Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

i am new to perl, i have a simple doubt, can i change the length of the array while using foreach loop, and can i shift or push elements into the array? if i shift the elements inside the array which element will the loop take for the next iteration, same way what is the case when i push?

Replies are listed 'Best First'.
Re: Question on arrays
by neshura (Chaplain) on Feb 12, 2000 at 03:58 UTC
    I agree with btrott. Messing with the array elements is a Bad Idea(tm). It is absurdly easy to set up an infinite loop (or worse -- worse being subtle bugs rather than obvious bugs). I don't have a book handy, but from what I can tell by testing a few configurations, for and foreach both do implicit array operations that are, to the programmer, black-boxed. Don't mess with the box if you are new to perl.

    The code btrott posts shows the implicit behavior. $_ is shifted/unshifted in every loop. Within the second loop, 'bar' is shifted from @arr into $_; as a result, the _explicit_ shift removes baz. When the loop ends, $_ is unshifted back onto the array but 'baz' is lost forever. @arr is reduced to 'foo bar quack'.

    Of course, this could come in handy for obfuscation.

[reply]
Re: Question on arrays
by japhy (Canon) on Feb 11, 2000 at 22:08 UTC
    When going through an array, a for() loop will keep track the index of the element it's at, so it knows what the next element will be. 
    for (@array) { ... }
# is kind of like
for ($i = 0; $_ = $array[$i], $i < @array; $i++) { ... }

    [download]
[reply]
[d/l]
Re: Question on arrays
by btrott (Parson) on Feb 12, 2000 at 00:54 UTC
    According to perldoc perlsyn: 
    If any part of LIST is an array, foreach will get very
confused if you add or remove elements within the loop
body, for example with splice. So don't do that.

    [download]
    So, it seems to me, you probably don't want to be adding or removing elements while within a for loop. Was that your question?

    For example, I just tested this w/ shift. I looped through an array using foreach and did a shift part of the way through: 

    my @arr = qw/foo bar baz quack/;
for (@arr) {
    print $_, "\n";
    shift @arr if $_ eq "bar";
}

    [download]
    Here's the output: 
    foo
bar
quack

    [download]
    So the shift shifted off "foo", which messed up the internal loop counter, which means that "baz" didn't get displayed. So this probably isn't the best idea.
[reply]
[d/l]
[select]

Back to Seekers of Perl Wisdom