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very different execution time
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very different execution time


  • Subject: very different execution time
  • From: Hans van der Meer <email@hidden>
  • Date: Fri, 28 Sep 2007 13:52:04 +0200

I have two versions of a class for the production of permutations.
In one the permutation is numbers in an int array, in the other objects in a NSMutableArray.
There is a huge difference in execution time between these two versions. When the array version takes 135 seconds, the corresponding calculation with NSMutableArray takes 700 seconds; using exchangeObjectAtIndex:withObjectAtIndex: takes even 25% more. Without the specific array operations the time needed is not much less then for the array version. I therefore conclude that the replace operations in NSMutableArray are responsible for the huge difference. However, given that in the one integers are assigned and in the other pointers, I have difficulties to understand the magnitude of the difference. Could it be the object message sending mechanism? Perhaps I have something wrong? If that difference is real and without cure, what causes it?
I hope someone will provide the answer.


The operations concerned are in a recursive method of which the core is:

if (steps[level] > 0) { i = --steps[level];
  unsigned int tmp = permutation[i];
  permutation[i] = permutation[i + 1]; permutation[i + 1] = tmp;
} else if (level > 0) {
  unsigned int tmp = permutation[0];
  for (i = 0; i < level; i++) { permutation[i] = permutation[i + 1]; }
  permutation[level] = tmp;
  [self nextlevel: level]; ...
}

and for the Foundation collection:
if (steps[level] > 0) { i = --steps[level];
id temp = [theValues objectAtIndex: i];
[theValues replaceObjectAtIndex: i withObject: [theValues objectAtIndex: (i + 1)]];
[theValues replaceObjectAtIndex: (i + 1) withObject: temp];
} else if (level > 0) {
id temp = [theValues objectAtIndex: 0];
for (i = 0; i < level; i++) {
[theValues replaceObjectAtIndex: i withObject: [theValues objectAtIndex: (i + 1)]];
}
[theValues replaceObjectAtIndex: level withObject: temp];
[self nextlevel: level]; ...
}


Hans van der Meer



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