in reply to while loop acting up, though I'm not sure how

You can see what's going on by minimally changing your code, like this: 
use strict;
use warnings;

my $expiry = time + 3600;

my $time;
my $previoustime;

sub mysleep {
      printf 'before sleeping: time = %d, $time = %d, %s', time, $time
+, "\n";
      sleep shift;
      printf 'after sleeping:  time = %d, $time = %d, %s', time, $time
+, "\n";
}

while ( $time = time
        and $time + 300 < $expiry
        and ( $time - $previoustime >= 10
              or mysleep (10 + $previoustime - $time)
            )
      ) {
  $previoustime = $time;
  warn $time;
}

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Re^2: while loop acting up, though I'm not sure how
by msh210 (Monk) on Nov 17, 2015 at 15:27 UTC

    That adds information, but I still don't see the bug. The output is

    Use of uninitialized value $previoustime in subtraction (-) at soonix.
+pl line 15.
1447773471 at MWE.pl line 22.
before sleeping: time = 1447773471, $time = 1447773471,
after sleeping:  time = 1447773481, $time = 1447773471,
1447773471 at MWE.pl line 22.
1447773481 at MWE.pl line 22.
before sleeping: time = 1447773481, $time = 1447773481,
after sleeping:  time = 1447773491, $time = 1447773481,
1447773481 at MWE.pl line 22.
1447773491 at MWE.pl line 22.
before sleeping: time = 1447773491, $time = 1447773491,
after sleeping:  time = 1447773501, $time = 1447773491,
1447773491 at MWE.pl line 22.
1447773501 at MWE.pl line 22.

    [download]

    etc. The reason for the second warn each time around is (presumably) because mysleep doesn't get called i.e. because !($time - $previoustime >= 10) i.e. (presumably) because $time - 10 < $previoustime. But that's not the case: after all, we've set $previoustime = $time (which we were just warned is time - 10) and then $time = time. So I still don't get what's going on.

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[d/l]
[select]
      • The first time, $previoustime is undefined (warning from "line 15"), hence ($time - $previoustime >= 10) is true and the rest of the or is ignored (first "line 22" warning), because the loop condition is true anyway.
      • Second time the condition is evaluated: Since in the previous iteration the condition was true without sleep, the time assignment happened within the same second as the previous one, resulting in a sleep 10
      • Third time: After the sleep, time has advanced by 10 seconds, ($time - $previoustime >= 10) being true again, same situation as in first iteration, i.e. no sleep.
      • Fourth iteration is just like the second
      The sleep is not evaluated (hence, not executed) every odd iteration because of the "short-circuit" nature of perl's boolean operators. davido explains this thoroughly in Perl Idioms Explained - && and || "Short Circuit" operators - despite the subject, this is also regarding and and or.
[reply]
[d/l]
[select]
        Thank you.
[reply]

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