在Objective C中随机化NSArray的规范方法 [英] canonical way to randomize an NSArray in Objective C

问题描述

是否有规范方法在Objective C中随机化数组？

Is there a canonical way to randomize an array in Objective C?

推荐答案

我的实用程序库在NSMutableArray上定义此类别这样做：

My utility library defines this category on NSMutableArray to do it:

@interface NSMutableArray (ArchUtils_Shuffle)
- (void)shuffle;
@end

// Chooses a random integer below n without bias.
// Computes m, a power of two slightly above n, and takes random() modulo m,
// then throws away the random number if it's between n and m.
// (More naive techniques, like taking random() modulo n, introduce a bias 
// towards smaller numbers in the range.)
static NSUInteger random_below(NSUInteger n) {
    NSUInteger m = 1;

    // Compute smallest power of two greater than n.
    // There's probably a faster solution than this loop, but bit-twiddling
    // isn't my specialty.
    do {
        m <<= 1;
    } while(m < n);

    NSUInteger ret;

    do {
        ret = random() % m;
    } while(ret >= n);

    return ret;
}

@implementation NSMutableArray (ArchUtils_Shuffle)

- (void)shuffle {
    // http://en.wikipedia.org/wiki/Knuth_shuffle

    for(NSUInteger i = [self count]; i > 1; i--) {
        NSUInteger j = random_below(i);
        [self exchangeObjectAtIndex:i-1 withObjectAtIndex:j];
    }
}

@end

制作确定你在调用它之前的某个时候播种随机数生成器（例如 srandom（time（NULL）））;否则输出不会非常随机。

Make sure you seed the random number generator (with e.g. srandom(time(NULL))) sometime before you call it; otherwise the output won't be very random.

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