GOTO Considered Harmful" letter or article.
Without some knowledge of the sorts of things that were done in Fortran (and in other languages, most of which have rightly fallen by the wayside), it is hard for us newcomers to understand the scope of the problem which Dijkstra was dealing with. Heck, I didn't start programming until ten years after that letter was published (but I did have the misfortune to program in Fortran IV for a while).
There is no such things as GOTO considered harmful.
GOTO is a tool, and as all tools, it can be used and abused.
There are, however, many tools in the programming world that have a tendency to be abused more than being used, and GOTO is one of them. the WITH statement of Delphi is another.
Personally I don't use either in typical code, but I've had the odd usage of both GOTO and WITH that were warranted, and an alternative solution would've contained more code.
The best solution would be for the compiler to just warn you that the keyword was tainted, and you'd have to stuff a couple of pragma directives around the statement to get rid of the warnings.
It's like telling your kids to not run with scissors. Scissors are not bad, but some usage of them are perhaps not the best way to keep your health.
Since I began doing a few things in the linux kernel, gotos don't bother me so much as they once did. At first I was sort of horrified to see they (kernel guys) added gotos into my code. I've since become accustomed to the use of gotos, in some limited contexts, and will now occasionally use them myself. Typically, it's a goto that jumps to the end of a function to do some kind of cleanup and bail out, rather than duplicating that same cleanup and bailout in several places in the function. And typically, it's not something large enough to hand off to another function -- e.g. freeing some locally (k)malloc'ed variables is a typical case.
I've written code that used setjmp/longjmp only once. It was in a MIDI drum sequencer program. Playback happened in a separate process from all user interaction, and the playback process used shared memory with the UI process to get the limited info it needed to do the playback. When the user wanted to stop playback, the playback process just did a longjmp "back to the beginning" to start over, rather than some complicated unwinding of wherever it happened to be executing when the user wanted it to stop. It worked great, was simple, and I never had any problems or bugs related to it in that instance.
setjmp/longjmp have their place -- but that place is one you'll not likely visit but once in a very long while.
Edit: I just looked at the code. It was actually siglongjmp() that I used, not longjmp (not that it's a big deal, but I had forgotten that siglongjmp even existed.)
It never was, as long as you were able to think for yourself.
Because goto can be used for confusing metaprogramming
Goto is both a high-level and a low-level control expression, and as a result it just doesn't have a appropriate design pattern suitable for most problems.
It's low-level in the sense that a goto is a primitive operation that implements something higher like while or foreach or something.
It's high-level in the sense that when used in certain ways it takes code that executes in a clear sequence, in an uninterrupted fashion, except for structured loops, and it changes it into pieces of logic that are, with enough gotos, a grab-bag of logic being dynamically reassembled.
So, there is a prosaic and an evil side to goto.
The prosaic side is that an upward pointing goto can implement a perfectly reasonable loop and a downward-pointing goto can do a perfectly reasonable break or return. Of course, an actual while, break, or return would be a lot more readable, as the poor human wouldn't have to simulate the effect of the goto in order to get the big picture. So, a bad idea in general.
The evil side involves a routine not using goto for while, break, or return, but using it for what's called spaghetti logic. In this case the goto-happy developer is constructing pieces of code out of a maze of goto's, and the only way to understand it is to simulate it mentally as a whole, a terribly tiring task when there are many goto's. I mean, imagine the trouble of evaluating code where the else is not precisely an inverse of the if, where nested ifs might allow in some things that were rejected by the outer if, etc, etc.
Finally, to really cover the subject, we should note that essentially all early languages except Algol initially made only single statements subject to their versions of if-then-else. So, the only way to do a conditional block was to goto around it using an inverse conditional. Insane, I know, but I've read some old specs. Remember that the first computers were programmed in binary machine code so I suppose any kind of an HLL was a lifesaver; I guess they weren't too picky about exactly what HLL features they got.
Having said all that I used to stick one goto into every program I wrote "just to annoy the purists".
If you're writing a VM in C, it turns out that using (gcc's) computed gotos like this:
char run(char *pc) {
void *opcodes[3] = {&&op_inc, &&op_lda_direct, &&op_hlt};
#define NEXT_INSTR(stride) goto *(opcodes[*(pc += stride)])
NEXT_INSTR(0);
op_inc:
++acc;
NEXT_INSTR(1);
op_lda_direct:
acc = ram[++pc];
NEXT_INSTR(1);
op_hlt:
return acc;
}
works much faster than the conventional switch inside a loop.
Denying the use of the GOTO statement to programmers is like telling a carpenter not to use a hammer as it Might damage the wall while he is hammering in a nail. A real programmer Knows How and When to use a GOTO. I’ve followed behind some of these so-called ‘Structured Programs’ I’ve see such Horrid code just to avoid using a GOTO, that I could shoot the programmer. Ok, In defense of the other side, I’ve seen some real spaghetti code too and again, those programmers should be shot too.
Here is just one small example of code I’ve found.
YORN = ''
LOOP
UNTIL YORN = 'Y' OR YORN = 'N' DO
CRT 'Is this correct? (Y/N) : ':
INPUT YORN
REPEAT
IF YORN = 'N' THEN
CRT 'Aborted!'
STOP
END
-----------------------OR----------------------
10: CRT 'Is this Correct (Y)es/(N)o ':
INPUT YORN
IF YORN='N' THEN
CRT 'Aborted!'
STOP
ENDIF
IF YORN<>'Y' THEN GOTO 10
"In this link http://kerneltrap.org/node/553/2131"
Ironically, eliminating the goto introduced a bug: the spinlock call was omitted.
