Why do I get "error: variable 'sum_r' set but not used [-Werror,-Wunused-but-set-variable]" when I set a variable inside a for loop?

Why do I get an error, error: variable 'sum_r' set but not used [-Werror,-Wunused-but-set-variable] when I set the variables sum_r sum_g, and sum_b in a for loop then try to change their values later in another for loop?

Here is my current code:

// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
    RGBTRIPLE new[height][width];

    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            int sum_r = 0;
            int sum_g = 0;
            int sum_b = 0;

            for (int ji = i - 1; ji < 3; ji++)
            {
                for (int jj = j - 1; jj < 3; jj++)
                {
                    int rgb_r = image[ji][jj].rgbtRed;
                    int rgb_g = image[ji][jj].rgbtGreen;
                    int rgb_b = image[ji][jj].rgbtBlue;

                    sum_r += rgb_r;
                    sum_g += rgb_g;
                    sum_b += rgb_b;
                }
            }
        }
    }
    return;
}

I have tried using pointers and addresses but I cannot seem to figure it out. Could someone explain why this is happening?

>Solution :

1. You do not need return in a void function
2. You are getting this message because you set those variables in the loop but you do not use it. Your function can be reduced to the no-op and the compiler is warning that you forgot to do something with your value.

optimizing compilers will rescue this function to:

void blur(int height, int width, RGBTRIPLE image[height][width])
{
}

https://godbolt.org/z/cEeejGKrh