in reply to == uniq

You could modify these implementations.

A related problem I once had fun with. Process a file and only print lines once (unlike uniq no assumption of sorted input). That isn't quite what you wanted, but here was the fastest version I could come up with in Perl: 

perl -ne 'print if 1 == ++$s{$_}' < in > out

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(Yes, details like where to place the plusses mattered.)

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(dkubb) Re: (2) Printing unique lines in a file
by dkubb (Deacon) on Feb 13, 2001 at 13:25 UTC

    Here's the fastest and shortest way I could find to process an input file, and print the unique lines inside it:

    perl -ne "$s{$_} ||= print" < in > out

    Explanation: The ||= operator has a behaviour called "short-circuit". If the left-hand side of the operation evaluates to true, the operation will short-circuit and stop - essentially the right-hand side will be ignored and never evaluated. But if the left side is false, then whatever the right-hand side evaluates to will be assigned to the left.

    In this case, we are processing each line from a file called "in". The first time we see $s{$_} it will evaluate to false, setting off the right hand side to be evaluted. This will print $_, return a 1, then set $s{$_} to 1.

    The next time we see $s{$_} inside the loop, it is true, so the ||= never again runs the right-hand side operation. This results in the above code only printing a line the first time it's seen, and no more.

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Re: Re (tilly) 1: == uniq
by MeowChow (Vicar) on Feb 13, 2001 at 08:54 UTC
    can't resist... 
    perl -ne '!$s{$_}++ && print' < in > out

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       MeowChow                                   
               s aamecha.s a..a\u$&owag.print
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[d/l]
      That executes more slowly. You have to waste time creating new variables because of using postincrement rather than preincrement.
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