Sort by the mode in an Array in Objective-C iPhone

Question

If I have an NSArray with some values in them. Is there a way using descriptors to sort it by the most frequent number in the array first and the least frequent number at the end,

Array has(
"3",
"2",
"1",
"3",
"3",
"7",
)

to

Array has(
"3",
"3",
"3",
"1",
"2",
"7",
)

Answer 1

@interface NSArray (Ext)
-(NSArray*) sortByMostFrequent ;
@end



@implementation NSArray (Ext)
-(NSArray*) sortByMostFrequent {
    NSMutableDictionary* frequencyDict = [NSMutableDictionary dictionary];
    for (id obj in self) {
        int frequency = [[frequencyDict valueForKey:obj] intValue];
        [frequencyDict setValue:[NSNumber numberWithInt:frequency+1] forKey:obj];
    }
    NSMutableArray* ary = [NSMutableArray arrayWithCapacity:self.count];
    for (id obj in self) {
        [ary addObject:[NSDictionary dictionaryWithObjectsAndKeys:
                        obj, @"Object",
                        [frequencyDict valueForKey:obj], @"Frequency",
                        nil]];
    }
    NSSortDescriptor* sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Frequency" ascending:NO];
    [ary sortUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
    [sortDescriptor release];
    return [ary valueForKey:@"Object"];
}
@end



/// example
NSArray* ary = [NSArray arrayWithObjects:@"3", @"2", @"1", @"3", @"3", @"7", nil];
NSLog(@"ary %@", [ary sortByMostFrequent]);