Implementing iterative autocorrelation process in C++ using for loops - c++

Question

I am implementing pitch tracking using an autocorrelation method in C++ but I am struggling to write the actual line of code which performs the autocorrelation.
I have an array containing a certain number ('values') of amplitude values of a pre-recorded signal, and I am performing the autocorrelation function on a set number (N) of these values.
In order to perform the autocorrelation I have taken the original array and reversed it so that point 0 = point N, point 1 = point N-1 etc, this array is called revarray
Here is what I want to do mathematically:
(array[0] * revarray[0])
(array[0] * revarray[1]) + (array[1] * revarray[0])
(array[0] * revarray[2]) + (array[1] * revarray[1]) + (array[2] * revarray[0])
(array[0] * revarray[3]) + (array[1] * revarray[2]) + (array[2] * revarray[1]) + (array[3] * revarray[0])
...and so on. This will be repeated for array[900]->array[1799] etc until autocorrelation has been performed on all of the samples in the array.
The number of times the autocorrelation is carried out is:
values / N = measurements
Here is the relevent section of my code so far
for (k = 0; k = measurements; ++k){
for (i = k*(N - 1), j = k*N; i >= 0; i--, j++){
revarray[j] = array[i];
for (a = k*N; a = k*(N - 1); ++a){
autocor[a]=0;
for (b = k*N; b = k*(N - 1); ++b){
autocor[a] += //**Here is where I'm confused**//
}
}
}
}
I know that I want to keep iteratively adding new values to autocor[a], but my problem is that the value that needs to be added to will keep changing. I've tried using an increasing count like so:
for (i = (k*N); i = k*(N-1); ++i){
autocor[i] += array[i] * revarray[i-1]
}
But I clearly know this won't work as when the new value is added to the previous autocor[i] this previous value will be incorrect, and when i=0 it will be impossible to calculate using revarray[i-1]
Any suggestions? Been struggling with this for a while now. I managed to get it working on just a single array (not taking N samples at a time) as seen here but I think using the inverted array is a much more efficient approach, I'm just struggling to implement the autocorrelation by taking sections of the entire signal.

Answer 1

It is not very clear to me, but I'll assume that you need to perform your iterations as many times as there are elements in that array (if it is indeed only half that much - adjust the code accordingly).
Also the N is assumed to mean the size of the array, so the index of the last element is N-1.
The loops would looks like that:
for(size_t i = 0; i < N; ++i){
autocorr[i] = 0;
for(size_t j = 0; j <= i; ++j){
const size_t idxA = j
, idxR = i - j; // direct and reverse indices in the array
autocorr[i] += array[idxA] * array[idxR];
}
}
Basically you run the outer loop as many times as there are elements in your array and for each of those iterations you run a shorter loop up to the current last index of the outer array.
All that is left to be done now is to properly calculate the indices of the array and revarray to perform the calculations and accummulate a running sum in the current outer loop's index.