Re: Sorting AoH by two keys

<=>, the spaceship operator, has an interesting property - when the two sides are equal, it returns 0. This is very useful, because then you say:

my $AoH = [ sort {
    $a->{under} <=> $b->{undef}
    || $a->{order} <=> $b->{order}
} @$sqldata ];
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This works because || evaluates it's left side, and if it's false (0 is false), it evaluates the right too. It returns the first true value it finds (or the last false one if it didn't).

Let's compare 2,2 and 2,1:

(2 <=> 2) == 0 so
0 || 2 <=> 1 evaluates to 2 <=> 1
(2 <=> 1) == 1 # the answer we wanted
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BTW, I noticed you have 'sqldata'... This smells like you're doing fetching the entire result table as an array ref, and then sorting it... Why not make it into

select ... order by under, order;
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This stuff is better (and probably more efficiently) handled on the server side.

Update: tphyahoo was right - but it was no typo... I wrote down $a twice when I should have written $b. Sorry!

-nuffin
zz zZ Z Z #!perl

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Re^2: Sorting AoH by two keys by bradcathey (Prior) on Jul 08, 2005 at 22:19 UTC
Perfect, thanks for the 'tutorial.' After I saw your solution I realized that I was basically asking the same question as I did in How to sort a AoH reference, but I wasn't understanding the `\|\|` as I stared at it. Now I see how it works in conjunction with the `<=>`. Copying code out of a book might get something to work, but it doesn't help in the long run unless it is truly understood. Which is why I didn't want to rely solely on the `SELECT...ORDER BY`. (BTW, you are quite perceptive). —Brad "The important work of moving the world forward does not wait to be done by perfect men." George Eliot	[reply] [d/l] [select]
Re^2: Sorting AoH by two keys by tphyahoo (Vicar) on Jul 09, 2005 at 16:19 UTC
nothingmuch, although the OP said this works, it didn't work for me. UPDATE: it was a typo. I think nothingmuch meant `my $sorted = [ sort { $a->{under} <=> $b->{under} \|\| $a->{order} <=> $b->{order} } @$aoh ];` [download]	[reply] [d/l]