StuPeas — Thu, 24 Aug 2006 09:30:39 +0000

Cheers Abhishek, Your the Man!!!

Abhishek Reddy — Thu, 24 Aug 2006 09:08:41 +0000

i measures the number of comparisons made, which can vary depending on the algorithm that the sort function uses. This is a fairly deep subject in computing, so I won't go into much detail here. It's well worth investigating, though.

Add something like print "$i: $a $b\n"; after your $i++; to see what's going on. It should print:

Which looks like mergesort.

You may get something different, such as quicksort, depending on your version of Perl (5.6 and earlier use quicksort, 5.7 uses mergesort, 5.8 flips a coin).

extra

StuPeas — Thu, 24 Aug 2006 08:56:49 +0000

I made a miskake and ran this code. I believe it may help to figure out the exact sequence of evevts taking place from the time the sort function starts to be executed.

sub sort_by_num
{
  $i++;
  return $a <=> $b;
}

$i=0;
@arr = (500,1,400,5);
 
foreach (sort sort_by_num(@arr))
   {
    print "$_\n;";
     
   }

print "i = $i";

The output is
1
;5
;400
;500
;i=5

If possible could you explain each stage in the program for me. I know its alot to ask, but ive been driven mad by this problem for 4 days now, and my course tutors are absolutely useless.

I would be very very grateful.

THANX
Stu

Thank you

StuPeas — Thu, 24 Aug 2006 07:25:16 +0000

Thanks alot.

I think I may need to know Exactly how the sort function works (which list values get compared to which and in what order, and how many times.

If i run this code then "i" ends up being 5,, This I dont understand and probably never will until I know how sort works.

[CODE]
#!c:\perl\bin\perl.exe

sub sort_by_num
{
$i++;
return $a <=> $b;
}

$i=0;
@arr = (500,1,400,5);

foreach (sort sort_by_num(@arr))
{
print "$_\n;"

}

print "i = $i";

[\CODE].

Abhishek Reddy — Thu, 24 Aug 2006 06:09:48 +0000

Your conclusion is reasonable -- I can see how the syntax might throw you off. In fact, this is a nuance of the sort function, as it doesn't behave the way Perl's syntax naturally implies. It is similar to C's sort(), if that is familiar to you.

The optional first argument to sort is a subroutine telling it what predicate to use when sorting the sequence. The subroutine isn't evaluated just once in the foreach header -- it's evaluated many times by sort, instead of comparing ASCII values. It only uses the default ASCII sort predicate when you supply no subroutine.

Read it as "sort @arr using my subroutine sort_by_num instead of your default".

To be clearer, you could rewrite the parens like so:
sort (sort_by_num @arr)'

Hope that helps.