Obj-C：计算NSNumber对象的NSArray的标准差？ [英] Obj-C: Calculate the standard deviation of an NSArray of NSNumber objects?

问题描述

如果我有一个 NSArray 的 NSNumber 对象，如何计算数字的标准差数组？

If I have an NSArray of NSNumber objects, how do I calculate the standard deviation of the numbers in the array?

推荐答案

假设可以安全地将所有NSNumbers处理为双倍长度的浮点数在这个范围的极端，有一些64位整数），我正确地记住了公式的第一个实现可以是：

Assuming it's safe to process all NSNumbers as double length floats (so you'll lose some precision if you've got some 64 bit integers at extreme ends of the range in there) and I've remembered the formula correctly a first implementation could be:

- (NSNumber *)meanOf:(NSArray *)array
{
    double runningTotal = 0.0;

    for(NSNumber *number in array)
    {
        runningTotal += [number doubleValue];
    }

    return [NSNumber numberWithDouble:(runningTotal / [array count])];
}

- (NSNumber *)standardDeviationOf:(NSArray *)array
{
    if(![array count]) return nil;

    double mean = [[self meanOf:array] doubleValue];
    double sumOfSquaredDifferences = 0.0;

    for(NSNumber *number in array)
    {
         double valueOfNumber = [number doubleValue];
         double difference = valueOfNumber - mean;
         sumOfSquaredDifferences += difference * difference;
    }

    return [NSNumber numberWithDouble:sqrt(sumOfSquaredDifferences / [array count])];
}

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