Okay, I am happy with speed now. Winning answers were a combination of:

  • the system 'sort' step mentioned by jethro and apl and maybe others (and searching only 1000 adjacent lines).

  • the 'abs' command of grep and maybe others.

    The rest of the stuff sounds fascinating and potenitally useful. Thanks, truthfully, to everyone. Soon I hope to do this on a global daily dataset where n=800million or so. If anyone wants to hammer me on better methods and such please feel free. Again, thanks to all.

    In reply to Re^3: fastest way to compare numerical strings? by Anonymous Monk
    in thread fastest way to compare numerical strings? by Anonymous Monk

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