http://qs321.pair.com?node_id=344274


in reply to Re: Re: CamelTrouble
in thread CamelTrouble

Well, I haven't read all of your code, so I can't say authoritatively that I know exactly what the bug is, but i have read the good_ones_go funtion, and fortunately your program is nice and moduler, and your variables names are fairly clear, (the only thing i needed to look up anywhere else in the script is what "@good_ones" contains, and what you pass to "good_ones_go" when you call it) so I think I have a pretty good idea what's going wrong.

Rather then just fixing it for you, let me ask you a question, what happens if you run this script.... 

# perl
use warnings;
use strict;
my @good_ones = (100..110);
foreach (0..$#good_ones) {
   print "$_: $good_ones[$_] ...";
   if (rand() < 0.5) {
       splice @good_ones,$_,1;
   }
   print "($#good_ones)\n";
}

[download]

(PS: you may be tempted to "fix" your script by adding a single call to "last;" ... I wouldn't recommend that. Consider what would happen if the camel moves one pixel, and that puts him close enough to pick up two monks.)

Replies are listed 'Best First'.
Re: Re: Re: Re: CamelTrouble
by mawe (Hermit) on Apr 13, 2004 at 16:31 UTC
    Thanks for the snippet! And thank you for NOT telling me how to fix it, I love sleepless nights ;-)
    After this night I read Vautrin's reply (after, very clever :-/) and tried this 
    ...
foreach (0..$#good_ones) {
    if (defined $good_ones[$_]) {
        print ...
...

    [download]
    The error messages are gone. Is this the solution? Maybe I'm silly, but I still have no idea why the code produced an error massage at all.
[reply]
[d/l]

      That's certainly a valid solution, but of course TMTOWTDI

      As for why the code produced the error in the first place, you are starting with an array of N elements, and you are iterating over a list of the indexes of the elements in that array. But while you iterate over those elements, you are deleting them from the orriginal array, which shortens it, but you've still got the indexes, so as you get to the end of your list of indexes, the last few indexes may not exist in the array anymore.

      Ie: if $#good_ones is 9, and the camel picks up $good_ones5 and you splice it out of the list, then when that foreach loop reaches 9, $good_ones9, it's going to be undef (because it's off the end of the array)

      Another valid solution would be to change your foreach(0..$#good_ones) to for(my $_=0;$_<=$#good_ones;), and only increment $_ if you do NOT splice the array. In that case, $#good_ones will be evaluated on each pass of the loop, and it will do the right thing as the array gets shorter and shorter.

[reply]
        Great explanation! Now even I can unerstand it ;-) Thank you!! (And thanks for your patience!)
[reply]

In Section Code Catacombs