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Defining events including variable values in Tk

With Tcl/Tk, you assign an action to a button when you define the button. The action is often specified by the -command option, and it's important to understand the difference between double quotes and braces in this context.

If you put the command in DOUBLE QUOTES, then any $ variable used within the command that's to be run will be interpreted using the variable's value at the time that the button is DEFINED.

If you put the command in CURLY BRACES, then any $ variable used within the command that's to be run will be interpreted at the time that the button is PRESSED.

For example:

#!/usr/bin/wish

proc action {ff ss} {
    puts "$ff and $ss"
    }

for {set k 1} {$k<5} {incr k} {
    button .first$k -text "First $k" -command "action one $k"
    button .second$k -text "Second $k" -command {action two $k}
    pack .first$k .second$k
    }
set k 7

All the buttons with "First" text will give different results when pressed - from 1 to 4. The buttons with "Second" text will all give the result 7 when pressed as that's the value that's been left in the variable k when the tcl/tk process enters its main loop.

earth-wind-and-fire:~/feb05 grahamellis$ wish wishdemo
one and 1
two and 7
one and 2
two and 7
one and 3
two and 7
one and 4
two and 7
earth-wind-and-fire:~/feb05 grahamellis$


See also Tk Events

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Related Material

Tcl/Tk - Events in Tk
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