Question

I have an array for instance,

    Array {
    3.0 at Index 0
    2.0 at Index 1
    3.5 at Index 2
    1.0 at Index 4
}

I would like to get sort it in ascending order without losing the index in the first place, like this,

    Array {
    1.0 at Index 4
    2.0 at Index 1
    3.0 at Index 0
    3.5 at Index 2
}

When I sort the array using this,

NSArray *sortedArray = [hArray sortedArrayUsingSelector:@selector(caseInsensitiveCompare:)];
[knnRecog sortUsingDescriptors:[NSArray arrayWithObject:sortAsc]];

I lose the index. Does anyone know a way to preserve the index after sorting the array? Thanks

Answer 1

You can do the sorting you require with a change to your underlying data structure. Consider using an array of arrays, e.g.:

{
    {3.0, Index 0},
    {2.0, Index 1},
    {3.5, Index 2},
    {1.0, Index 4}
}

and given a sorting function:

NSComparisonResult customCompareFunction(NSArray* first, NSArray* second, void* context)
{
    id firstValue = [first objectAtIndex:0];
    id secondValue = [second objectAtIndex:0];
    return [firstValue compare:secondValue];
}

You can sort it like so:

NSArray* myArray = [NSArray arrayWithObjects:
                    [NSArray arrayWithObjects:
                     [NSNumber numberWithFloat:3.0f],
                     [NSNumber numberWithInt:0], nil],
                    [NSArray arrayWithObjects:
                     [NSNumber numberWithFloat:2.0f],
                     [NSNumber numberWithInt:1], nil],
                    [NSArray arrayWithObjects:
                     [NSNumber numberWithFloat:3.5f],
                     [NSNumber numberWithInt:2], nil],
                    [NSArray arrayWithObjects:
                     [NSNumber numberWithFloat:1.0f],
                     [NSNumber numberWithInt:4], nil]];

NSArray* sortedArray = [myArray sortedArrayUsingFunction:customCompareFunction context:NULL];

NSLog(@"Sorted array: %@", sortedArray);

Which prints:

Sorted array: (
(
1,
4
),
(
2,
1
),
(
3,
0
),
(
3.5,
2
)
)