Use of uninitialized value $_ in pattern match (m//)?

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

Dear monks,

I've been using the code below to match some files in the path described in the code. However I'm getting a "Use of uninitialized value $_ in pattern match (m//)" error.

#!/usr/bin/perl/

use strict;
use warnings;

my $i;
my $j;
my $k;
my $l;
my $id = "AC";

my $moved;

for ($i=2; $i<=3; $i++) {
my $j = sprintf ("%04d", $i);
  for ($k=1; $k<=3; $k++) {
  my $l = sprintf ("%05d", $k);
  my $path;
  $path = "/home/results/initial/$id$j/F${l}";
  chdir "$path";
  /MV(\d{4})/;  
  $moved = $1;
  print "$moved\n";
  }
 }
[download]

I seem to be getting this error even though I have changed directory into the directory specified in the path - which is something I don't want to do, I want to give the absolute path each time, but I was just trying to see if it would solve my problem. I also ran the script using perl -wc to check syntax/warnings but it comes back as "syntax correct". Also, if I do print `ls $path/MV*`; it finds the files fine and prints them. What do you think could be the source of my problem?

Comment on Use of uninitialized value $_ in pattern match (m//)? Select or Download Code

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Re: Use of uninitialized value $_ in pattern match (m//)? by hippo (Archbishop) on Feb 29, 2016 at 15:19 UTC
The problem is that you have not set `$_` anywhere either explicitly or implicitly. From your description it looks like you want to match against a filename so perhaps it is glob that you really want rather than `m//`?	[reply] [d/l] [select]
Re: Use of uninitialized value $_ in pattern match (m//)? by Corion (Patriarch) on Feb 29, 2016 at 15:20 UTC
What is your match trying to match against? Usually, you have a variable to the left of the match statement. If you don't have that, Perl will use the default topic, `$_` to match. What is the value of `$_` and where do you set it?	[reply] [d/l] [select]
Re^2: Use of uninitialized value $_ in pattern match (m//)? by fasoli (Beadle) on Feb 29, 2016 at 15:36 UTC
Hi, thanks for your time! I am trying to match against filenames, from files that are in the path I specify. I modified my script to `$match =~/MV(\d{4})/; $moved = $1; print "$moved \n";` [download] but although I initialise `my $match` outside of the loops, I still get the same error as described initially :(	[reply] [d/l] [select]
Re^3: Use of uninitialized value $_ in pattern match (m//)? by poj (Abbot) on Feb 29, 2016 at 15:46 UTC
Try `#!/usr/bin/perl/ use strict; use warnings; my $id = "AC"; my $root = '/home/results/initial/'; for my $i (2..3){ for my $k (1..3){ my $path = $root.sprintf "%s%04d/F%05d",$id,$i,$k; print "$path\n"; for (glob "$path/MV*"){ print "$1\n" if /MV(\d{4})/; } } }` [download] poj	[reply] [d/l]
Re^3: Use of uninitialized value $_ in pattern match (m//)? by Corion (Patriarch) on Feb 29, 2016 at 15:42 UTC
What is the value in `$match`? What do you expect the value in `$match` to be? As hippo already mentioned, you seem to be highly confused about how to read entries in a directory. See glob, or opendir and readdir.	[reply] [d/l] [select]
Re^3: Use of uninitialized value $_ in pattern match (m//)? by AnomalousMonk (Archbishop) on Feb 29, 2016 at 16:22 UTC
... I still get the same error as described initially ... From the OP: '... I'm getting a "Use of uninitialized value $_ in pattern match (m//)" error.' If you're really still getting this exact "error" (a warning, actually), then you still have a `/MV(\d{4})/;` statement lurking somewhere in your code that is still trying to implicitly match against the `$_` variable which you have still not initialized. What line number is associated with the warning message? Look at (or near) that line for the offending statement. Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]