[lug] perl question

[rm at fabula.de]

On Tue, Jun 04, 2002 at 10:15:27AM +0000, j davis wrote:
> 
> 
> hi,
> 
>  this little piece of perl removes dups from a list..
>  so they tell me. does anyone here speak extreme laymen
> ?
> 

is this a question/riddle? 

 Ok, the first question would be: do you need to preserve the
 order of the list?

 # given the following list 

 my @list = ('a', 'b', 'c', 'd', 'b', 'f', 'e', 'e');

-No, i don't. Then:

 # read the list into an hash
 # If an item is seen more than once, the hash value
 # for it will be > 1, but we don't care ...
 my %seen = undef;
 map { $seen{$_}++ } @list;
 
 # ... since we get the list via
 @unique_list = keys %seen;

-Yes, i want it ordered. Then:
  my @unique_list = grep {
                          if(!$seen{$_})
                            {
                              $seen{$_}++;
                              $_;
                            }
                        } @list;

Ok, this _is_ a bit cryptic, but it works.
Basically, you use the hash as a marker storage, for each item
in the list, look up whetherwe have 'seen' it allready, if not,
return it (i.e. pass it on to @unique_list) and mark it, else just
pass on to the next list entry.

 Hope that helps

    Ralf 
 
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