Flow-control structures: Difference between revisions

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The <tt>after</tt> facility can be used to execute some code at some future time asynchronously, like this

after 1000 {myroutine x}

<lang tcl>after 1000 {myroutine x}</lang>

which will call "<tt>myroutine</tt>" with parameter "<tt>x</tt>" 1000ms from 'now'; no matter what other code might be running at the time (i.e. "<tt>after</tt>" schedules the execution, then returns and continues program flow with the following code).

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The scheduled task can be removed from the scheduler for example with

after cancel myroutine

<lang tcl>after cancel myroutine</lang>

(other ways are possible).

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The correct way to schedule some regularly recurring task in TCL is to incorporate a self-scheduling at the end of the routine. For example the following will produce a clock whose display is updated once a second:

<lang tcl>package require Tk

proc update {} {

.clockface configure -text [clock format [clock seconds]]

after 1000 update ; # call yourself in a second

}

# now just create the 'clockface' and call ;update' once:

pack [label .clockface]

update

update</lang>

=== loop control ===

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close $f

}</lang>

=== custom control structures ===

A novel aspect of Tcl is that it's relatively easy to create new control structures (more detail at http://wiki.tcl.tk/685).

~~Eg.~~ ~~defining~~ a command to perform some operation for each line of an input file:

For example, this example defines a command to perform some operation for each line of an input file:

<lang tcl>proc forfilelines {linevar filename code} {

upvar $linevar line ; # connect local variable line to caller's variable

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close $filechan

}</lang>

Now use it to print the length of each line of file "mydata.txt":

Now we can use it to print the length of each line of file "mydata.txt":

<lang tcl>forfilelines myline mydata.txt {

puts [string length $myline]