在NSNumbers的NSArray中寻找最小和最大值

问题描述

比较包含floats中包含NSNumbers的NSArray的所有值以找到最大值和最小值的有效方法是什么?

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

有什么想法可以在Objective-C中做到又好又快?

Any ideas how to do this nice and quick in Objective-C?

推荐答案

如果执行速度(而不是编程速度)很重要，则显式循环最快.我对1000000个随机数的数组进行了以下测试:

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:

版本1:对数组进行排序:

Version 1: sort the array:

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

版本2:使用"doubleValue"进行键值编码:

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

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

版本3:使用自我"进行键值编码:

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

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

第4版:显式循环:

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

版本5:块枚举:

__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

测试程序创建一个包含1000000个随机数的数组，然后应用所有排序相同阵列的技术.上面的时间是一次运行的输出，但是我进行了大约20次运行，每次运行的结果非常相似.我还更改了5种排序方法的应用顺序，以排除缓存效果.

The test program creates an array of 1000000 random numbers and then applies all sortingtechniques 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.

更新:我现在创建了(希望)更好的测试程序.完整的源代码在这里: https://gist.github.com/anonymous/5356982 .平均排序次数1000000个随机数组成的数组(以秒为单位，在3.1 GHz Core i5 iMac上，发布编译):

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 anarray 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

更新2:正如人们所看到的，快速枚举比块枚举快(在此处也进行了说明:).

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/).

编辑:以下是完全错误的，因为正如Hot Licks正确地注意到的那样，我忘记了初始化用作锁的对象，因此根本不进行同步.使用lock = [[NSObject alloc] init];时，并发枚举如此之慢我不敢显示结果.也许更快的同步机制可能帮助...)

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 slowthat I dare not to show the result. Perhaps a faster synchronization mechanism mighthelp ...)

如果将NSEnumerationConcurrent选项添加到块枚举:

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

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