For vs. While

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

I've been messing with my "Japh" thing (I tried to space it out for this node as best I could) and I noticed something peculiar (to me, at least):

V(qw/74 401 211 79/);
sub V{while(@_){
$v**=$V?$#_:$V;
print(
    map{
        $V<$v?
            chr(shift):
                chr(reverse(pop))}$_)
        ,$V+=$#_%2!=1?1:0}}
[download]

If I swap the while for a for, it only prints out "Ja".
Why might it do that?
I thought I may be cutting it off early by pop'ing off of the end, but that doesn't make any sense to me.

Thanks in advanced!




V(qw/74 401 211 79/);sub V{while(@_){$v**=$V?$#_:$V;
print(map{$V<$v?chr(shift):chr(reverse(pop))}$_),$V+=$#_%2!=1?1:0}}
[download]

Comment on For vs. While Select or Download Code

Replies are listed 'Best First'.
Re: For vs. While by ikegami (Patriarch) on Dec 05, 2006 at 23:57 UTC
`for (@array)` loops until the number of iterations is greater than the number of elements in the array. When you reached the 3rd iteration of your `for` loop, `@_` only has two elements (because of `shift` and `pop`), so the loop exits. `sub f1 { while (@_) { print(shift); } print("\n"); } sub f2 { for (@_) { print(shift); } print("\n"); } sub f3 { for (map $_, @_) { print(shift); } print("\n"); } f1(map/./g, Japh); # Japh f2(map/./g, Japh); # Ja f3(map/./g, Japh); # Japh` [download] `f2` iterates over the elements of `@_`. `f3` iterates over a premade list on the stack, so it doesn't matter that `@_` changes.	[reply] [d/l] [select]
Re^2: For vs. While by Andrew_Levenson (Hermit) on Dec 06, 2006 at 00:54 UTC
Ahh, that makes perfect sense. Thanks! `V(qw/74 401 211 79/);sub V{while(@_){$v**=$V?$#_:$V; print(map{$V<$v?chr(shift):chr(reverse(pop))}$_),$V+=$#_%2!=1?1:0}}` [download]	[reply] [d/l]
Re^3: For vs. While by ikegami (Patriarch) on Dec 06, 2006 at 01:10 UTC
That behaviour is specifically for `for (@array)`. `for (@array)` is different than other forms of `for (LIST)`. As soon as one iterates over anything but just an array, it's the size of the list on the stack that matters. The real difference is that `for (@array)` is optimized to avoid flattening `@array` onto the stack. Instead, `for (@array)` navigates `@array` in place. For all other expressions, a list is formed on the stack and `for` iterates over that. The following show the difference between iterating in place, and iterating over a list on the stack. `sub f4 { for (@_) { print($_); shift; } print("\n"); } sub f5 { for ((), @_) { print($_); shift; } print("\n"); } f4(map/./g, Japh); # Jp f5(map/./g, Japh); # Japh` [download]	[reply] [d/l] [select]
Re: For vs. While by Joost (Canon) on Dec 05, 2006 at 23:56 UTC
Pop and shift without arguments work on the @_ array (in subroutines at least), and modifying arrays update: that is; adding or removing elements - modifying elements is fine while looping through them with for(each) is IIRC not even officially supported. Even though it appears to do more or less what I'd expect. In short it doesn't work for exactly the reason you mentioned. :-) "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply]


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