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Puzzle in sort

Posted on 2000-04-29
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Last Modified: 2010-03-05
Hi,

I am not familiar with perl, and has came across this line for a sort

foreach $key (sort {$info{$b} <=> $info{$a}}keys(%info))
{

I was told that this is used to sort a hash list by using other fields that the key, how is it so?

Thanks in Advance.
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Question by:sata
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3 Comments
 
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Expert Comment

by:ozo
ID: 2762184
perldoc -f sort
perldoc -q sort
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guadalupe earned 320 total points
ID: 2762350
OK the deal is this ... first $a and $b are identifiers to the keys supplied by the keys fuction applied to the hash %info at the right of the statement.  The <=> is a numerical comparative operator... that means that we understand the values of the hash to be numeric.  If we wanted to do alphabetic comparisons we would change the <=> for cmp    Then  we yor comment about  
"hash list by using other fields that the key"

is so because you will notice that the values to the left and right of the comparison operator are indeed not the keys but the values...  as mentioned above $a and $b identify the keys and as in any hash operation $hash{key} will be the value, so
$info{$b} and $info{$a} are indeed the values of the hash corresponding to the keys   $a   and $b ...

the real magic is in the $a, $b and sort these mantain two distinct keys and keep sorting untill the whole thing is sorted and there is just perl magic which I don't pretend to understand the internal workings of but hey thats what perl is for...if you wanted it another way you would be using C!  The final thing:  foreach $key....
All the rest before returns a sorted list of keys so you just take them one at a time and go!!!  The only other thing is that if I remember correctly
$info{$b} <=> $info{$a} will give lesser to greater  either way if it orders the wrong way from wat you wnt you can always chnge it to:

$info{$a} <=> $info{$b}  and the same if you were using cmp:

$info{$a} cmp $info{$b}...

Hope this has made things a bit clearer!
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by:sata
ID: 2765368
Thanks for the answer!!
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