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

Here is another version I've used some time ago.

NSArray *numbers = [NSArray arrayWithObjects:[NSNumber numberWithInt:...],
                                             [NSNumber numberWithInt:...],
                                             [NSNumber numberWithInt:...], nil];

// Compute array average
int total = 0;
int count = [numbers count];

for (NSNumber *item in numbers) {

    total += [item intValue];
}

double average = 1.0 * total / count;

// Sum difference squares
double diff, diffTotal = 0;

for (NSNumber *item in numbers) {

    diff = [item doubleValue] - average;
    diffTotal += diff * diff;
}

// Set variance (average from total differences)
double variance = diffTotal / count; // -1 if sample std deviation

// Standard Deviation, the square root of variance
double stdDeviation = sqrt(variance);