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Re: Date to be sorted in descending and time in ascending

by roboticus (Canon)
on May 18, 2012 at 09:48 UTC ( #971256=note: print w/ replies, xml ) Need Help??


in reply to Date to be sorted in descending and time in ascending

karthik7887:

You just need a custom sort method. Break the string into the date part and time part, and give the proper comparisons. Something like this:

my @sorted = sort { substr($b,0,8) <=> substr($a,0,8) || substr($a,8) +<=> substr($b,8) } @unsorted;

The first chunk breaks the date part off of the string, and sorts it in reverse order (note $b is on the left, $a on the right). In the case when the dates are the same, it breaks the time off the string and sorts it in ascending order ($a on the left, $b on the right).

...roboticus

When your only tool is a hammer, all problems look like your thumb.


Comment on Re: Date to be sorted in descending and time in ascending
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Re^2: Date to be sorted in descending and time in ascending
by aaron_baugher (Deacon) on May 18, 2012 at 21:22 UTC

    This means that the substr function will be called four times for each comparison, which could add up if it's a large array. That makes it a good candidate for a Schwartzian Transform. (You probably know that; I'm adding it for the original poster.) UPDATE: I was completely wrong about this; see below.

    #!/usr/bin/env perl use Modern::Perl; use Data::Dumper; my @data = <DATA>; @data = map { $_->[0] } sort { $b->[1][0] <=> $a->[1][0] or $a->[1][1] <=> $b->[1][1] } map { [ $_, [substr( $_, 0, 8 ), substr( $_, 8)]] } @data; say @data; __DATA__ 20090405022300 20080405022600 20090405022900 20080405023500 20050405005000 20080405022500 20090405022500 20020405081200 20010405000000 20090405022100

    UPDATE: While I understand the Schwartzian Transform in theory and think it's one of the coolest things ever, I haven't had much call to actually use it, so I did a benchmark for this case against the simple sort of substr calls. I was a little surprised to see that the repeated substr calls beat my ST, testing with array sizes from 10 to 1_000_000. In fact, the ST took about twice as long in all tests. I guess four substr calls (or two if the first comparison returns a value so the second comparison isn't necessary) don't qualify as expensive enough to make the overhead of the ST worth it here. Darn it.

    Aaron B.
    Available for small or large Perl jobs; see my home node.

      aaron_baugher:

      Hmmm, I'm surprised. I figured at a million strings that the Schwartzian Transform would win. But you show another good lesson: Measure, don't guess. While both you and I expected the transform to win at a million strings, measurement trumps expectation.

      ...roboticus

      When your only tool is a hammer, all problems look like your thumb.

        Yep. For further curiosity, I did a count on how many substr calls there were. For the ST, of course, you have two for each element, one to get the first 8 chars and one to get the rest. So for a million-element array, that's 2M calls. But for the sort-on-substr version, I got about 37M substr calls. That'll vary some depending on how unsorted the original array is, but that's probably a good ballpark number.

        That's a lot more substr calls, but I guess it's still less work than building an entire new million-element array (with each element a reference to a two-element array), as the ST requires.

        It does give me a (very rough) rule of thumb, though: for the ST to be more efficient, the alternative probably needs to do the equivalent of 8-10 substr calls. So just a few core functions probably won't qualify, but a longer series of functions, or some fairly complex regexes, or certainly any sort of file or database lookups, probably will. And there's always measurement to see for sure.

        Aaron B.
        Available for small or large Perl jobs; see my home node.

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