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I came across this yesterday and was wondering what the relationship of pass-by-value, pass-by-reference and pass-reference-by-value is. How do they all behave? Which programming language uses which?
Additionally:
I have also occasionally heard other terms like pass-by-pointer, pass-by-copy, pass-by-object...how do they all fit in?
It's a made-up term that refers to something that is actually pass-by-value, but where people always get confused because the only values in the language are references (pointers to things). Many modern languages work this way, including Java (for non-primitives), JavaScript, Python, Ruby, Scheme, Smalltalk, etc.
Beginners always ask questions on StackOverflow about why it's not pass-by-reference, because they don't understand what pass-by-reference really is. So to satisfy them some people make up a new term that they also don't understand, but makes it seem like they learned something, and which sounds vaguely like both pass-by-value and pass-by-reference.
Use of terminology varies widely between different language communities. For example, the Java community almost always describes this semantics as pass-by-value, while the Python and Ruby communities rarely describe it as pass-by-value, even though they are semantically identical.
Let's start with the basics. You probably already know this or at least think you do. Surprisingly many people are actually misusing these terms quite a bit (which certainly doesn't help with the confusion).
What is pass-by-value and pass-by-reference?
I am not going to tell you the answer just yet and explain it using examples instead. You will see why later.
Imagine you are trying to learn a programming language and a friend tells you that he knows a great book on the topic. You have two options:
borrow the book from him or
buy your own copy.
Case 1: If you borrow his book and you make notes in the margins (and add some bookmarks, references, drawings, ...) your friend can later benefit from those notes and bookmarks as well. Sharing is caring. The world is great!
Case 2: But if, on the other hand, your friend is a bit of a neat freak or holds his books very dear he might not appreciate your notes and even less the occasional coffee stain or how you fold the corners for bookmarking. You should really get your own book and everybody can happily treat his book the way he likes. The world is great!
These two cases are - you guessed it - just like pass-by-reference and pass-by-value.
The difference is whether the caller of the function can see changes that the callee makes.
Let's look at a code example.
Say you have the following code snippet:
function largePrint(text):
text = makeUpperCase(text)
print(text)
myText = "Hello"
largePrint(myText)
print(myText)
and you run it in a pass-by-value language the output will be
HELLO
Hello
while in a pass-by-reference language the output will be
HELLO
HELLO
If you pass-by-reference the variable "myText" gets changed by the function. If you just pass the value of the variable the function cannot change it.
If you need another example, this one using websites got quite a few upvotes.
Okay, now that we have the basics down let's move on to...
What is pass-reference-by-value?
Let's use our book example again. Assume that you have decided to borrow the book from your friend although you know how much he loves his books. Let us also assume that you then - being the total klutz that you are - spilled a gallon of coffee over it. Oh, my...
Not to worry, your brain starts up and realizes that you can just buy your friend a new book. It was in such good condition he won't even notice!
What does that story have to do with pass-reference-by-value?
Well, imagine on the other hand that your friend loved his books so muchs he doesn't want to lend it to you. He does however agree to bring the book and show it to you. The problem is, you can still spill a gallon of coffee over it but now you can't buy him a new book anymore. He would notice!
While this may sound dreadful (and probably has you sweating as if you had drunk that gallon of coffee) but it is actually quite common and a perfectly good way to share books call functions. It is sometimes called pass-reference-by-value.
How does the code example fare?
In a pass-reference-by-value function the output of our snippet is
HELLO
Hello
just like in the pass-by-value case. But if we change the code a little bit:
function largePrint(text):
text.toUpperCase()
print(text)
myText = "Hello"
largePrint(myText)
print(myText)
The output becomes:
HELLO
HELLO
just like in the pass-by-reference case. The reason is the same as in the book example: We can change the variable (or book) by calling text.toUpperCase() (or by spilling coffee on it). BUT we cannot change the object that the variable references anymore (text = makeUppercase(text) has no effect); in other words we cannot use a different book.
So summarizing we get:
pass-by-value:
You get your own book, your own copy of the variable and whatever you do with it the original owner will never know.
pass-by-reference:
You use the same book as your friend or the same variable as the caller. If you change the variable it changes outside your function.
pass-reference-by-value
You use the same book as your friend but he doesn't let it out of his sight. You can change the book but not EXchange it. In variable terms that means that you cannot change what the variable references but you can change that which the variable references.
