Unable to use if condition inside foreach loop using perl?

finddata has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Unable to use if condition inside foreach loop using perl? by Corion (Patriarch) on Mar 23, 2017 at 09:41 UTC
`if(my $pattern =~ /^REVISION_LOCATION:/){` [download] Please explain in plain English what this line is supposed to do. Please explain every character or word you used there and describe its meaning and effect.	[reply] [d/l]
Re^2: Unable to use if condition inside foreach loop using perl? by finddata (Sexton) on Mar 23, 2017 at 09:44 UTC
There am checking for if the line starts with the word REVISION_LOCATION then the loop as follows.	[reply]
Re^3: Unable to use if condition inside foreach loop using perl? by marto (Cardinal) on Mar 23, 2017 at 09:49 UTC
Honestly, I think you should review your previous questions, the responses you have had and take the advice given many times about learning the basics. Repeating the same mistakes means you're not learning from them.	[reply]
Re^4: Unable to use if condition inside foreach loop using perl? by afoken (Chancellor) on Mar 23, 2017 at 17:47 UTC
Re^5: Unable to use if condition inside foreach loop using perl? by AnomalousMonk (Archbishop) on Mar 23, 2017 at 18:38 UTC
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Re^5: Unable to use if condition inside foreach loop using perl? by marto (Cardinal) on Mar 24, 2017 at 09:42 UTC
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Re^5: Unable to use if condition inside foreach loop using perl? by LanX (Saint) on Mar 23, 2017 at 17:56 UTC
Re^5: Unable to use if condition inside foreach loop using perl? by Anonymous Monk on Mar 23, 2017 at 20:33 UTC
Re^3: Unable to use if condition inside foreach loop using perl? by Corion (Patriarch) on Mar 23, 2017 at 09:47 UTC
Please explain every character or word you used there and describe its meaning and effect. In your reply, I cannot see how you explained for example the word `my`, the word `$pattern_line` and some others. Please try again.	[reply] [d/l] [select]
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Re: Unable to use if condition inside foreach loop using perl? by haukex (Archbishop) on Mar 23, 2017 at 09:45 UTC
The `=~` operator applies the pattern on the right-hand side to the value on the left-hand side. This means the value on the left-hand side should be the line you are matching against. Try `if($temp_line =~ /^REVISION_LOCATION:/)` Update: Added link	[reply] [d/l] [select]
Re: Unable to use if condition inside foreach loop using perl? by thanos1983 (Parson) on Mar 23, 2017 at 12:18 UTC
Hello finddata I think everyone is trying to explain that the way that you have defined your regular expression will not work. The reason: `foreach my $temp_line(@store_files) { if(my $pattern =~ /^REVISION_LOCATION:/){` [download] Simply you are comparing against an empty string. It is like you are doing `if('' =~ /^REVISION_LOCATION:/){`, when you should be doing something like: `if($temp_line =~ /^REVISION_LOCATION:/){`. If still is not clear enough read the perlrequick it will give a quick understanding about regex. Sample of replicating your error: #!/usr/bin/perl use strict; use warnings; my $dir = shift // '.'; opendir my $dh, $dir or die "Could not open '$dir' for reading '$!'\n"; my @things = grep {$_ ne '.' and $_ ne '..'} readdir $dh; foreach my $thing (@things) { if(my $pattern =~ /^test/){ print $thing . "\n"; } } closedir $dh or die "Could not close dir hanlde '$dh': '$!'\n"; __END__ Monks$ perl test.pl Use of uninitialized value $pattern in pattern match (m//) at test.pl +line 11. Use of uninitialized value $pattern in pattern match (m//) at test.pl +line 11. [download] I would also recommend to refactor a bit your code an use readdir . Sample from my localhost with no error: `#!/usr/bin/perl use strict; use warnings; my $dir = shift // '.'; opendir my $dh, $dir or die "Could not open '$dir' for reading '$!'\n"; my @things = grep {$_ ne '.' and $_ ne '..'} readdir $dh; foreach my $thing (@things) { if($thing =~ /^test/){ print $thing . "\n"; } } closedir $dh or die "Could not close dir hanlde '$dh': '$!'\n"; __END__ Monks$ perl test.pl test.pl~ test.pl` [download] Seeking for Perl wisdom...on the process of learning...not there...yet!	[reply] [d/l] [select]


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