How expensive is the NSArray count method?

Question

I am in a situation that I need to get the items in an array and time is sensitive. I have the option of using a separate variable to hold the current count or just use NSMutableArray's count method.

ex: if (myArray.count == ... ) or if (myArrayCount == ...)

How expensive is it to get the counting of items from the count method of an array?

Answer 1

The correct answer is, there is no difference in speed, so access the count of the array as you wish my child :)

Answer 2

Fetching NSArray's count method is no more expensive then fetching a local variable in which you've stored this value. It's not calculated when it's called. It's calculated when the array is created and stored.

For NSMutableArray, the only difference is that the property is recalculated any time you modify the contents of the array. The end result is still the same--when you call count, the number returned was precalculated. It's just returning the precalculated number it already stored.

Storing count in a variable, particularly for an NSMutableArray is actually a worse option because the size of the array could change, and access the count in this variable is not faster whatsoever. It only provides the added risk of potential inaccuracy.

The best way to prove to yourself that this is a preset value that is not calculated upon the count method being called is to create two arrays. One array has only a few elements. The other array has tens of thousands of elements. Now time how long it takes count to return. You'll find the time for count to return is identical no matter the size of the array.