I have to agree that the spaceship operator is special ! But the sequence of
sorting is still a very mysterious thing to me. I could imagine sorting is
just like a factory conveyancy belt, pushing the elements thru some sorting
machine.
To find out what's exactly going on, I've decided to take a peep into the
spaceship and this is what I got which I still do not understand precisely
how it sorts. I would appreciate very much if someone could offer an
explanation on the undermentioned results.
Thanks !
@a = qw ( 12 34 14 0 35 44 2);
@b = sort spaceship @a;
print join ',', @b;
sub spaceship {
print "\$a = $a whilst \$b = $b\n";
$a<=>$b;
};
Results :-
$a = 14 whilst $b = 0
$a = 0 whilst $b = 35
$a = 14 whilst $b = 35
$a = 34 whilst $b = 14
$a = 14 whilst $b = 44
$a = 14 whilst $b = 2
$a = 12 whilst $b = 14
$a = 44 whilst $b = 35
$a = 34 whilst $b = 44
$a = 34 whilst $b = 35
$a = 2 whilst $b = 12
$a = 0 whilst $b = 12
$a = 0 whilst $b = 2
0,2,12,14,34,35,44
----- Original Message -----
From: "Jonathan E. Paton" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
Sent: Sunday, November 11, 2001 6:21 PM
Subject: Re: explanation needed on sort using {$a<=>$b}
> > @a= qw (68 3 5 67 54 23 69 );
> >
> > @b = sort {-1} @a; ### what happens here !
> > results = 5,3,68,67,69,23,54
> >
>
> Hey look, its reversed the order of the numbers either side
> of the number 67! Extremely useful :P You shouldn't use
> this sort, since it breaks for other quantities of numbers.
>
> > @c = sort {$a <=> $b} @a; ### what happens here !
> > results = 3,5,23,54,67,68,69
>
> Sorts numerically.
>
> > I know that sort by default sort in ascii order, I
> > wanted to know what exactly happens to the "spaceship"
> > operator. Could somebody help me to visualize what's
> > taking place.
> >
> > Thanks
>
> Sort needs a subroutine which returns -1, 0, 1 as the
> result of comparing two elements. The values mean (where
> $a and $b is a pair of values from the array):
>
> -1 = put $a before $b
> 0 = doesn't matter which way to put $a and $b (i.e equal)
> 1 = put $a after $b
>
> Now, the lovely <=> operator returns the following:
>
> -1 - when $a is less than $b
> 0 - when $a is equal to $b
> 1 = when $a is greater than $b
>
> Combine these:
>
> When $a is less than $b put $a first.
> When $a is same as $b leave alone.
> When $a is greater than $b put $a second.
>
> Result:
>
> You have a numerical sort! You don't really need to know
> the impliementation of the sort (I was going to explain it,
> but my explaination seemed too hazy).
>
> Jonathan Paton
>
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