Shuffling an array of NSNumbers & converting back to ints - objective-c

Question

I have an NSMutableArray urlArray of size n, I want to randomly choose 4 of these URLs from the total number of elements in the array.
However I don't want to shuffle urlArray directly, I'd prefer to make an "indexArray" [0 ... (n-1)] & shuffle these, & then use the first 4 elements of the shuffled indexArray to decide which elements I choose from urlArray.
First off I created the indexArray as follows:
for (int i = 0; i < numberOfStems; i++) {
[indexArray addObject:[NSNumber numberWithInteger:i]];
}
This allowed me to shuffle my indexArray, so far so good. Because I used the
[NSNumber numberWithInteger:i] method, the elements in the shuffled indexArray are NSNumbers.
Is there a way to convert the NSNumber objects in indexArray into ints?
I attempted to use the intValue function but this didn't appear to be what I needed.
I also tried creating a c style array but that wasn't so successful either - I'd like to stick with objective-c syntax if possible.
Any ideas? any hints appreciated :)

Answer 1

Why don't you just create a normal c array, shuffle that and then use the first four integers in the array as the for random index?
something like
int* index = malloc(numberOfStems*sizeof(int));
for (int i = 0; i < numberOfStems; ++i)
{
index[i] = i;
}
for (int i = numberOfStems - 1; i > 0; --i)
{
int randomIndex = arc4random() % i;
int tmp = index[i];
index[i] = index[randomIndex];
index[randomIndex] = tmp;
}
now use index to access the URL's
EDITED: updated algorithm (although not really related to OP question)

Answer 2

For a temporary array that stores only integers and gets thrown away after a relatively short task I would definitely prefer a C-style array: this would avoid a great deal of overhead, and is also simple to read.
int *array = (int*)malloc(sizeof(int)*numberOfStems);
for (int i = 0 ; i != numberOfStems ; i++) {
array[i] = i;
}
// Do the shuffle
// Pick first four, and do whatever you need to do
// ...
// Now that you are done with the array, do not forget to free it:
free(array);