So now when I show you the following Python code:
def appendSeven(list):
list.append(7)
def replaceWithNewList(list):
list = ["a", "b", "c"]
firstList = [1, 2, 3, 4, 5, 6]
print(firstList)
appendSeven(firstList)
print(firstList)
secondList = ["x", "y", "z"]
print(secondList)
replaceWithNewList(secondList)
print(secondList)
and tell you that the output is this
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6, 7]
['x', 'y', 'z']
['x', 'y', 'z']
what do you say?
Indeed, you are correct! Python uses pass-reference-by-value! (And so is Java and Ruby and Scheme and...)
Conclusion:
Was this extremely difficult? No.
Then why are so many people confused by it? (I'm not going to link to all the other questions about "How do I pass a variable by reference in Python" or "Is Java pass-by-reference?" and so on...)
Well, I see several reasons:
Saying pass-value-by-reference is really long (and it behaves almost the same as pass-by-reference anyway so get of my back and stop splitting hairs!)
There are nasty special cases that complicate the issue
like primitive types (which are often pass-by-value in pass-value-by-reference languages)
immutable types (which cannot be changed so passing them by-reference doesn't get you anywhere)
Some people just like to use different names:
pass-by-copy for pass-by-value because if you paid attention, you need your own book, so a copy has to be made. (But that is an implementation detail, other tricks like copy-on-write might be used to only sometimes make a copy.)
pass-by-reference for pass-reference-by-value because, after all, you did get a reference and that means you can change the object that the caller had (even if you cannot EXchange it)
pass-by-reference for pass-reference-by-value because almost no language actually has pass-by-reference (notable Exceptions (that I know): C++, C# but you have to use special syntax)
pass-by-sharing for pass-reference-by-value (remember our book example? that name is actually way better and everybody should use it!)
pass-by-object for pass-reference-by-value because allegedly Barbara Liskov first named it and used that, so that's its birthname)
pass-by-pointer for pass-by-reference because C doesn't have pass-by-reference and that is how it allows you to accomplish the same thing.
If there is anything you think I should add, please let me know. If I made a mistake, PLEASE LET ME KNOW!
For those of you who still haven't got enough: http://en.wikipedia.org/wiki/Evaluation_strategy
I am learning C from 'Programming in C' by Stephen Kochan.
Though the author is careful from the beginning only not to confuse the students with jargon, but occasionally he has used few terms without explaining their meaning. I have figured out the meaning of many such terms with the help of internet.
However, I could not understand the exactly meaning of the phrase 'language construct', and unfortunately the web doesn't provide a good explanation.
Considering I am a beginner, what does 'language construct' mean?
First, you need to understand what a constructed language Formal Language is. All programming languages are constructed formal languages (read the reference). You may then read a little bit about compiler construction, including this reference as well.
Going back to your question, consider this: The English language (a natural language) has tokens 'A-Z/0-9/,;"...' which we use to build "words" and we use languages rules to build sentences out of words. So, in the English language, a construct is what we build out of tokens.
Consider this brick-and-mortar example: Imagine if you set out to build a house, the basic materials you might use are: sand, iron, wood, cement, water (just five for simplicity). Anything you build out of these 4 or 5+ items would be a "construct", which in turn helps you build your house.
I have intentionally omitted details to further simplify the answer; hope this is helpful.
A language construct is a piece of language syntax. For example, the following is a language construct in C that lets you control the flow of a program:
if ( condition ) {
/* when condition is true */
} else {
/* when condition is false */
}
They usually use the term language construct because these are parts of most programming languages, but may be written differently, depending on the language. For example, a similar language construct in bourne shell would be:
if COMMAND; then
# when command returns 0
else
# when command returns anything else
fi
The function of this construct is the same, however, the way it's written is a bit different.
Hope this helps. If you need more detail, you may want to do a bit more research. As one of the comments suggests, Wikipedia may be helpful.
They are the base units from which the language is built up. They can't be used as a function rollback. They are directly called by the parser.
