in reply to For loop not executing all the values

Perl's for loop loops over a list. You have a single item in the list - the first line in the file. Instead you need a while loop which loops while a condition is true. Actually, you need just a little more work than that. You should loop until the file read returns undef. Consider:

#!/usr/bin/perl
use strict;
use warnings;

my $fileName = "file_detail.txt";
open my $fOut, '>', $fileName or die "Can't create '$fileName': $!\n";
print $fOut <<CONTENT;
123
abc
12ab3c
CONTENT
close $fOut;

open (my $fh, "<", $fileName);

while (defined (my $value = <$fh>)) {
    chomp ($value);
    if ($value eq "abc") {
        print "matched 'abc'\n";
    } elsif ($value eq "123") {
        print "matched '123'\n";
    }
}

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Prints:

matched '123'
matched 'abc'

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Premature optimization is the root of all job security

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Re^2: For loop not executing all the values
by Athanasius (Archbishop) on Sep 01, 2015 at 08:37 UTC

    Hello GrandFather,

    Instead you need a while loop which loops while a condition is true. Actually, you need just a little more work than that. You should loop until the file read returns undef.

    Well, according to perlsyn#Compound-Statements, “The while statement executes the block as long as the expression is true.” So far, so good. But if that were the whole story, this:

    #! perl
use strict;
use warnings;

print "Contents of \$/: |$/|\n";
print ">$_<\n" while <DATA>;

__DATA__
7
42
0

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    would print only the first two lines, since the final line — which is not terminated by a newline — is '0' and therefore false. But the final line is printed, because with the <> operator (or its equivalent readline call) the while condition is tested for definedness, not truth. This is documented in perlop#IO/Operators:

    In these loop constructs, the assigned value ... is then tested to see whether it is defined. The defined test avoids problems where the line has a string value that would be treated as false by Perl; for example a "" or a "0" with no trailing newline.

    All of which is (apparently) undercut by the op’s assertion, below, that adding an explicit test for definedness solved his problem. I don’t know how to explain that. An early version of Perl? A strange value for $/? I do know that perlop#IO/Operators says the following are equivalent:

    while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }

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    and immediately goes on to imply that explicit assignment to a lexical variable “behaves similarly” to implicit assignment to the default variable, $_.

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

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[d/l]
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      Hmm, that looks like cargo culting on my part. I had the impression that in the past the explicit assignment broke the special handling for <> in while loop expressions. A quick look through the Perl documentation doesn't support that impression for any version of Perl I've ever used so there is a bunch of code I can tidy up!

      I'll leave my post above as is to avoid confusion for future readers.

      Premature optimization is the root of all job security
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Re^2: For loop not executing all the values
by parthodas (Acolyte) on Sep 01, 2015 at 07:25 UTC
    Thanks for the help. Using while loop with defined helped. I tried while, but it was also exiting after the first execution. After putting defined, it is working.
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