Replace, move objects in NSMutableArray

I Have an array which I have already set the capacity to 5. What I want to do is, when I add an object and the capacity exceeds 5 then it moves the whole array structure of objects by one. For example 1,2,3,4,5 (array). Then when 6 comes to be added It must replace 5 and delete one to result in 2,3,4,5,6 and so on.. So in other words it has to save only last 5 objects. BUT as in the actual coding it will save objects with coordinates in an array with capacity of 100+ so it has to be quick and efficient. I tried with int to see how it progresses:

      NSMutableArray *array = [[NSMutableArray alloc] initWithCapacity:5];

            int i;
            for (i = 0; i<=20;i++ ) {
                NSNumber *number = [NSNumber numberWithInt:i];
                if (i>=5) {
                [array insertObject:number atIndex:5];
                }else{
                [array addObject:number];
                }
                NSLog(@"array = %@, count = %i",array, array.count);
            }

but it stores all 20 in sorting like 1,2,3,4,20,19,18,17...6 any help would be appreciated! Thank you in advance..

If index is already occupied, the objects at index and beyond are shifted by adding 1 to their indices to make room. so what you can use is replaceObjectAtIndex:withObject: this will replace the object. but if you want to remove the first object you do it your self. first check the length of the array. if greater than 5 remove the first element and shift all the objects to the the lower index. then add the new object.

if ([array count]>=5)
{
[array removeObjectAtIndex:index];

[array insertObject:new object atIndex :5];