for(n) Tcl Built-In Commands for(n) ______________________________________________________________________________
NAME
for - 'For' loop
SYNOPSIS
for start test next body
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DESCRIPTION
For is a looping command, similar in structure to the C for statement.
The start, next, and body arguments must be Tcl command strings, and
test is an expression string. The for command first invokes the Tcl
interpreter to execute start. Then it repeatedly evaluates test as an
expression; if the result is non-zero it invokes the Tcl interpreter on
body, then invokes the Tcl interpreter on next, then repeats the loop.
The command terminates when test evaluates to 0. If a continue command
is invoked within body then any remaining commands in the current exe-
cution of body are skipped; processing continues by invoking the Tcl
interpreter on next, then evaluating test, and so on. If a break com-
mand is invoked within body or next, then the for command will return
immediately. The operation of break and continue are similar to the
corresponding statements in C. For returns an empty string.
Note: test should almost always be enclosed in braces. If not, vari-
able substitutions will be made before the for command starts execut-
ing, which means that variable changes made by the loop body will not
be considered in the expression. This is likely to result in an infi-
nite loop. If test is enclosed in braces, variable substitutions are
delayed until the expression is evaluated (before each loop iteration),
so changes in the variables will be visible. See below for an example:
EXAMPLES
Print a line for each of the integers from 0 to 9:
for {set x 0} {$x<10} {incr x} {
puts "x is $x"
}
Either loop infinitely or not at all because the expression being eval-
uated is actually the constant, or even generate an error! The actual
behaviour will depend on whether the variable x exists before the for
command is run and whether its value is a value that is less than or
greater than/equal to ten, and this is because the expression will be
substituted before the for command is executed.
for {set x 0} $x<10 {incr x} {
puts "x is $x"
}
Print out the powers of two from 1 to 1024:
for {set x 1} {$x<=1024} {set x [expr {$x * 2}]} {
puts "x is $x"
}
SEE ALSO
break(n), continue(n), foreach(n), while(n)
KEYWORDS
boolean, for, iteration, loop
Tcl for(n)
tcl 8.6.5 - Generated Fri Mar 4 15:35:36 CST 2016