Updated 2012-10-15 11:00:06 by dkf


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

Where you want to create a loop that continues forever, the standard idiom is:
while 1 { ... }

(Where 1 may be substituted with any other true value.) In this case, it is expected that the loop body will contain something that causes the loop to exit by some other means (e.g., break, return, exit, ...)

The only case where the test need not be braced is the endless loop (which should contain a break command inside):
 while 1 {...}

Or a return--there could be returns instead of break. Also, CL can imagine writing
      while $test_variable {...}

as a collapsed test-forever combination.

While discussing at the Tcl'ers Chat a while that would be safe in safe interpreters GPS came up with these:
 $ tclsh8.4
 % rename while real.while
 % proc do.while {cond body} { if {$cond} {uplevel 2 $body} }
 % proc while {cond body} { set start [clock seconds]; real.while 1 { do.while $cond $body; if {[clock seconds] >= ($start + 600)} { return -code error "too much time used"}}}

GPS And yes I know that $cond should be uplevel'd...

GPS Note that 600 seconds is somewhat crazy. I think 30 or 20 is good.

GPS Another trick would be to prevent calls to while while within a while.

GPS There could be a global lock (protected from set) that would just return -code error "existing while running" if the lock is active.

dkf Why the do.while in there?

GPS just seemed easier to type out do.while and think of this as units rather than one big proc

GPS Here's another without the do.while:
 % rename while real.while
 % proc while {cond body} { set start [clock seconds]; real.while 1 { if {[uplevel [linsert $cond 0 expr]]} { uplevel $body } else return; if {[clock seconds] > ($start + 10)} { return -code error "time limit exceeded in while loop"}}}

US See also DoS

See also edit