Re: Re: When does while() test for defined vs truth

Ah, no wonder I couldn't find it... its actually in perlop2.

That still doesn't explain the difference between the last two examples I gave:

# This tests for definedness
while($x = <*.pl>) {}

# This tests for truthfulness
1 while($x = <*.pl>)
[download]

Is that a bug, or should the two forms of while() behave differently?

-Blake

Comment on Re: Re: When does while() test for defined vs truth Download Code

Replies are listed 'Best First'.
Re: Re: Re: When does while() test for defined vs truth by japhy (Canon) on Dec 17, 2001 at 08:35 UTC
When Perl sees you using `<FH>` or `$var = <FH>` in a `while` (or `until`), it will insert the `defined()` for you. From perlop in bleadperl: The following lines are equivalent: `while (defined($_ = <STDIN>)) { print; } while ($_ = <STDIN>) { print; } while (<STDIN>) { print; }` [download] (...) In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "" or a "0" with no trailing newline. _____________________________________________________ Jeff`[japhy]`Pinyan: Perl, regex, and perl hacker. `s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;`	[reply] [d/l] [select]
Re4: When does while() test for defined vs truth by blakem (Monsignor) on Dec 17, 2001 at 08:46 UTC
I noticed that the behavior was consistant for filehandle reads, which is why I found the behavior of globbing perplexing... The following lines are equivalent to each other: `while (<STDIN>) { print; } # tests for definedness print while <STDIN>; # tests for definedness` [download] But these two are not equivalent to each other: `while ($_ = <>) { print } # tests for definedness print while ($_ = <>); # tests for truthfulness` [download] Update -- Here is an example in action: `% mkdir newdir % cd newdir % touch 0 1 2 % ls 0 1 2 % perl -le 'while ($_ = <>) { print }' 0 1 2 % perl -le 'print while ($_ = <>)' [note nothing is printed here]` [download] -Blake	[reply] [d/l] [select]