Updated 2016-10-01 19:16:28 by pooryorick

break, a built-in Tcl command, aborts a looping command.

See Also  edit

continue
return
for
foreach
while

Synopsis  edit

break

Description  edit

This command is typically invoked inside the body of a looping command such as for or foreach or while. It returns a TCL_BREAK code, which causes a break exception to occur. The exception causes the current script to be aborted out to the innermost containing loop command, which then aborts its execution and returns normally. Break exceptions are also handled in a few other situations, such as the catch command, Tk event bindings, and the outermost scripts of procedure bodies.

Documentation  edit

official reference

Examples  edit

Print a line for each of the integers from 0 to 5:
for {set x 0} {$x<10} {incr x} {
    if {$x > 5} {
        break
    }
    puts "x is $x"
}

Discussion  edit

One surprising use of break is to make sure a foreach loop runs exactly once (because we're only interested in the list assignment, not the actual body):
foreach {a b} [list $b $a] break ;# (1)

RS 2006-04-20: This code for swapping two variables can however be had simpler:
foreach a $b b $a break          ;# (2)

Lars H: No, that sets a to the first element of b and b to the first element of a, which isn't in general the same thing as swapping the values.

RS: True. I was thinking in terms of scalars only... For values parsable as lists, approach (1) is better.

justinalanbass: This has nothing to do with break; the body can just as effectively be replaced with anything.
foreach a $b b $a {set c 1}

RLE (2015-04-30): This is not the same, the break is in fact important. This idiom is the traditional way to perform an lassign in Tcl 8.4 and before. To emulate lassign, you want the foreach to only iterate once, therefore the break is required to assure only one iteration.

With your alternate version, the foreach loop will iterate a number of times equal to the list length of the longer list, the variables will receive the last elements of the lists (not the first elements), and one of the variables might even end up empty (if the lists are of different lengths).

Your version also introduces a subtle data dependent heisenbug. If the contents of $a and $b are simple scalars, it will appear to work properly. And will likely work for quite a long time. But the moment a string that is not parseable as a list happens to be placed into one (or both) of the variables, you'll get what appears to be a weird error message about "can not parse list from string". Murphy's law postulates that this data dependency will surface several years after you wrote the code, in the middle of the night, while you are away on a lengthy vacation, causing all kinds of havoc for something that up until that point had worked just fine. Or that it will surface several years later, when you modify something unrelated elsewhere in your code, and happen to create a non-parseable string for the foreach in the process. At which point you'll get a 'spooky bug at a distance' situation arising because your change and the bug will seem wholly unrelated.

escargo 2004-07-13: I recently noted that in Icon, the break expression takes an optional expression just like the return expression does. This allows a bit more information to come out of a break. (It's also probably not what Tcl considers an exception.)

AMG: Perl's break expression takes an argument which gives the number of levels to break. This avoids one common need for goto: breaking out of nested loops. Tcl has another way to do this, though:
try {
    foreach x {a b c} {
        foreach y {d c f} {
            puts [list $x $y]
            if {$x eq $y} {
                return -level 0 -code 5
            }
        }
    }
} on 5 {} {}

Some sugar:
proc loop {script} {tailcall try $script on 5 {} {}}
proc stop {} {return -code 5}
loop {
    foreach x {a b c} {
        foreach y {d c f} {
            puts [list $x $y]
            if {$x eq $y} {
                stop
            }
        }
    }
}