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Postings from November 2022
support@openmbox.net writes:
> please see this ops:
>
> $ echo -n 0 > 1.txt
>
> 1.txt has only one line without eof.
>
> but the script below still got true for matching 0.
>
> $ cat test.pl
> use strict;
>
> open HD,"1.txt" or die $!;
> while(<HD>){
> print "hi";
> }
>
>
> which will print hi.
>
> can you help further?
>
I see -- so it's really about how readline operator (<FH>) works, or how
it works within a while loop. I'm not sure if I could explain it better
than quoting some documentation.
Indeed the readline operator returns the line and may set $_ as side-effect,
but when being tested in `while` -- and only in `while` -- perl
compiler does something extra and put an `defined` operator in there.
If you follow link in the documentation of readline [1] to "perlop: I/O
Operatorns" [2], you'll find this statement saying:
Thus the following lines are equivalent:
while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;
[1]: https://perldoc.perl.org/functions/readline
[2]: https://perldoc.perl.org/perlop#I%2FO-Operators
And the paragraph right before them contains some text describing the
same thing.
Such effect may be verified by compiling test.pl with -MO=Deparse:
# perl -MO=Deparse test.pl
use strict;
die $! unless open HD, '1.txt';
while (defined($_ = readline HD)) {
print 'Hi';
}
test.pl syntax OK
You could see that <HD> is compiled to be "readline", and an extra
"defined" operator appears.
(See `perldoc B::Deparse` for more about -MO=Deparse)
You could furthermore play with it and see that no that extra "defined"
operator would be added to `if (<FH>)`, `unless (<FH>)`, `until (<FH>)`,
or when the while-condition is a bit more complicated (for
example: `while(! <FH>)`). This is a special case only for `while (<FH>)`
I guess this is made special to make it convenient for iterating through
the entire file line by line. Otherwise when line values are false-y by
chance, the loop ends early and mostly likely that'll be a surprise.
--
Cheers,
Kang-min Liu
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