while

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 that 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, continue, for, foreach