expression value in Perl's while loop

theNbomr · 08-30-2011, 03:09 PM

Quote:

Originally Posted by anomie

Sorry, but this is getting silly. So the expectation was that reading in a 0 should have ended the while() loop? Then how the fudge would you ever read a 0 from a file?

Well, in the context of the OP's code/question, the value would still be read. Only the matter of terminating the loop was in question. Two different matters.
I think we now all understand what was originally in question, and the explanation for the apparent paradox.
--- rod.

password636 · 08-30-2011, 09:36 PM

Finally I think I've found the right answer to my question. And sorry guys I don't think you all get the right point

@theNbomr
Empty string, "0", '0' and 0 (integer) are all evaluated False in the while conditional. It's not about forcing the integer context, it's a scalar context.

@David the H.
You are close. But there is no such "the whole expression being evaluated" thing. Just like C, such kind of conditional depends on the value of the expression, not the expression itself.

Post the right answer here so that we can all learn something

Code:

       The following lines are equivalent:

           while (defined($_ = <STDIN>)) { print; }
           while ($_ = <STDIN>) { print; }
           while (<STDIN>) { print; }
           for (;<STDIN>;) { print; }
           print while defined($_ = <STDIN>);
           print while ($_ = <STDIN>);
           print while <STDIN>;

       This also behaves similarly, but avoids $_ :

           while (my $line = <STDIN>) { print $line }

       In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see
       whether it is defined.  The defined test avoids problems where line has a string value that would be treated as false
       by Perl, for example a "" or a "0" with no trailing newline.  If you really mean for such values to terminate the
       loop, they should be tested for explicitly:

           while (($_ = <STDIN>) ne '0') { ... }
           while (<STDIN>) { last unless $_; ... }

       In other boolean contexts, "<filehandle>" without an explicit "defined" test or comparison elicit a warning if the
       "use warnings" pragma or the -w command-line switch (the $^W variable) is in effect.

This could be found in perldoc perlop - I/O operators.