I am programming in tcl\tk.
The code is showing following error:
"missing operand at _#_
in expression "+_#_""
entry .e1 -textvar a
entry .e2 -textvar b
message .m -textvar c
button .b -text "press here" -command "set c [expr $a+$b]"
The error is showing in the last line. I am running it in tclsh and showing the same error. I have also tried using it in function proc but the same error is popping up.
I am trying to do arithmetic operations using tk.
When the 4th line is evaluated by the interpreter it expands the contents of the quoted part and will execute the expr command with the contents of a and b expanded. However, these variables have no value at that point in time. You meant to evaluate that command when the user clicks the button but it is being evaluated when the button is created.
The quick fix is you need:
button .b -text "press here" -command {set c [expr {$a + $b}]}
provided a and b are global as the command will be evaluated in the global namespace when the button is clicked.
The longer fix is you need to read the Tcl man page quite carefully with attention to the description of the differences between quoted "" and grouped {} expressions in Tcl.
Related
Let's consider the following code:
package require Tk
proc test {} {
foreach n {
1 2 3 4 5 6 7 8 9
} {
pack [button ._$n -text $n -command {puts $n}]
}
}
test
When one of the buttons invoked, "n" is unknown.
I found a away to address this by changing {puts $n} to "puts $n", but not sure this is a correct approach.
The callback for the button command is executed in the global scope, and there's no n variable there.
If you add global n command into that proc, then the error message won't appear, but each button will print the same value of n.
As you intent is to associate the value of n for each button, you need to pick a different quoting mechanism: braces prevent variable expansion. See https://www.tcl-lang.org/man/tcl8.6/TclCmd/Tcl.htm -- Shawn's comment gave you the answer.
You have to bind the current value of $n to the callback at the time you set it; your code as it stands uses whatever happens to be in the global n variable at the time that the callback is invoked (i.e., it is either the same for all the buttons or an error).
The list command is designed to be perfect for doing that binding; it generates a list, yes, but it also guarantees to generate a substitution-free command that has the words that are its arguments. That is, the script/command call:
eval [list $a $b $c]
is guaranteed to be the same as:
$a $b $c
for any values at all. This is an exceptionally useful property for almost any kind of code generation since it lets you make a script — that can be a call to a procedure for anything complicated — and pass any values over, entirely safely. In your case, this means that you should change change:
pack [button ._$n -text $n -command {puts $n}]
to:
pack [button ._$n -text $n -command [list puts $n]]
Thanks for your answers!
I ended up with the following possible ways:
"puts $n" - good for simple cases, but a bit non-intuitive
[list puts $n] - good, especially for lists
[subst {puts $n}] - good, straightforward substitution
button .mltext.button -text "Apply" -command {set top_tlbl [update_text $top_tlbl $spc ] }
I get an error :
can't read "spc": no such variable
while executing
"update_text $top_tlbl $spc "
invoked from within
".mltext.button invoke"
How can I pass the values of the variables to the update_text function?
Maybe I can start by understanding this :
(System32) 3 % expr 2 + 2
4
(System32) 4 % list expr 2 + 2
expr 2 + 2
(System32) 5 % [list expr 2 + 2]
invalid command name "expr 2 + 2"
(System32) 6 %
According to me, the last one should generate expr 2 + 2, which is the same as the first command - so, why does TCL have a problem?
Thanks..
In your examples with expr, we start by counting the words.
expr 2 + 2 has four words: expr, 2, + and 2. The first one is the command name, expr, and the others are passed as arguments; the documentation for expr says that it concatenates its arguments and evaluates the resulting expression.
list expr 2 + 2 has five words: list, expr, 2, + and 2. The first one is the command name, list, and the others are passed as arguments; the documentation for list say that it returns the list that has its arguments as elements. Though we don't see it here explicitly, what this does is introduce exactly the quoting needed to make a single substitution-free command. Good Tcl code uses list quite a lot when generating code.
[list expr 2 + 2] has one word, which is the result of calling list expr 2 + 2. If you just feed that into Tcl directly, it has a weird but legal command name, and you probably don't have a command called that. Hence you get an error.
Now let's consider:
button .mltext.button -text "Apply" -command {set top_tlbl [update_text $top_tlbl $spc]}
From your comments, you're calling this inside a procedure. This means that you need to bind the variables inside the callback (which is evaluated inside uplevel #0, and probably long after your current procedure has returned) without doing the callback immediately. Curiously, it looks like you're changing top_tlbl at runtime too.
