Finding out NSArray/NSMutableArray changes' indices

Question

I have a NSMutableArray oldArray. Now, at one point, this NSMutableArray object gets updated with another NSMutableArray,

Answer 1

If both arrays are already sorted in ascending order, you can find the added and removed elements with a single loop over both arrays (using two independent pointers into the array):

NSArray *oldArray = @[@"a",@"b",@"d",@"e",@"g"];
NSArray *newArray = @[@"a",@"c",@"d",@"e",@"f",@"h"];

NSMutableArray *removedArray = [NSMutableArray array];
NSMutableArray *addedArray = [NSMutableArray array];

NSUInteger iold = 0; // index into oldArray
NSUInteger inew = 0; // index into newArray

while (iold < [oldArray count] && inew < [newArray count]) {
    // Compare "current" element of old and new array:
    NSComparisonResult c = [oldArray[iold] compare:newArray[inew]];
    if (c == NSOrderedAscending) {
        // oldArray[iold] has been removed
        [removedArray addObject:@(iold)];
        iold++;
    } else if (c == NSOrderedDescending) {
        // newArray[inew] has been added
        [addedArray addObject:@(inew)];
        inew++;
    } else {
        // oldArray[iold] == newArray[inew]
        iold++, inew++;
    }
}
// Process remaining elements of old array:
while (iold < [oldArray count]) {
    [removedArray addObject:@(iold)];
    iold++;
}
// Process remaining elements of new array:
while (inew < [newArray count]) {
    [addedArray addObject:@(inew)];
    inew++;
}

NSLog(@"removed: %@", removedArray);
NSLog(@"added: %@", addedArray);

Output:

removed: (
    1,
    4
)
added: (
    1,
    4,
    5
)