I'm learning about Tcl just now. I've seen just a bit of it, I see for instance to create a variable (and initialize it) you can do
set varname value
I am familiarizing with the fact that basically everything is a string, such as "value" above, but "varname" gets kind of a special treatment I guess because of the "set" built-in function, so varname is not interpreted as a string but rather as a name.
I can later on access the value with $varname, and this is fine to me, it is used to specify varname is not to be considered as a string.
I'm now reading about lists and a couple commands make me a bit confused
set colors {"aqua" "maroon" "cyan"}
puts "list length is [llength $colors]"
lappend colors "purple"
So clearly "lappend" is another one of such functions like set that can interpret the first argument as a name and not a string, but then why didn't they make it llength the same (no need for $)?
I'm thinking that it's just a convention that, in general, when you "read" a variable you need the $ while you don't for "writing".
A different look at the question: what Tcl commands are appropriate for list literals?
It's valid to count the elements of a list literal:
llength {my dog has fleas}
But it doesn't make sense to append a new element to a literal
lappend {my dog has fleas} and ticks
(That is actually valid Tcl, but it sets the odd variable ${my dog has fleas})
this is more sensible:
set mydog {my dog has fleas}
lappend mydog and ticks
Names are strings. Or rather a string is a name because it is used as a name. And $ in Tcl means “read this variable right now”, unlike in some other languages where it really means “here is a variable name”.
The $blah syntax for reading from a variable is convenient syntax that approximately stands in for doing [set blah] (with just one argument). For simple names, they become the same bytecode, but the $… form doesn't handle all the weird edge cases (usually with generated names) that the other one does. If a command (such as set, lappend, unset or incr) takes a variable name, it's because it is going to write to that variable and it will typically be documented to take a varName (variable name, of course) or something like that. Things that just read the value (e.g., llength or lindex) will take the value directly and not the name of a variable, and it is up to the caller to provide the value using whatever they want, perhaps $blah or [call something].
In particular, if you have:
proc ListRangeBy {from to {by 1}} {
set result {}
for {set x $from} {$x <= $to} {incr x $by} {
lappend result $x
}
return $result
}
then you can do:
llength [ListRangeBy 3 77 8]
and
set listVar [ListRangeBy 3 77 8]
llength $listVar
and get exactly the same value out of the llength. The llength doesn't need to know anything special about what is going on.
Related
I have set 3 global variables as below in a file FILE1:
set VAR1 2
set VAR2 3
set VAR3 4
Now I want to use these 3 variables in another file FILE2 in iterative way:
Means, something like this:
for {set a 1} {$a < 4} {incr a} {
$::VAR$a
}
where VAR$a - should be incremented each time to VAR1,VAR2,VAR3 etc...
But if I try like this using the global variable I get error in tcl
Any better solution for this?
Either make your meaning clearer to the interpreter
set ::VAR$a
(you are aware that this is just getting the variable's value without doing anything with the value, that is, a pointless operation, right?)
Or use an array, which is basically a two-part variable name:
set ::VAR($a)
in which case you need to initialize as an array:
set VAR(1) 2
etc, or
array set VAR {1 2 2 3 3 4}
The reason why $::VAR$a doesn't always work is AFAICT that the variable substitution becomes ambiguous. Given these definitions:
set foobar 1
set a foo
set b bar
what should $a$b substitute into? To avoid ambiguity, the substitution rules are kept simple: the first substitution stops before the second dollar sign, and the whole expression evaluates to the string foobar. How about $$a$b to substitute the value of foobar, then? No, a dollar-sign followed directly by a character that can't be a part of a variable name means that the first dollar sign becomes just a dollar sign: you get $foobar. The best way to handle this is to reduce the levels of substitution using the set command to get a value: set $a$b. Bottom line: variable substitution using $ does not always work well, but the set always does the job.
Documentation:
set,
Summary of Tcl language syntax
In my scripts, when using a variable, I generally empty the contents of a variable to ensure that the list appends are clean. Something like the following
set var1 [list]
foreach var2 {a b c} {
lappend var1 $var2
}
But it seems like unsetting the variable first would accomplish the same thing. Something like this
unset -nocomplain var1
foreach var2 {a b c} {
lappend var1 $var2
}
Is there any advantage for using one vs the other?
It doesn't make any difference in this case. If I was to write such a loop in my own code, I would be more likely to use set var {} since that is the empty list literal (as well as being the empty string, the empty dictionary, the empty script, etc.) but there isn't any execution time difference to speak of. It just reflects how I think about scripts.
Of course, if you are doing something where it does matter, use the right one for that case.
As Donal wrote, set var {}. Internally, the same value will be assigned regardless of whether you assign {}, [list] etc. Yes, it will shimmer, and no, it won't be a problem.
Regarding set vs unset: while you can use them as you see fit, they mostly serve different patterns. In the assign-empty-value pattern, you want a variable ready for writing or reading, with a predefined, empty value. In the remove-from-scope pattern you want the name to be unused (it won't be unusable: you can still assign to / create it). Unless you're after something like the second pattern, you probably won't have much serious use for unset.
I can't understand how assignments and use of variables work in Tcl.
Namely:
If I do something like
set a 5
set b 10
and I do
set c [$a + $b]
Following what internet says:
You obtain the results of a command by placing the command in square
brackets ([]). This is the functional equivalent of the back single
quote (`) in sh programming, or using the return value of a function
in C.
So my statement should set c to 15, right?
If yes, what's the difference with
set c [expr $a + $b]
?
If no, what does that statement do?
