Scroll to navigation

while(3tcl) Tcl Built-In Commands 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







  

    
    Tcl