Question

Currently I’m trying to find a compact way to average a matrix. The obvious solution is to sum the matrix, then divide by the number of elements. I have, however, come across a method on the apple developer website that claims this can be done in a simpler way using valueForKeyPath. This is linked here:

http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/KeyValueCoding/Articles/CollectionOperators.html

Here is the example I’m currently working on to try and get it to work:

-(void)arrayAverager
{
   NSMutableArray *myArray = [NSMutableArray arrayWithCapacity:25];
   [myArray addObject:[NSNumber numberWithInteger:myValue]];
   NSNumber *averageValue = [myArray valueForKeyPath:@"@avg.self"];
   NSLog(@"avg  = %@", averageValue);

} 

The problem is: instead of averaging the array it merely prints out the elements in the array 1 by 1.

UPDATE

-(void) pixelAverager

{
    //Put x-coordinate value of all wanted pixels into an array
    NSMutableArray *xCoordinateArray = [NSMutableArray arrayWithCapacity:25];
    [xCoordinateArray addObject:[NSNumber numberWithInteger:xCoordinate]];
    NSLog(@"avg = %@", [xCoordinateArray valueForKeyPath:@"@avg.intValue"]);
 }

Answer 1

You need to use @avg.floatValue (or @avg.doubleValue, or what have you). The @avg operator will average the property of the objects in the array specified by the name after the dot. The documentation is confusing on this point, but that is what:

to get the values specified by the property specified by the key path
to the right of the operator

Is saying. Since you have a collection of NSNumber objects, you use one of the *value accessors, e.g. floatValue to get the value of each object. As an example:

#include <Foundation/Foundation.h>

int main(void) {
  NSMutableArray *ma = [NSMutableArray array];
  [ma addObject:[NSNumber numberWithFloat:1.0]];
  [ma addObject:[NSNumber numberWithFloat:2.0]];
  [ma addObject:[NSNumber numberWithFloat:3.0]];

  NSLog(@"avg = %@", [ma valueForKeyPath:@"@avg.floatValue"]);

  return 0;
}

Compiling and running this code returns:

$ clang avg.m -framework Foundation -o avg
steve:~/code/tmp
$ ./avg 
2013-01-18 12:33:15.500 avg[32190:707] avg = 2
steve:~/code/tmp

The nice thing about this approach is that this work for any collection, homogenous or otherwise, as long as all objects respond to the specified method, @avg will work.

EDIT

As pointed in the comments, the OP’s problem is that he is averaging a collection with one element, and thus it appears to simply print the contents of the collection. For a collection of NSNumber objects, @avg.self works just fine.