Tcl's a really strict language at its core; it always follows the rules. For your case, we can therefore analyse it like this:
set c [$a + $b]
That's three words, set (i.e., the standard “write to a variable” command), c, and what we get from evaluating the contents of the brackets in [$a + $b]. That in turn is a script formed by a single command invocation with another three words, the contents of the a variable (5), +, and the contents of the b variable (10). That the values look like numbers is irrelevant: the rules are the same in all cases.
Since you probably haven't got a command called 5, that will give you an error. On the other hand, if you did this beforehand:
proc 5 {x y} {
return "flarblegarble fleek"
}
then your script would “work”, writing some (clearly defined) utter nonsense words into the c variable. If you want to evaluate a somewhat mathematical expression, you use the expr command; that's it's one job in life, to concatenate all its arguments (with a space between them) and evaluate the result as an expression using the documented little expression language that it understands.
You virtually always want to put braces around the expression, FWIW.
There are other ways to make what you wrote do what you expect, but don't do them. They're slow. OTOH, if you are willing to put the + first, you can make stuff go fast with minimum interference:
# Get extra commands available for Lisp-like math...
namespace path ::tcl::mathop
set c [+ $a $b]
If you're not a fan of Lisp-style prefix math, use expr. It's what most Tcl programmers do, after all.
set c [$a + $b]
Running the above command, you will get invalid command name "5" error message.
For mathematical operations, we should rely on expr only as Tcl treats everything as string.
set c [expr $a + $b]
In this case, the value of a and b is passed and addition is performed.
Here, it is always safe and recommended to brace the expressions as,
set c [expr {$a+$b}]
To avoid any possible surprises in the evaluation.
Update 1 :
In Tcl, everything is based on commands. It can a user-defined proc or existing built-in commands such as lindex. Using a bare-word of string will trigger a command call. Similarly, usage of [ and ] will also trigger the same.
In your case, $a replaced with the value of the variable a and since they are enclosed within square brackets, it triggers command call and since there is no command with the name 5, you are getting the error.
I have to print multiple variables in a single puts like this
puts "$n1_$n2_$n3_$n4"
where n1 , n2 , n3 , n4 are 4 variables.
It wont print and will show error n1_ : no such variable
Expected output should be something like this (example)
01_abc_21_akdd
Variable names in Tcl can be any string in Tcl, there are no restrictions but if you want to use special characters (those not in the range of a-z, 0-9 and _, and letters in different languages depending on the platform and locale), you have to either brace the expression names or use other workarounds (like with the answer of Hoodiecrow).
What this means is that if you have a variable named abc.d, and if you use $abc.d, the Tcl engine will try to find the variable $abc because . is not a 'normal' character.
But if you have a variable named abc and use $abcd, or $abc_d, then the engine will start looking for the variables abcd or abc_d and not abc.
Because of this, you will have to use braces between the variable name for example:
${n1}
The reason why putting a backslash works is that \ is not a 'normal' character and after reading the above, it should be a little more obvious how things worked.
There are a few things that yet can go in variable names which don't need bracing and still mean something, except that something is 'special':
::: This is usually used for scoping purposes. For instance if you have a global variable named my_var, you can also use $::my_var to refer to it. Here :: tells Tcl that my_var is a global variable. Note that if there are more than two : in a row they will not add up:
% set ::y 5
5
% set ::::y
5
% set :::y
5
:: is usually used to define the namespace the variable is in. For example, $mine::var is a variable called var in the namespace with a name of mine.
(): These are used for arrays. $arr(key) is a variable with two parts: the array name arr and the key name key. Note: you can have an array named and a key named because...
% set () abc
abc
% puts $()
abc
% array get ""
{} abc
There might be some more, but those are the basics you could look out for.
Two other ways:
puts "${n1}_${n2}_${n3}_${n4}"
puts [format "%s_%s_%s_%s" $n1 $n2 $n3 $n4]
Documentation: format
(Note: the 'Hoodiecrow' mentioned in Jerry's answer is me, I used that nick earlier.)
I am a newbie in TCL Programming. I was having confusion about curly braces, answer to this question tcl curly braces cleared most of my doubts.
I can understand $var, {var}, and {$var}, But recently I came across another use of curly braces, ${var}. How is this interpreted by TCL?
I have seen this is used when accessing variables in namespaces when namespaces name is in variable.
for example:
set x myNamespace ;#myNamespace is name of namespace
puts [set ${x}::var1] ;#var1 is variable in the namespace
It gives error when you don't use curly braces around 'x'.
And I also don't understand the difference between {a b c} and [list a b c], what is the difference in result of interpretation of these two commands by TCL interpretation.
elaborated explanation would be highly appreciated.
See rule 8 of the manual. It allows you to have variable names that might get mis-interpreted. For instance:
% set dotted.name 1
1
% puts $dotted.name
can't read "dotted": no such variable
% puts ${dotted.name}
1
Read section 8 carefully as it actually explains all this quite explicitly.
Update to answer edited question
In the example you provide using a namespace name in a variable you must consider section 8 part 1: a variable name includes letters, digits, underscores and namespace separators. This means that x::var1 is a valid variable name. So $x::var1 will attempt to dereference the var1 variable in the x namespace. As this is not what you meant, you must dereference your x variable separately. There are two ways to do this. You can either use the set command or the dollar operator.
set x myNamespace
puts [set ${x}::var1]
puts [set [set x]::var1]
The two puts statements are equivalent here with the second version showing an explicit separate pass to obtain the value of the x variable which is then substituted into the expression for the outer set command. The same occurs in the first version but just uses the grouping operator to restrict the effect of the dollar to the x variable name.