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)