Finding the smallest and biggest value in NSArray of NSNumbers

Question

What's an effective and great way to compare all the values of NSArray that contains NSNumbersfrom floats to find the biggest one and the smallest one?

Any ideas how to do this nice and quick in objective-c?

There is no such thing as an NSArray that contains floats. It would have to be an NSArray that contains NSNumbers. – matt Apr 10 '13 at 18:43 — matt, Apr 10 '13 at 18:43
@matt Thanks! I've corrected the question name – Sergey Grischyov Apr 10 '13 at 22:11 — Sergey Grischyov, Apr 10 '13 at 22:11

Martin R · Accepted Answer · 2013-07-23 20:32:07Z

If execution speed (not programming speed) is important, then an explicit loop is the fastest. I made the following tests with an array of 1000000 random numbers:

Version 1: sort the array:

NSArray *sorted1 = [numbers sortedArrayUsingSelector:@selector(compare:)];
// 1.585 seconds

Version 2: Key-value coding, using "doubleValue":

NSNumber *max=[numbers valueForKeyPath:@"@max.doubleValue"];
NSNumber *min=[numbers valueForKeyPath:@"@min.doubleValue"];
// 0.778 seconds

Version 3: Key-value coding, using "self":

NSNumber *max=[numbers valueForKeyPath:@"@max.self"];
NSNumber *min=[numbers valueForKeyPath:@"@min.self"];
// 0.390 seconds

Version 4: Explicit loop:

float xmax = -MAXFLOAT;
float xmin = MAXFLOAT;
for (NSNumber *num in numbers) {
    float x = num.floatValue;
    if (x < xmin) xmin = x;
    if (x > xmax) xmax = x;
}
// 0.019 seconds

Version 5: Block enumeration:

__block float xmax = -MAXFLOAT;
__block float xmin = MAXFLOAT;
[numbers enumerateObjectsUsingBlock:^(NSNumber *num, NSUInteger idx, BOOL *stop) {
    float x = num.floatValue;
    if (x < xmin) xmin = x;
    if (x > xmax) xmax = x;
}];
// 0.024 seconds

The test program creates an array of 1000000 random numbers and then applies all sorting techniques to the same array. The timings above are the output of one run, but I make about 20 runs with very similar results in each run. I also changed the order in which the 5 sorting methods are applied to exclude caching effects.

Update: I have now created a (hopefully) better test program. The full source code is here: https://gist.github.com/anonymous/5356982. The average times for sorting an array of 1000000 random numbers are (in seconds, on an 3.1 GHz Core i5 iMac, release compile):

Sorting      1.404
KVO1         1.087
KVO2         0.367
Fast enum    0.017
Block enum   0.021

Update 2: As one can see, fast enumeration is faster than block enumeration (which is also stated here: http://blog.bignerdranch.com/2337-incremental-arrayification/).

EDIT: The following is completely wrong, because I forgot to initialize the object used as lock, as Hot Licks correctly noticed, so that no synchronization is done at all. And with lock = [[NSObject alloc] init]; the concurrent enumeration is so slow that I dare not to show the result. Perhaps a faster synchronization mechanism might help ...)

This changes dramatically if you add the NSEnumerationConcurrent option to the block enumeration:

__block float xmax = -MAXFLOAT;
__block float xmin = MAXFLOAT;
id lock;
[numbers enumerateObjectsWithOptions:NSEnumerationConcurrent usingBlock:^(NSNumber *num, NSUInteger idx, BOOL *stop) {
    float x = num.floatValue;
    @synchronized(lock) {
        if (x < xmin) xmin = x;
        if (x > xmax) xmax = x;
    }
}];

The timing here is

Concurrent enum  0.009

so it is about twice as fast as fast enumeration. The result is probably not representative because it depends on the number of threads available. But interesting anyway! Note that I have used the "easiest-to-use" synchronization method, which might not be the fastest.

often block enumeration is faster than fast enumeration. did u try that? how often did u execute your examples? – vikingosegundo Apr 10 '13 at 16:50 — vikingosegundo, Apr 10 '13 at 16:50
@vikingosegundo: I have updated the answer. Block enumeration is slower than fast enumeration here. – Martin R Apr 10 '13 at 17:00 — Martin R, Apr 10 '13 at 17:00
How often did u execute the codes? one or few runs wont give you a good result. – vikingosegundo Apr 10 '13 at 17:01 — vikingosegundo, Apr 10 '13 at 17:01
So explicit loop is fastest!!! Quite unbelievable – Anoop Vaidya Apr 10 '13 at 17:05 — Anoop Vaidya, Apr 10 '13 at 17:05
@vikingosegundo: I will improve the test program to determine the average execution time for each method, and then report the result later. – Martin R Apr 10 '13 at 17:06 — Martin R, Apr 10 '13 at 17:06

不及格的程序员-八神

NSArray enumeration performance examined

Which kinds of enumeration do exist?

Linear enumeration

Graphs

Conclusions

An oddity

Concurrent enumeration

The graphs

Conclusions

Allocation

How did I measure?

Finding the smallest and biggest value in NSArray of NSNumbers

5 Answers

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