88 lines
2.9 KiB
Plaintext
88 lines
2.9 KiB
Plaintext
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1997 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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.TH for n "" Tcl "Tcl Built-In Commands"
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.so man.macros
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.BS
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'\" Note: do not modify the .SH NAME line immediately below!
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.SH NAME
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for \- 'For' loop
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.SH SYNOPSIS
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\fBfor \fIstart test next body\fR
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.BE
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.SH DESCRIPTION
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.PP
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\fBFor\fR is a looping command, similar in structure to the C
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\fBfor\fR statement. The \fIstart\fR, \fInext\fR, and
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\fIbody\fR arguments must be Tcl command strings, and \fItest\fR
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is an expression string.
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The \fBfor\fR command first invokes the Tcl interpreter to
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execute \fIstart\fR. Then it repeatedly evaluates \fItest\fR as
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an expression; if the result is non-zero it invokes the Tcl
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interpreter on \fIbody\fR, then invokes the Tcl interpreter on \fInext\fR,
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then repeats the loop. The command terminates when \fItest\fR evaluates
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to 0. If a \fBcontinue\fR command is invoked within \fIbody\fR then
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any remaining commands in the current execution of \fIbody\fR are skipped;
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processing continues by invoking the Tcl interpreter on \fInext\fR, then
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evaluating \fItest\fR, and so on. If a \fBbreak\fR command is invoked
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within \fIbody\fR
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or \fInext\fR,
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then the \fBfor\fR command will
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return immediately.
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The operation of \fBbreak\fR and \fBcontinue\fR are similar to the
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corresponding statements in C.
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\fBFor\fR returns an empty string.
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.PP
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Note: \fItest\fR should almost always be enclosed in braces. If not,
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variable substitutions will be made before the \fBfor\fR
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command starts executing, which means that variable changes
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made by the loop body will not be considered in the expression.
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This is likely to result in an infinite loop. If \fItest\fR is
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enclosed in braces, variable substitutions are delayed until the
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expression is evaluated (before
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each loop iteration), so changes in the variables will be visible.
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See below for an example:
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.SH EXAMPLES
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.PP
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Print a line for each of the integers from 0 to 9:
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.PP
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.CS
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\fBfor\fR {set x 0} {$x<10} {incr x} {
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puts "x is $x"
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}
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.CE
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.PP
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Either loop infinitely or not at all because the expression being
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evaluated is actually the constant, or even generate an error! The
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actual behaviour will depend on whether the variable \fIx\fR exists
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before the \fBfor\fR command is run and whether its value is a value
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that is less than or greater than/equal to ten, and this is because
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the expression will be substituted before the \fBfor\fR command is
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executed.
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.PP
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.CS
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\fBfor\fR {set x 0} $x<10 {incr x} {
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puts "x is $x"
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}
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.CE
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.PP
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Print out the powers of two from 1 to 1024:
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.PP
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.CS
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\fBfor\fR {set x 1} {$x<=1024} {set x [expr {$x * 2}]} {
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puts "x is $x"
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}
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.CE
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.SH "SEE ALSO"
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break(n), continue(n), foreach(n), while(n)
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.SH KEYWORDS
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boolean, for, iteration, loop
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'\" Local Variables:
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'\" mode: nroff
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'\" End:
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