source: downloads/tcl8.5.2/doc/for.n @ 35

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'\" RCS: @(#) $Id: for.n,v 1.9 2007/12/13 15:22:32 dgp Exp $
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.so man.macros
.TH for n "" Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
for \- 'For' loop
.SH SYNOPSIS
\fBfor \fIstart test next body\fR
.BE

.SH DESCRIPTION
.PP
\fBFor\fR is a looping command, similar in structure to the C
\fBfor\fR statement.  The \fIstart\fR, \fInext\fR, and
\fIbody\fR arguments must be Tcl command strings, and \fItest\fR
is an expression string.
The \fBfor\fR command first invokes the Tcl interpreter to
execute \fIstart\fR.  Then it repeatedly evaluates \fItest\fR as
an expression; if the result is non-zero it invokes the Tcl
interpreter on \fIbody\fR, then invokes the Tcl interpreter on \fInext\fR,
then repeats the loop.  The command terminates when \fItest\fR evaluates
to 0.  If a \fBcontinue\fR command is invoked within \fIbody\fR then
any remaining commands in the current execution of \fIbody\fR are skipped;
processing continues by invoking the Tcl interpreter on \fInext\fR, then
evaluating \fItest\fR, and so on.  If a \fBbreak\fR command is invoked
within \fIbody\fR
or \fInext\fR,
then the \fBfor\fR command will
return immediately.
The operation of \fBbreak\fR and \fBcontinue\fR are similar to the
corresponding statements in C.
\fBFor\fR returns an empty string.
.PP
Note: \fItest\fR should almost always be enclosed in braces.  If not,
variable substitutions will be made before the \fBfor\fR
command starts executing, which means that variable changes
made by the loop body will not be considered in the expression.
This is likely to result in an infinite loop.  If \fItest\fR 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:
.SH EXAMPLES
Print a line for each of the integers from 0 to 10:
.CS
for {set x 0} {$x<10} {incr x} {
   puts "x is $x"
}
.CE
.PP
Either loop infinitely or not at all because the expression being
evaluated is actually the constant, or even generate an error!  The
actual behaviour will depend on whether the variable \fIx\fR exists
before the \fBfor\fR 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 \fBfor\fR command is
executed.
.CS
for {set x 0} $x<10 {incr x} {
   puts "x is $x"
}
.CE
.PP
Print out the powers of two from 1 to 1024:
.CS
for {set x 1} {$x<=1024} {set x [expr {$x * 2}]} {
   puts "x is $x"
}
.CE

.SH "SEE ALSO"
break, continue, foreach, while

.SH KEYWORDS
for, iteration, looping