objective-c - Objective-C : Calculate the standard deviation of an NSArray of NSNumber objects?

Question

如果我有 NSNumber 对象的 NSArray，我该如何计算数组中数字的标准偏差？

Answer 1

假设将所有 NSNumbers 处理为双倍长度 float 是安全的(所以如果你在范围的末端有一些 64 位整数，你会失去一些精度)并且我首先正确地记住了公式实现可以是:

- (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])];
}