in reply to while (<FP>) conditionals

My only guess as to why it continues is because <FP> actually returns 0 and a newline character.

It does, and a string containing “0\n” is true, not false:

14:26 >perl -we "my $s = qq[0\n]; printf qq[%s\n], ($s ? 'true' : 'fal
+se');"
true

14:31 >

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To see the special behaviour described by ++davido, you can set the $/ variable (the input record separator) to read 1 character at a time, and then try a C-style for-loop (which does not have the special behaviour):

#! perl
use strict;
use warnings;

local $/ = \1;

for ($_ = <DATA>; $_; $_ = <DATA>)
{
    print "<$_>\n";
}

__DATA__
1
0
42

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— the loop terminates on the 0. To get the desired behaviour, change it to:

for ($_ = <DATA>; defined $_; $_ = <DATA>)

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and a little experimentation will show you that this is now equivalent to the special forms:

print "<$_>\n" for <DATA>;

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and

print "<$_>\n" while <DATA>;

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Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

Replies are listed 'Best First'.
Re^2: while (<FP>) conditionals
by jktstance (Novice) on Feb 05, 2014 at 14:46 UTC
    It does, thank you. I tried running your first test: 
    perl -we "my $s = qq[0\n]; printf qq[%s\n], ($s ? 'true' : 'fal
+se');

    [download]
    All I ever got were the following errors: 
    syntax error at -e line 1, near "my  ="
Search pattern not terminated or ternary operator parsed as search pat
+tern at -e line 1.

    [download]
    However, when I put it into a script and ran it, it worked fine. Also, your second test does not work for me: 
    #! perl
use strict;
use warnings;

local $/ = \1;

for ($_ = <DATA>; $_; $_ = <DATA>)
{
    print "<$_>\n";
}

__DATA__
1
0
42

    [download]
    When I run it, I get the following: 
    <1>
<
>

    [download]
    But at this point, my question is answered. Now we're just toying around!
[reply]
[d/l]
[select]
      When typing code on the command line, you often have to escape some characters depending on your system.
      For example, the error message shows that: my $s = got munged by your shell into my  = before it was passed to perl.
[reply]
[d/l]
[select]
        Another reason we should not use one-liners in replies if the OS is not specified.
        لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ
[reply]
        I escaped the two $s instances with backslashes and it worked fine. I'm still not sure why %s didn't need to be escaped. As you can probably tell, I'm not too well versed in the linux shell either...
[reply]
      Also, your second test does not work for me ... When I run it, I get the following:

      But that’s exactly what you should get! local $/ = \1; makes the diamond operator read one character at a time. The first character is 1, which is printed. The second character is a newline, which is also printed (I put the angle brackets in to show that a new line has been printed). The third character is 0, and because the C-style for-loop is not magic, this is “false” and the loop terminates without printing anything more.

      Hope that helps,

      Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

[reply]
[d/l]
[select]
        OK, yes, I skipped over the 1-character point. It's actually only printing two things). Thanks, all straight now!
[reply]

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