while - Execute script repeatedly as long as a condition is met
while test body
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
}
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"
}
break(3tcl),
continue(3tcl),
for(3tcl),
foreach(3tcl)
boolean, loop, test, while