Let's start by thinking about the innermost part. We can generate it with list just fine (ignoring the different lifetime bindings):
list update_text $top_tlbl $spc
Now we've just got to make the other parts work as well. That's where it gets fiddly and you end up with something like this:
… -command "set top_tlbl \[[list update_text $top_tlbl $spc]\]"
Now let's fix the lifetime stuff:
… -command "set top_tlbl \[update_text \$top_tlbl [list $spc]\]"
This sort of thing is more than a bit error-prone in complicated callbacks! At that point, it's much easier (and good practice) to have a little helper procedure:
proc do_update_text {varname value} {
upvar #0 $varname var
set var [update_text $var $value]
}
Then you can do (note: passing the name of top_tlbl, not the value):
… -command [list do_update_text top_tlbl $spc]
The use of global variable names might be considered to be problematic, except you've got to remember that the global namespace is mainly owned by your application. If you want to store variables in globals for your app, do it! You have the complete right.
Library code needs to be a bit more careful of course. The usual techniques there include using variables in other namespaces (Tk does this internally) or in objects. The code doesn't really get that much more complicated; it's still building on the practice of using helper procedures listed above. Passing a namespaced variable name is pretty easy though:
… -command [list do_update_text ::mynamespace::top_tlbl $spc]
-command {set top_tlbl [update_text $top_tlbl $spc]} will be invoked in global context when the button is clicked. Is your variable spc accessible in global context? Try to specify the variable's namespace explicitly, for example $::spc.
expr 2 + 2 and "expr 2 + 2" are not the same. Tcl identifiers can contain spaces and "expr 2 + 2" is the whole identifier, which is not found.
You should probably just call update_text and have top_tlbl be a global and set from the called function rather than returned.
Its generally simplest to just try and call a simple function and use list to ensure everything is quoted correctly.
button .mltext.button -text "Apply" \
-command [list update_text $top_tlbl $spc]
This will capture the current values of top_tlbl and spc when you define this button and its command. If you want the values at the time you press the button then you should probably be passing the names of the variables and using the variable or global commands.
Anyway, the rules are that things inside curly braces {} have no substututions performed on them. So {$v} is passed to the callee as exactly that - dollar v. Using list or double quotes lets variable substitution be performed and so you pass the value of the variable. List properly handles quoting for cases where the variable value might contain spaces and so on.
button .mltext.button -text "Apply" -command {set top_tlbl [update_text $top_tlbl $spc ] }
I get an error :
can't read "spc": no such variable
while executing
"update_text $top_tlbl $spc "
invoked from within
".mltext.button invoke"
How can I pass the values of the variables to the update_text function?
Maybe I can start by understanding this :
(System32) 3 % expr 2 + 2
4
(System32) 4 % list expr 2 + 2
expr 2 + 2
(System32) 5 % [list expr 2 + 2]
invalid command name "expr 2 + 2"
(System32) 6 %
According to me, the last one should generate expr 2 + 2, which is the same as the first command - so, why does TCL have a problem?
Thanks..
In your examples with expr, we start by counting the words.
expr 2 + 2 has four words: expr, 2, + and 2. The first one is the command name, expr, and the others are passed as arguments; the documentation for expr says that it concatenates its arguments and evaluates the resulting expression.
list expr 2 + 2 has five words: list, expr, 2, + and 2. The first one is the command name, list, and the others are passed as arguments; the documentation for list say that it returns the list that has its arguments as elements. Though we don't see it here explicitly, what this does is introduce exactly the quoting needed to make a single substitution-free command. Good Tcl code uses list quite a lot when generating code.
[list expr 2 + 2] has one word, which is the result of calling list expr 2 + 2. If you just feed that into Tcl directly, it has a weird but legal command name, and you probably don't have a command called that. Hence you get an error.
Now let's consider:
button .mltext.button -text "Apply" -command {set top_tlbl [update_text $top_tlbl $spc]}
From your comments, you're calling this inside a procedure. This means that you need to bind the variables inside the callback (which is evaluated inside uplevel #0, and probably long after your current procedure has returned) without doing the callback immediately. Curiously, it looks like you're changing top_tlbl at runtime too.
Let's start by thinking about the innermost part. We can generate it with list just fine (ignoring the different lifetime bindings):
list update_text $top_tlbl $spc
Now we've just got to make the other parts work as well. That's where it gets fiddly and you end up with something like this:
… -command "set top_tlbl \[[list update_text $top_tlbl $spc]\]"
Now let's fix the lifetime stuff:
… -command "set top_tlbl \[update_text \$top_tlbl [list $spc]\]"
This sort of thing is more than a bit error-prone in complicated callbacks! At that point, it's much easier (and good practice) to have a little helper procedure:
proc do_update_text {varname value} {
upvar #0 $varname var
set var [update_text $var $value]
}
Then you can do (note: passing the name of top_tlbl, not the value):
… -command [list do_update_text top_tlbl $spc]
The use of global variable names might be considered to be problematic, except you've got to remember that the global namespace is mainly owned by your application. If you want to store variables in globals for your app, do it! You have the complete right.
