How to find the median value of NSNumbers in an NSArray? - iphone

Question

I'm trying to calculate the median of a (small) set of NSNumbers in an NSArray. Every object in the NSArray is a NSNumber.
Here is what I'm trying, but it's not working:
NSNumber *median = [smallNSArray valueForKeyPath:#"#median.floatValue"];

Answer 1

NSArray *sorted = [smallNSArray sortedArrayUsingSelector:#selector(compare:)]; // Sort the array by value
NSUInteger middle = [sorted count] / 2; // Find the index of the middle element
NSNumber *median = [sorted objectAtIndex:middle]; // Get the middle element
You can get fancier. For example, the median of a set with an even number of numbers is technically the average of the middle two numbers. You could also wrap this up into a neat one-line method in a category on NSArray:
#interface NSArray (Statistics)
- (id)median;
#end
#implementation NSArray (Statistics)
- (id)median
{
return [[self sortedArrayUsingSelector:#selector(compare:)] objectAtIndex:[self count] / 2];
}
#end

Answer 2

For anyone who has the unusual need for this function, here's a category method on NSArray that will work with both an odd and an even number of elements:
NSARRAY CATEGORY METHOD
- (float)median {
if (self.count == 1) return [self[0] floatValue];
float result = 0;
NSUInteger middle;
NSArray * sorted = [self sortedArrayUsingSelector:#selector(compare:)];
if (self.count % 2 != 0) { //odd number of members
middle = (sorted.count / 2);
result = [[sorted objectAtIndex:middle] floatValue];
}
else {
middle = (sorted.count / 2) - 1;
result = [[#[[sorted objectAtIndex:middle], [sorted objectAtIndex:middle + 1]] valueForKeyPath:#"#avg.self"] floatValue];
}
return result;
}
TEST
NSArray * singleElement = #[#1];
NSArray * oddNumberOfElements = #[#3, #5, #7, #12, #13, #14, #19, #20, #21, #22, #23, #29, #39, #40, #56];
NSArray * evenNumberOfElements = #[#3, #5, #7, #12, #13, #14, #19, #20, #21, #22, #23, #29, #40, #56];
NSLog(
#"oddNumberOfElements: %f, evenNumberOfElements: %f singleElement: %f",
[oddNumberOfElements median], [evenNumberOfElements median], [singleElement median]
);
//oddNumberOfElements: 20.000000, evenNumberOfElements: 19.500000 singleElement: 1.000000

Answer 3

Swift Extension
extension Array where Element: Comparable {
var median: Element {
return self.sort(<)[self.count / 2]
}
}