while(3tcl)
NAME
while - Execute script repeatedly as long as a condition is met
SYNOPSIS
while test body ______________________________________________________________________________
DESCRIPTION
The while command evaluates test as an expression (in the same way that
expr evaluates its argument). The value of the expression must a
proper boolean value; if it is a true value then body is executed by
passing it to the Tcl interpreter. Once body has been executed then
test is evaluated again, and the process repeats until eventually test
evaluates to a false boolean value. Continue commands may be executed
inside body to terminate the current iteration of the loop, and break
commands may be executed inside body to cause immediate termination of
the while command. The while command always returns an empty string.
Note: test should almost always be enclosed in braces. If not,
variable substitutions will be made before the while 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 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. For an
example, try the following script with and without the braces around
$x<10:
set x 0
while {$x<10} {
puts "x is $x"
incr x
}
EXAMPLE
Read lines from a channel until we get to the end of the stream, and
print them out with a line-number prepended:
set lineCount 0
while {[gets $chan line] >= 0} {
puts "[incr lineCount]: $line"
}
SEE ALSO
break(3tcl), continue(3tcl), for(3tcl), foreach(3tcl)
KEYWORDS
boolean value, loop, test, while
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