in reply to Re^5: loop surprise
in thread loop surprise

Maybe it is a code smell...

But what I was trying to do is to find the smallest number in a certain range satisfying some condition and then go on using that found value.

Doing it like 

my $i;

for $i (1..10) {
  last if &condition($i);
}

&do_more_stuff_with_the_value_found($i);

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Looked (at the time) to be a straightforward solution and I was suprised that it did not work.

This is just for explaining where this comes from (I am not a really good programmer I am afraid...)

And even accepting that this is not the way to do it (there are of course many other ways), it is neverthless surprising that this actually does work: 

my $i;

for ($i=1; $i<=10; $i) {
  last if &condition($i);
}

&do_more_stuff_with_the_value_found($i);

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This is to my eyes at least an inconsistency (and thus a wart). But whatever, this is how it was, is and will be...

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Re^7: loop surprise
by Your Mother (Archbishop) on Apr 04, 2018 at 00:38 UTC

    That use case is completely sensible. Your original was a string overwritten with a count and reminded me of the awful code I've had to deal with over the years where there is one $i or $output and friends shared around huge scopes or even more than one script, concatted or overwritten willy nilly. Your use case in a small scope seems totally kosher though it's outside my style or sensibility.

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Re^7: loop surprise
by Skeeve (Parson) on Apr 04, 2018 at 10:02 UTC

    I can remember that long time (say: 30 or more years) ago, I also wrote loops like that.

    But now that you mention it, and thinking about it, I notice always write those kind of loops like: 

    
my $interesting_index= 'not existing';

for ($i=1; $i<=10; ++$i) {
  next unless &condition($i);
  $interesting_index= $i;
  last;
}
# spit an error if $interesting_index is still 'not existing'

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    The reason behind this is simply that your loop might not have found a valid value.

    s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
    +.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e
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      There are many variants of how to do this. My inclination might be: 
      my $interesting_index;
my $i = 0;

while (!defined($interesting_index) and $i++ <= 10)
{   
    $interesting_index = $i if condition($i);
}

# spit an error if $interesting_index is still 'not defined'

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      If the purpose of the loop is to find an index, then I would try to make that fact prominent at the start of the loop. That way you don't have to read the body of the code to figure out the purpose/intent.

      In Perl often there are other ways to do something similar to this functionality with the various List::utils. My Perl code rarely uses any $i subscripts.

      update: maybe I have an "off by one" error in original post. I think it is also possible to use the C style loop: 

      for (my $i=1; !defined($interesting_index) and $i<=10; $i++){}

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Re^7: loop surprise
by BillKSmith (Monsignor) on Apr 04, 2018 at 04:18 UTC
    I have made this mistake in the past. I now usually avoid the issue by using a function such as "first_index" in List::MoreUtils.
    Bill
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