Scripting question

[Jonathan McKeown]

On Thursday 13 September 2007 20:35, Roland Smith wrote:
> On Thu, Sep 13, 2007 at 10:16:40AM -0700, Kurt Buff wrote:
> > I'm trying to do some text file manipulation, and it's driving me nuts.
[snip]
> > I've looked at sort and uniq, and I've googled a fair bit but can't
> > seem to find anything that would do this.
> >
> > I don't have the perl skills, though that would be ideal.
> >
> > Any help out there?
>
> #!/usr/bin/perl
> while (<>) {
>     # Assuming no whitespace in addresses; kill everything after the first
>     # space 
>     s/ .*$//; 
>     # Store the name & count in a hash
>     $names{$_}++;
> }
> # Go over the hash
> while (($name,$count) = each(%names)) {
>   if ($count == 1) {
>       # print unique names.
>       print $name, "\n";
>   }
> }

Another approach in Perl would be:

#!/usr/bin/perl
my (%names, %dups);
while (<>) {
    my ($key) = split;
    $dups{$key} = 1 if $names{$key};
    $names{$key} = 1;
}
delete @names{keys %dups};
#
# keys %names is now an unordered list of only non-repeated elements
# keys %dups is an unordered list of only repeated elements

split splits on whitespace, returning a list of fields which can be assigned 
to a list of variables. Here we only want to capture the first field: split 
is more efficient for this than using a regex. The first occurrence of $key 
is in parens because it's actually a list of one variable name.

We build two hashes, one, %name, keyed by the original names (this is the 
classic way to reduce duplicates to single occurrences, since the duplicated 
keys overwrite the originals), and one, %dup, whose keys are names already 
appearing in %names - the duplicated entries. Having done that we use a hash 
slice to delete from %names all the keys of %dups, which leaves the keys of 
%names holding all the entries which only appear once (and the keys of %dups 
all the duplicated entries if that's useful).

Jonathan