Objective-C, sorting muti-dimensional arrays

Question

I've been having some trouble with sorting multi-dimensional arrays in Objective-C. I basically have an array of which every element is an array of the form:

(NSString, NSDate, NSString, NSString)

such that my top level array has the form:

(
(NSString, NSDate, NSString, NSString),
(NSString, NSDate, NSString, NSString),
(NSString, NSDate, NSString, NSString),
(NSString, NSDate, NSString, NSString),
...
)

I would like to be able to sort the elements of the top level array based on any of of their own elements. I wrote the following code which does this, but has proven to be far too inefficient for the large data sets I am dealing with.

-(NSMutableArray *) sortArrayByDate:(NSMutableArray *) unsortedArray {

    NSMutableArray * sortedArray = [[NSMutableArray alloc ] init ];

    while ([unsortedArray count]>0) {
        int topIndex = 0;
        NSDate * topDate = [[NSDate alloc] initWithString:@"1970-01-01 00:00:00 +0600"];
        for(int j=0;j<[unsortedArray count];j++) {
            NSDate * targetDate = [[unsortedArray objectAtIndex:j] objectAtIndex:1];
            if ([targetDate compare:topDate] == NSOrderedDescending) {
                topDate = targetDate;
                topIndex = j;
            }            
        }
        [sortedArray addObject:[unsortedArray objectAtIndex:topIndex]];
        [unsortedArray removeObjectAtIndex:topIndex];        
    }    
    return sortedArray;
}

Can anyone please make a suggestion on how to accomplish this task using the more established methods of either sortUsingSelector or sortUsingDescriptor? If I was sorting a 1D array I think it would be something like:

[unsortedArray sortUsingSelector: @selector(compare:)]

but how do I tell it to sort using the nth value of the array I am passing it?

-Many thanks

-Many Thanks

Answer 1

in most cases, you would create an object:

@interface MONObject : NSObject
{
  NSString * a;
  NSDate * b;
  NSString * c;
  NSString * d;
}
...

then teach it to compare itself to others, then use those objects in the array. logical organization of data and implementation.

Answer 2

You can also use blocks-based method sortUsingComparator: in NSMutableArray like this –

[myArray sortUsingComparator:^(id first, id second){
    id firstObject = [first objectAtIndex:1];
    id secondObject = [second objectAtIndex:1];

    return [firstObject compare:secondObject];
}]

You also have a parallel method sortedArrayUsingComparator: in NSArray which will spew out a sorted array.

Sorting by varying indices

typedef NSComparator (^ComparatorFactory)(id);

ComparatorFactory comparatorForIndex = ^NSComparator(id context) {
    NSInteger index = [(NSNumber*)context integerValue];
    NSComparator comparator = ^(id first, id second) {
        id firstObject = [first objectAtIndex:index];
        id secondObject = [second objectAtIndex:index];

        return [firstObject compare:secondObject];
    };

    return [[comparator copy] autorelease];
};

[myArray sortUsingComparator:comparatorForIndex([NSNumber numberWithInteger:1])];

Depending on the index passed, the array will sort based on objects at that index. This is pretty basic code but you can add to this.