Objective C: Array with size repopulate - ios

Question

I need an array where I can store 3 values, and calculate the average of it.
After 3s the array is full, I would like to refresh every value starting from begin [0] of the array. So every 3s the the array is updated and I have the most recent average. For the moment when I add an object the array keeps growing.
This is the code I have at the moment:
//Inside function which loops every second so I can access myValue
self.myArray = [[NSMutableArray alloc] initWithCapacity:3];
[self.myArray addObject:[NSNumber numberWithDouble:myValue]];
for(int i = 0; i < self.myArray.count; i++) {
NSLog(#"Array %d: %#", i, self.myArray[i]);
}
Output
Array 0: 2
Array 1: 4
Array 2: 5
Array 3: 6
Array 4: 1

Answer 1

Just check if the size of your array is larger than 3. If so, remove the oldest element.
Here is a possible implementation:
Alloc and init the array:
self.myArray = [[NSMutableArray alloc] init];
Insert a new object and delete the oldest one:
[self.myArray insertObject:object atIndex:0];
if ( self.myArray.count > 3 )
{
[self.myArray removeLastObject];
}

Answer 2

It looks like you're trying to calculate a running average that gets updated every second. Here's a simple alternative approach that may work for you.
Initialize the average:
runningAverage = myValue;
Inside the loop:
const int WEIGHT = 3;
runningAverage = (runningAverage * (WEIGHT - 1) + myValue) / WEIGHT;
Here's what the output would look like:
Input: 1, 2, 3, 4, 101, 6, 7, 8
(Using an array)
Output: -, -, 2, 3, 36, 37, 38, 7
(Using running average)
Output: 1, 1.33, 1.88, 2.59, 35.4, 25.6, 19.4, 15.6