It includes all the syntax, semantics and coding styles of a language.
For more clarification you may refer to this question.
Wikipedia definition:
A language construct is a syntactically allowable part of a program that may be formed from one or more lexical tokens in accordance with the rules of a programming language.
The term Language Constructs is often used as a synonym for control structure, and should not be confused with a function.
Without seeing the context that the phrase is used in, I cannot be sure, but generally the phrase 'language construct' just means the combination of keywords, grammar and structure of a coding language. Basically, how to format/write/construct a piece of code.
Let say you want to create a class containing methods and properties, so:
Construct is an architecture of a class you are about to create. The architecture of the class consists of methods and properties created by you by using predefined utilities (such as: 'if', 'else', 'switch', 'break', etc)
That's my take on construct.
In reference to a programming language
Language Constructs mean the basic constructs of a programming languge e.g
1. Conditions (if, else, switch)
2. Loops (For, While, Do-while) etc
C is a structural language so while compiling your code everything thing goes statement by statement. Thus it becomes necessary to place your statement properly. This placing i.e. putting your statement correctly is your language construct else there may be syntax error or logical error.
Language constructs according to the GCSE book are basic building block of a programming language. that are
1. Sequential,
2. Selection, if, if/else
3. Iteration, while, for
Language construct is a piece of language syntax.
Example:
A declaration of a variable is a language construct:
{
int a; // declaration of a variable "a"
}
A language construct is a piece of syntax that the compiler has intimate knowledge about, usually because it needs to handle it specially. Typical examples of language constructs are the short-circuiting operators found in many imperative languages. Because these operators require lazy evaluation in an otherwise eager language, they must be handled specially by the compiler.
So, a stricter definition of a language construct may be: a syntactical form that is handled specially by the compiler, having functionality that cannot be implemented by a user.
Given some (English) word that we shall assume is a plural, is it possible to derive the singular form? I'd like to avoid lookup/dictionary tables if possible.
Some examples:
Examples -> Example a simple 's' suffix
Glitch -> Glitches 'es' suffix, as opposed to above
Countries -> Country 'ies' suffix.
Sheep -> Sheep no change: possible fallback for indeterminate values
Or, this seems to be a fairly exhaustive list.
Suggestions of libraries in language x are fine, as long as they are open-source (ie, so that someone can examine them to determine how to do it in language y)
It really depends on what you mean by 'programmatically'. Part of English works on easy to understand rules, and part doesn't. It has to do mainly with frequency. For a brief overview, you can read Pinker's "Words and Rules", but do yourself a favor and don't take the whole generative theory of linguistics entirely to heart. There's a lot more empiricism there than that school of thought really lends to the pursuit.
A lot of English can be statistically lemmatized. By the way, stemming or lemmatization is the term you're looking for. One of the most effective lemmatizers which work off of statistical rules bootstrapped with frequency-based exceptions is the Morpha Lemmatizer. You can give this a shot if you have a project that requires this type of simplification of strings which represent specific terms in English.
There are even more naive approaches that accomplish much with respect to normalizing related terms. Take a look at the Porter Stemmer, which is effective enough to cluster together most terms in English.
Going from singular to plural, English plural form is actually pretty regular compared to some other European languages I have a passing familiarity with. In German for example, working out the plural form is really complicated (eg Land -> Länder). I think there are roughly 20-30 exceptions and the rest follow a fairly simple ruleset:
-y -> -ies (family -> families)
-us -> -i (cactus -> cacti)
-s -> -ses (loss -> losses)
otherwise add -s
That being said, plural to singular form becomes that much harder because the reverse cases have ambiguities. For example:
pies: is it py or pie?
ski: is it singular or plural for 'skus'?
molasses: is it singular or plural for 'molasse' or 'molass'?
So it can be done but you're going to have a much larger list of exceptions and you're going to have to store a lot of false positives (ie things that appear plural but aren't).
Is "axes" the plural of "ax" or of "axis"? Even a human cannot tell without context.
You can take a look at Inflector.net - my port of Rails' inflection class.
No - English isn't a language which sticks to many rules.
