Re^4: how do I "initialize" $

... with strict; and warnings; active, when reading from the predefined DATA file handle, you will get a warning if you don't have the while (defined (my $line =<DATA>)){} syntax ... a line with just "0" could evaluate to "false".

I'm confused about what you intended to say in the quoted text. A
while (<DATA>) { ... }
is always implicitly
while (defined($_ = <DATA>)) { ... }
or more fundamentally
while (defined($_ = readline(DATA))) { ... }
for DATA or any other filehandle. Even if a '0' with no newline is the last character in a file or __DATA__ or __END__ block, no warning will be printed. (IIRC, this became true in a very early, but still non-zero, sub-version of Perl 5 — or was it true even for 5.0.0?) (Update: The following code was tested under ActiveState Perl version 5.8.9.)

File t_read_DATA_1.pl:

# t_read_DATA_1.pl                                          12oct18waw

use warnings;
use strict;

while (<DATA>) {
    print;
    }

__DATA__
line the first
second line
penultimate line is line 3
0
[download]

Output:

c:\@Work\Perl\monks\Marshall>od -t x1 t_read_DATA_1.pl | tail
0000120 72 6e 69 6e 67 73 3b 0d 0a 75 73 65 20 73 74 72
0000140 69 63 74 3b 0d 0a 0d 0a 77 68 69 6c 65 20 28 3c
0000160 44 41 54 41 3e 29 20 7b 0d 0a 20 20 20 20 70 72
0000200 69 6e 74 3b 0d 0a 20 20 20 20 7d 0d 0a 0d 0a 5f
0000220 5f 44 41 54 41 5f 5f 0d 0a 6c 69 6e 65 20 74 68
0000240 65 20 66 69 72 73 74 0d 0a 73 65 63 6f 6e 64 20
0000260 6c 69 6e 65 0d 0a 70 65 6e 75 6c 74 69 6d 61 74
0000300 65 20 6c 69 6e 65 20 69 73 20 6c 69 6e 65 20 33
0000320 0d 0a 30
0000323

c:\@Work\Perl\monks\Marshall>perl t_read_DATA_1.pl
line the first
second line
penultimate line is line 3
0
[download]

Another way to see this is by deparsing:

c:\@Work\Perl\monks\Marshall>perl -MO=Deparse,-p t_read_DATA_1.pl
use warnings;
use strict 'refs';
while (defined(($_ = <DATA>))) {
    print($_);
}
__DATA__
line the first
second line
penultimate line is line 3
t_read_DATA_1.pl syntax OK
0
[download]

See O and B::Deparse. (I think the final "0" appears in console output before the "syntax OK" message because the "0" is not terminated by a newline and doesn't get flushed until the process ends. The "syntax" message may also be going to STDERR. Or something like that...)

Update: I really should have mentioned that the preceding code was run under ActiveState Perl version 5.8.9, the ancientest Perl I hold in captivity.

Give a man a fish: <%-{-{-{-<

Comment on Re^4: how do I "initialize" $_ (updated) Select or Download Code

Replies are listed 'Best First'.
Re^5: how do I "initialize" $_ by Marshall (Canon) on Oct 13, 2018 at 04:21 UTC
I tested this with my Perl `C:\Users\mmtho\Documents\PerlProjects>perl -v This is perl 5, version 24, subversion 3 (v5.24.3) built for MSWin32-x +64-multi-thread (with 1 registered patch, see perl -V for more detail) ...` [download] And I found out that Perl has changed how it deals with <DATA>. This works with Perl 5.24: `#!/usr/bin/perl use strict; use warnings; while (my $line = <DATA>) { print $line; } __DATA__ aBC XYAS` [download]	[reply] [d/l] [select]
Re^6: how do I "initialize" $_ by AnomalousMonk (Archbishop) on Oct 13, 2018 at 06:21 UTC
In an oversight since corrected, I neglected to mention that the code here was run under Perl version 5.8.9. I tried `while (my $line = <DATA>) { print $line; }` [download] with my original `__DATA__` both with and without a newline after the 0 on the final line and with strictures and warnings fully enabled and under 5.8.9 and saw no difference in behavior (and no warnings). The reason is easy to see from a deparse: `c:\@Work\Perl\monks\Marshall>perl -MO=Deparse,-p t_read_DATA_2.pl use warnings; use strict 'refs'; print("perl version: $] \n\n"); while (defined((my $line = <DATA>))) { do { print($line) }; } __DATA__ line the first second line penultimate line is line 3 t_read_DATA_2.pl syntax OK 0` [download] The defined test is automatically added. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^7: how do I "initialize" $_ by Marshall (Canon) on Oct 13, 2018 at 09:51 UTC
Interesting. In any event apparently defined() on the <DATA> file handle is not needed - or at least not any more. Update: I haven't been able to reproduce this warning on my current Perl 5.24 version. From what I remember the warning was unusually specific in what it said - this was very specific. To my knowledge, I never did actually try Perl code that would cause the problem that was warned about. This was a warning about something that could potentially happen - not an error. This situation did not occur in my source code. I just took Perl at "its word" and adjusted my source code so that this warning went away. I think of Perl as a living, breathing, biological organism that evolves over time. It is completely possible that this was an extraneous warning about something that didn't really matter. At this point, I don't know. It could be that this warning was just emitted in one version of Perl and corrected in the next version. I've worked with a lot of versions of Perl. I don't have nor would I have saved the exact text of this warning.	[reply]