Obj-C: Calculate the standard deviation of an NSArray of NSNumber objects?

Question

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

Answer 1

There is some good code on Rosetta Code for this. To go through your NSArray (instead of C array like they have in their example), just use this code along with their implementation of SDAccum:

- (double)computeStandardDeviationWithArray:(NSArray *)numberArray
{
    double sd;
    SDAccum *sdacc = [[SDAccum alloc] init];

    for(NSNumber *aNumber in numberArray)
    {
        sd = [sdacc value: [aNumber doubleValue]];
    }

    [sdacc release];
    return sd;
}