Library code needs to be a bit more careful of course. The usual techniques there include using variables in other namespaces (Tk does this internally) or in objects. The code doesn't really get that much more complicated; it's still building on the practice of using helper procedures listed above. Passing a namespaced variable name is pretty easy though:
… -command [list do_update_text ::mynamespace::top_tlbl $spc]
-command {set top_tlbl [update_text $top_tlbl $spc]} will be invoked in global context when the button is clicked. Is your variable spc accessible in global context? Try to specify the variable's namespace explicitly, for example $::spc.
expr 2 + 2 and "expr 2 + 2" are not the same. Tcl identifiers can contain spaces and "expr 2 + 2" is the whole identifier, which is not found.
You should probably just call update_text and have top_tlbl be a global and set from the called function rather than returned.
Its generally simplest to just try and call a simple function and use list to ensure everything is quoted correctly.
button .mltext.button -text "Apply" \
-command [list update_text $top_tlbl $spc]
This will capture the current values of top_tlbl and spc when you define this button and its command. If you want the values at the time you press the button then you should probably be passing the names of the variables and using the variable or global commands.
Anyway, the rules are that things inside curly braces {} have no substututions performed on them. So {$v} is passed to the callee as exactly that - dollar v. Using list or double quotes lets variable substitution be performed and so you pass the value of the variable. List properly handles quoting for cases where the variable value might contain spaces and so on.
I have a code like this
proc press2 {v sbit} {
puts $v
puts $sbit
}
:
:
button .t.ok2 -text "OKI" -command "press2 $v $sbit"
with this I get the error wrong # args: should be "press2 v sbit"
if I change it to button .t.ok2 -text "OKI" -command {press2 $v $sbit} I get can't read "v": no such variable and finally I tried button .t.ok2 -text "OKI" -command [press2 $v $sbit] which doesn't give any errors but doesn't work also. Just asking is there any good documentation available for TCL/TK ? The usual man pages and googling isnt helping me much. I am doing much by trial and error.
Passing multiple arguments to a procedure is easy, but the "correct" way depends on what you want:
Early Binding: If you want to pass the current values later (e.g. if you create the widgets in a loop) you need to use list:
button .t.ok2 -text OKI -command [list press2 $v $sbit]
list creates a command that is free from any further substitution*.
Late Binding: If you want pass the value when this command is executed, simply brace it with {}
button .t.ok2 -text OKI -command {press2 $v $sbit}
The variables v and sbit are subsituted when the button is pressed. You only have access to global variables (or variables in a namespace, but not local variables).
* Tk's bind replaces % and a following character with something special. This is done using string substitution, not Tcl substitution, so list does not guard against this.
I'm creating a small piece of GUI that is a must complete for the progression of the flow. What I want is to create a proc that creates a GUI and returns 1 or 0 when the GUI is closed and then the flow continues, like this:
first part of the code
...
...
if {![open_gui]} {
return
}
second part of the code
...
...
The GUI is simple 3 entries with a save and cancel buttons, if the save button is pressed, then some values should be stored to the data model and the function to return 1, if the cancel button is pressed, or the GUI is closed by closing the window then nothing should happen and the proc to return 0.
Is this possible?
Right now what I did is to break the code into two peaces, (code_part_1 and code_part_2) I run the first part, then open the GUI and the save button calls the second part, while the cancel just closes the GUI:
code_part_1
open_gui_split
And the function open_gui_split is:
proc open_gui_split {} {
# ...
set save_b [button $win.save_b -text save -command [list code_part_2]
# ...
}
* - All the code presented is only a representation of the architecture and not the real code.
It is entirely possible to create commands that run a Tk GUI, waiting for response from a user and returning that value. The key to doing that is the tkwait command:
proc popUpButton {w} {
toplevel $w
pack [button $w.b -text "push me" -command [list destroy $w]]
# This waits in the event loop until $w is destroyed...
tkwait window $w
return "button was pushed"
}
puts "about to pop up the button"
puts ">>[popUpButton]<<"
puts "popped up the button"
tkwait comes in three varieties:
tkwait window $w waits for the window $w to be destroyed.
tkwait visibility $w waits for the window $w to become visible (but doesn't work on platforms other than Unix/X11).
tkwait variable $varname waits for variable $varname to be set; it's just like plain Tcl vwait (and in fact vwait was originally tkwait variable before the integration of the event loop into Tcl).
Be aware that re-entering the event loop increases the stack depth and can cause your code to get very confused if you are not careful. You will probably want to use focus and grab to ensure that users only interact with the popped up dialog.
Finally, to see a more complete example of how this all works, look at the source to tk_dialog (that's exactly the version from Tk 8.4.19, direct from our repository) which is just plain old Tcl code and does the sort of thing you're after. It's a much more completely worked example than I want to write, showing off things like how to get a value to return that's based on user input.