Pointer/Array Indexing Returning Unexpected Values - c++

Question

I am implementing code for convolution in C++ (I know it exists already but I'm just doing it for practice since I'm a beginner), and while I can get the correct output, there are certain methods I'm trying that are giving unexpected output depending on how I access the values of the convolution that I store in an array and I'm not sure why.
The function code that works, whether I access the values by array indexing or with pointer incrementing, is:
void conv(int M, int* h, int L, int* x, int* y) {
int n, m = 0;
for (n = 0; n < L + M - 1; n++) {
for (m = std::max(0, n - L + 1); m <= std::min(n, M - 1); m++) {
*(y+n) += *(h + m) * *(x + n - m);
};
std::cout << "using array index: " << std::endl;
std::cout << "n = " << n << " " << "y = " << y[n] << " " << std::endl;
std::cout << std::endl;
std::cout << "using pointer: " << std::endl;
std::cout << "n = " << n << " " << "y = " << *(y+n) << " " << std::endl;
std::cout << std::endl;
//y++;
}
}
However, if I make slight changes to this (numbered below):
void conv(int M, int* h, int L, int* x, int* y) {
int n, m = 0;
for (n = 0; n < L + M - 1; n++) {
for (m = std::max(0, n - L + 1); m <= std::min(n, M - 1); m++) {
*y += *(h + m) * *(x + n - m); //[1]
};
std::cout << "using array index: " << std::endl;
std::cout << "n = " << n << " " << "y = " << y[n] << " " << std::endl;
std::cout << std::endl;
std::cout << "using pointer: " << std::endl;
std::cout << "n = " << n << " " << "y = " << *y << " " << std::endl; //[2]
std::cout << std::endl;
y++; //[3]
}
}
In this case, only accessing the values via pointer provides the correct output, while accessing it via array indexing provides random garbage.
My test code is:
int main()
{
const int M = 5; const int L = 6;
int y[M + L - 1] = {};
int x[L] = { 1, -2, 5, 3, 8, -4 };
int h[M] = { 1,2,3,4,5 };
int* yPtr = y; int* hPtr = h; int* xPtr = x;
conv(M, hPtr, L, xPtr, yPtr);
std::cout << "value after leaving conv" << std::endl;
for (int i = 0; i < M+L-1; i++) {
std::cout << "i = " << i << " " << "y = " << y[i] << std::endl;
}
}
which always provides the correct output even when accessing the array elements in the for loop of the conv provides the incorrect output.
For reference, the correct output is y = {1, 0, 4, 11, 26, 31, 53, 35, 24, -20}.
What am I doing wrong in the second example of conv to be getting the wrong values when using array indexing?

Answer 1

In the second version of the code, you are incrementing y as you go through the loop, so y[n] in the second version is equivalent to y[2*n] in the first. Once n reaches half the size of the array, y[n] is past the end of the array, thus garbage. *y is equivalent to y[0].

Answer 2

Your example is sufficiently weird to be a little difficult to read, but from your second version, this is fishy:
std::cout << "n = " << n << " " << "y = " << y[n] << " " << std::endl;
You're incrementing y as you go, so y[n] is going to go to weird places fast.
I saved Y as int * yOrig = y; and then used that, and I think I'm getting the output you expect, but I'm not sure.