NSArray计数方法有多贵？

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

正确的答案是，速度没有区别，所以你希望我的孩子访问数组的计数:)

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.

Answer 3

As a correction to everyone above, NSArray does not have a count property. It has a count method. The method itself either physically counts all of the elements within the array or is a getter for a private variable the array stores. Unless you plan on subclassing NSArray and create a higher efficient system for counting dynamic and/or static arrays... you're not going to get better performance than using the count method on an NSArray. As a matter of fact, you should count on the fact that Apple has already optimized this method to it's max. My main ponder after this is that if you are doing an asynchronous call and your focus is optimizing the count of an NSArray how do you not know that you are seriously doing something wrong. If you are performing some high performance hitting method on the main thread or such... you should consider optimizing that. The performance hit of iterating and counting through the array using NSArray's count method should in no way effect your performance to any noticeable rate.

You should read up more on performance for NSArrays and NSMutableArrays if this is truly a concern for you. You can start here: link

Answer 4

If you need to get the item**s** then getting the count is not time critical. You'd also want to look at fast enumeration, or using enumeration with dispatch blocks, especially with parallel execution.

Answer 5

Edit:

Asa's is the most correct answer. I misunderstood the question.

Asa is right because the compiler will automatically optimize this and use the fastest way on its own.

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TheGamingArt is correct about NSArray being as optimal as could be. However, this is only for obj-c.

Don't forget you have access to c and c++ which means you can use vectors which should be only 'slightly' faster considering it won't use obj-c messaging. However, it wouldn't surprise me if the difference isn't noticeable. c++ vector benchmarks: http://baptiste-wicht.com/posts/2012/12/cpp-benchmark-vector-list-deque.html

This is a good example of Premature Optimization ( http://c2.com/cgi/wiki?PrematureOptimization ). I suggest you look into GCD or NSOperations ( http://www.raywenderlich.com/19788/how-to-use-nsoperations-and-nsoperationqueues )