If all you need to do is to calculate the average of a segment of the array just wire the array to the array subset function and specify the start and length of the subset you want. Wire the output to the average function (or a sum-function and then divide by the length).
If the arrays are very large using the subset function might require a lot of memory, in that case use the index array element in a loop with a shift register and add the elements you want in the loop (keep the previous result in the shift reg), then divide the output by the number of elements. The latter procedure works also nicely if you need the average of elements separated from eachother in the array.