The original paper should be thought of as "Unconditional GOTO Considered Harmful". It was in particular advocating a form of programming based on conditional (if) and iterative (while) constructs, rather than the test-and-jump common to early code. goto is still useful in some languages or circumstances, where no appropriate control structure exists.
About the only place I agree Goto could be used is when you need to deal with errors, and each particular point an error occurs requires special handling.
For instance, if you're grabbing resources and using semaphores or mutexes, you have to grab them in order and you should always release them in the opposite manner.
Some code requires a very odd pattern of grabbing these resources, and you can't just write an easily maintained and understood control structure to correctly handle both the grabbing and releasing of these resources to avoid deadlock.
It's always possible to do it right without goto, but in this case and a few others Goto is actually the better solution primarily for readability and maintainability.
-Adam
One modern GOTO usage is by the C# compiler to create state machines for enumerators defined by yield return.
GOTO is something that should be used by compilers and not programmers.
Until C and C++ (amongst other culprits) have labelled breaks and continues, goto will continue to have a role.
If GOTO itself were evil, compilers would be evil, because they generate JMPs. If jumping into a block of code, especially following a pointer, were inherently evil, the RETurn instruction would be evil. Rather, the evil is in the potential for abuse.
At times I have had to write apps that had to keep track of a number of objects where each object had to follow an intricate sequence of states in response to events, but the whole thing was definitely single-thread. A typical sequence of states, if represented in pseudo-code would be:
request something
wait for it to be done
while some condition
request something
wait for it
if one response
while another condition
request something
wait for it
do something
endwhile
request one more thing
wait for it
else if some other response
... some other similar sequence ...
... etc, etc.
endwhile
I'm sure this is not new, but the way I handled it in C(++) was to define some macros:
#define WAIT(n) do{state=(n); enque(this); return; L##n:;}while(0)
#define DONE state = -1
#define DISPATCH0 if state < 0) return;
#define DISPATCH1 if(state==1) goto L1; DISPATCH0
#define DISPATCH2 if(state==2) goto L2; DISPATCH1
#define DISPATCH3 if(state==3) goto L3; DISPATCH2
#define DISPATCH4 if(state==4) goto L4; DISPATCH3
... as needed ...
Then (assuming state is initially 0) the structured state machine above turns into the structured code:
{
DISPATCH4; // or as high a number as needed
request something;
WAIT(1); // each WAIT has a different number
while (some condition){
request something;
WAIT(2);
if (one response){
while (another condition){
request something;
WAIT(3);
do something;
}
request one more thing;
WAIT(4);
}
else if (some other response){
... some other similar sequence ...
}
... etc, etc.
}
DONE;
}
With a variation on this, there can be CALL and RETURN, so some state machines can act like subroutines of other state machines.
Is it unusual? Yes. Does it take some learning on the part of the maintainer? Yes. Does that learning pay off? I think so. Could it be done without GOTOs that jump into blocks? Nope.
I actually found myself forced to use a goto, because I literally couldn't think of a better (faster) way to write this code:
I had a complex object, and I needed to do some operation on it. If the object was in one state, then I could do a quick version of the operation, otherwise I had to do a slow version of the operation. The thing was that in some cases, in the middle of the slow operation, it was possible to realise that this could have been done with the fast operation.
SomeObject someObject;
if (someObject.IsComplex()) // this test is trivial
{
// begin slow calculations here
if (result of calculations)
{
// just discovered that I could use the fast calculation !
goto Fast_Calculations;
}
// do the rest of the slow calculations here
return;
}
if (someObject.IsmediumComplex()) // this test is slightly less trivial
{
Fast_Calculations:
// Do fast calculations
return;
}
// object is simple, no calculations needed.
This was in a speed critical piece of realtime UI code, so I honestly think that a GOTO was justified here.
Hugo
One thing I've not seen from any of the answers here is that a 'goto' solution is often more efficient than one of the structured programming solutions often mentioned.
Consider the many-nested-loops case, where using 'goto' instead of a bunch of if(breakVariable) sections is obviously more efficient. The solution "Put your loops in a function and use return" is often totally unreasonable. In the likely case that the loops are using local variables, you now have to pass them all through function parameters, potentially handling loads of extra headaches that arise from that.
Now consider the cleanup case, which I've used myself quite often, and is so common as to have presumably been responsible for the try{} catch {} structure not available in many languages. The number of checks and extra variables that are required to accomplish the same thing are far worse than the one or two instructions to make the jump, and again, the additional function solution is not a solution at all. You can't tell me that's more manageable or more readable.
Now code space, stack usage, and execution time may not matter enough in many situations to many programmers, but when you're in an embedded environment with only 2KB of code space to work with, 50 bytes of extra instructions to avoid one clearly defined 'goto' is just laughable, and this is not as rare a situation as many high-level programmers believe.
The statement that 'goto is harmful' was very helpful in moving towards structured programming, even if it was always an over-generalization. At this point, we've all heard it enough to be wary of using it (as we should). When it's obviously the right tool for the job, we don't need to be scared of it.
I avoid it since a coworker/manager will undoubtedly question its use either in a code review or when they stumble across it. While I think it has uses (the error handling case for example) - you'll run afoul of some other developer who will have some type of problem with it.
It’s not worth it.
Almost all situations where a goto can be used, you can do the same using other constructs. Goto is used by the compiler anyway.
I personally never use it explicitly, don't ever need to.
You can use it for breaking from a deeply nested loop, but most of the time your code can be refactored to be cleaner without deeply nested loops.