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Postings from November 2022
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?
November 20, 2022 at 11:19 PM, "Kang-min Liu" <gugod@gugod.org> wrote:
>
> support@openmbox.net writes:
>
> >
> > May i ask a question about reading file?
> >
> > while(0){ print 'hi' }
> >
> > Will never print hi.
> >
> > cat 1.txt:
> > 0
> >
> > open FH, '1.txt' or die;
> > while(<FH>) { print 'hi' }
> >
> > This will print hi.
> >
> > Since $_ == 0 here, why while become true?
> >
>
> This seems to be more about what are true-y and what are false-y.
>
> If the content of 1.txt is one line containing a character "0" follewed
> by the newline character, that means doing <FH> would make $_
> contain "0\n", that's two characters, not one.
>
> "0\n" would be a true-y value while it is still numerically the same as
> 0 when testing with the operator `==`. The `==` operator always convert
> both of its operants to numerical values before doing the
> comparison. Similarly `eq` operator always converts both operants to
> string before doing the comparison.
>
> There are a lot of ways to make variables being numerically 0 while also
> being a true-y value. Most of them are strings with leading zeros. You
> could try editing the following program to play around:
>
> use strict;
>
> $_ = "0\n";
>
> print "true-y\n" if $_;
>
> print "== 0\n" if $_ == 0;
> print "eq 0\n" if $_ eq 0;
>
> print "eq \"0\"\n" if $_ eq "0";
> print "== \"0\"\n" if $_ == "0";
>
> There are also a lot of values with leading "0" which would be true-y,
> but fail all 4 other if-s. :-)
>
> BTW, really, don't actually write `eq 0` or `== "0"`, they just look weird.
>
> --
> Cheers,
> Kang-min Liu
>
> --
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