[Perl-beginners] RE: remove duplicate lines

Extremely.

He's using just a hash, namely %seen. What he's doing is
reading all the files on the command line (or standard input
if there are no filename arguments) in list context, i.e.
he turns them into a large list of lines. This list is then
"filtered" by the grep command. This will return a subset of
the argument list - namely those items for which the grep
expression is true. That expression is "!$seen{$_}++".
$_ will be on of the lines. The first time a unique line
is evaluated in this expression, $seen{$_} does not exist
and the value is undefined. The ! operator forces this into
a boolean false and returns boolean true, i.e. the line will
be part of the result set. the post-increment ++ operator
will then create a hash key with the line as key and assign
it (undef)+1 = 1 as a value. So when the line is encountered
again, it will evaluate to a positive number, which will be
negated to false by the ! operator, which in turn means that
the line will not be put into the result set twice.
It has lots of useful code snippets, and they are commented
well enough to follow with a basic grasp of Perl. I would
not want to miss it in my library. It also used to be part
of a Book/CD set called the Perl Bookshelf. AFAIK it's out
of print but you might be able to pick it up at a used book
store (or onlinle at ABEbooks) cheaply.

Alternatively, if you want to learn about Perl style, check
out Joseph N. Hall's Effective Perl Programming.

HTH,
Thomas

The first edition went out of print, but the second hasn't:

<http://www.oreilly.com/catalog/perlckbk2/>

And AFAIK it has been part of each edition of the Perl CD Bookshelf,
including the current 4th edition:

<http://www.oreilly.com/catalog/perlcdbs4/>
Or the forthcoming, more up-to-date _Perl Best Practices_:

<http://www.oreilly.com/catalog/perlbp/>

Though in this case you'll have to wait until July or August for it to
be released, but I have no doubt that it'll be worth the wait.

Yes, anything by Dr. Conway will be interesting to say the least! :-)


John

The expression !$seen{$_}++ uses the hash %seen, if it was using a hashref it
would be written as !$seen->{$_}++
Yes, it is described in section 4.7 "Extracting Unique Elements from a List"
(this is in the second edition.)
Yes it is a good book. A lot of the stuff in the book can also be found in
the FAQ files that are included with Perl.

perldoc -q "How can I remove duplicate elements from a list or array"

perldoc perlfaq



John