Perl hash while loop cannot find the key value

Question

I am confused by one perl question, anyone has some idea?

I use one hash structure to store the keys and values like:
$hash{1} - > a;
$hash{2} - > b;
$hash{3} - > c;
$hash{4} - > d;
....
more than 1000 lines. I give a name like %hash

and then, I plan to have one loop statement to search for all keys to see whether it will match with the value from the file.

  for example, below is the file content:
  first line 1
  second line 2
  nothing
  another line 3

my logic is:

while(read line){

    while (($key, $value) = each (%hash))
    {
      if ($line =~/$key/i){
         print "found";
   }
}

so my expectation is :
first line 1     - > return found
second line 2     - > return found
nothing
another line 3    - > return found
....

However, during my testing, only first line and second line return found, for 'another line3', the 
program does not return 'found'
Note: the hash has more than 1000 records.

So I try to debug it and add some count inside and find out for those found case, the loop has run like 600 or 700 times, but for the 'another line3' case, it only runs around 300 times and just exit the loop and did not return found.

any idea why it happens like that?

and I have done one more testing is if my hash structure is small, like only 10 keys, the logic works.

and I try to use foreach, and It looks like foreach does not have this kind of issue.

user_2678168 · Answer

The pseudo code you give should work fine, but there might be a subtle problem.

If after you found your key and print it out you end the while loop, the next time each is called, it will continue where you left. Put it in other words "each" is an iterator that stores its state in the hash it iterates over.

In http://blogs.perl.org/users/rurban/2014/04/do-not-use-each.html the author explains this in more detail. His conclusion:

So each should be treated as in php: Avoid it like a plague. Only use it in optimized cases where you know what you are doing.

user_12313309 · Answer

The problem is not very well articulated by OP, provided sample data are poor for demonstration purpose.

Following sample code is an attempt based on provided problem description by OP.

Recreate filter hash from DATA block, compose $re_filter consisting of filter hash keys, walk through a file given as an argument on command line to filter out lines matching $re_filter.

use strict;
use warnings;

my $data = do { local $/;  };
my %hash = split ' ', $data;

my $re_filter = join('|',keys %hash);

/$re_filter/ && print for <>;

__DATA__
1   a
2   b
3   c
4   d

Input data file content

first line 1
second line 2
nothing
another line 3

Output

first line 1
second line 2
another line 3

Perl hash while loop cannot find the key value

2 Answers