66 lines
2.2 KiB
Plaintext
66 lines
2.2 KiB
Plaintext
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'\"
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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 while 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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while \- Execute script repeatedly as long as a condition is met
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.SH SYNOPSIS
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\fBwhile \fItest body\fR
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.BE
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.SH DESCRIPTION
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.PP
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The \fBwhile\fR command evaluates \fItest\fR as an expression
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(in the same way that \fBexpr\fR evaluates its argument).
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The value of the expression must a proper boolean
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value; if it is a true value
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then \fIbody\fR is executed by passing it to the Tcl interpreter.
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Once \fIbody\fR has been executed then \fItest\fR is evaluated
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again, and the process repeats until eventually \fItest\fR
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evaluates to a false boolean value. \fBContinue\fR
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commands may be executed inside \fIbody\fR to terminate the current
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iteration of the loop, and \fBbreak\fR
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commands may be executed inside \fIbody\fR to cause immediate
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termination of the \fBwhile\fR command. The \fBwhile\fR command
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always 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 \fBwhile\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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For an example, try the following script with and without the braces
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around \fB$x<10\fR:
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.PP
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.CS
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set x 0
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\fBwhile\fR {$x<10} {
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puts "x is $x"
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incr x
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}
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.CE
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.SH EXAMPLE
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.PP
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Read lines from a channel until we get to the end of the stream, and
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print them out with a line-number prepended:
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.PP
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.CS
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set lineCount 0
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\fBwhile\fR {[gets $chan line] >= 0} {
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puts "[incr lineCount]: $line"
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}
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.CE
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.SH "SEE ALSO"
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break(n), continue(n), for(n), foreach(n)
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.SH KEYWORDS
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boolean, loop, test, while
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