I think your best bet is either:
use a dictionary of common words and their plurals (or group them by their plural rule, eg: group words where you just add an S, words where you add ES, words where you drop a Y and add IES...)
rethink your application
It is not possible, as nickf has already said. It would be simple for the classes of words you have described, but what about all the words that end with s naturally? My name, Marius, for example, is not plural of Mariu. Same with Bus I guess. Pluralization of words in English is a one way function (a hash function), and you usually need the rest of the sentence or paragraph for context.
Ok, this may seem like a silly question, but it is seriously bugging me. Hoping some fellow programmer has a good word for it!
Thing is, I am making an ExpressionBuilder class to help me build up expressions to use with LinqToSQL. And my problem is about how word myself when describing what two methods. And it kind of is a problem in general for me too when talking about it. Here is the issue:
You have an Expression<Func<T, bool>>, A. Later you get another one, B. You are now going to combine that B with A using && / AndAlso or || / OrElse. So for example like this:
A = A && B;
Alright. So, what did you just do there? What is the verb for what you did with B to A? If you think in a series of this stuff, like A = A && B && C && D && E && ..., you could sort of say that you then "add" F to that series. But that wouldn't really be correct either I feel...
What I feed would be most "correct" would be that you take B and you "and" it to/with A. You take B and you "or" it to/with A. But can "and" and "or" be used as a verb?? Is that considered ok? Feels like incredibly bad English... but maybe it is ok in a programming environment? Or?
If I was speaking to a mathematician, I would probably use terms like "perform a logical conjunction" (or disjunction).
If I was speaking to a fellow programmer, I would use "and" and "or" as verbs directly.
If I was speaking with my mom, I would probably just find pen and paper and start drawing Venn diagrams.
In logic AND is the conjunction operator, so you are conjoining A and B. OR is disjoining.
I think it is perfectly ok to use "and" as a verb in this case. You and'd A and B. It just seems bad due to the words AND and OR themselves. If you talk about it with XOR though, it doesn't sound so bad to say you XOR'd something yet you're effectively saying the same thing.
Compound?
Naming is always one of the hardest things.
If you are adding (e.g. numbers, or items to a set/list) then I'd say "Add"
If you are concatenating (e.g. strings) then I'd say "Append"
Alternatively... if you are just "adding" another item to a list... "Push" works too
Does the output of A feed into the input of B?
If so I'd use 'chain', or 'compose' in the sense of functional composition
Otherwise, if they're independant functions which are being combined, then maybe 'cat' as shorthand for concatenate.
In general, this is composition of functions. Since these functions are all predicates, you're putting them together with the various logical operations, so the specific composition would be conjunction, disjunction, etc. All the basic set theory terms I forgot since college!
How about logically connect?
-- http://en.wikipedia.org/wiki/Logical_connective
I'd go with Set Notation (Venn Diagrams) when explaining it.
AND: A intersected with B
OR: A unioned with B
http://www.purplemath.com/modules/venndiag2.htm
How can I recognize when a user has missed a space when entering a search term? For example, if the user enters "usbcable", I want to search for "usb cable". I'm doing a REGEX search in MySQL to match full words.
I have a table with every term used in a search, so I know that "usb" and "cable" are valid terms. Is there a way to construct a WHERE clause that will give me all the rows where the term matches part of the string?
Something like this:
SELECT st.term
FROM SearchTerms st
WHERE 'usbcable' LIKE '%' + st.term + '%'
Or any other ideas?
Text Segmentation is a part of Natural Language Processing, and is what you're looking for in this specific example. It's used in search engines and spell checkers, so you might have some luck with example source code looking at open source spell checkers and search engines.
Spell checking might be the correct paradigm to consider anyway, as you first need to know whether it's a legitimate word or not before trying to pry it apart.
-Adam
Posted in the comments, but I thought it important to bring up as an answer:
Does that query not work? – Simon Buchan
Followed by:
Well, I should've tested it before I
posted. That query does not work, but
with a CONCAT it does, like so: WHERE
'usbcable' LIKE Concat('%', st.term,
'%'). I think this is the simplest,
and most relevant (specific to my
site), way to go. – arnie0674
Certainly far easier than text segmentation for this application...
-Adam