Why can't I define a count variable outside of my loop in this function (C++)

Question

This is a very basic question, I know, but I was solving the following simple exercise:

my2Darray is a two-dimensional array of doubles with nRows rows and nCols columns. Write a function that sums all the elements in each column and returns them in an array called totalsByColumn. Write a second function that sums all the elements in each row, and returns them in an array called totalsByRow.

My code works just fine now, but I was very curious as to why the following function wasn't working:

void colSum (int arrayArg[nRows][nCols]) {

    static int sumofCols[nCols] = {};
    int rowcount = 0;

    for (int i = 0; i < nCols; i++) {
        for ( ; rowcount < nRows; rowcount++) {
            sumofCols[i] += arrayArg[rowcount][i];
        }
        cout << sumofCols[i] << endl;
    }
}

Why is it that I can't create this rowcount variable outside of my nested for loops? Only the first column of the two rows from the array I used as argument to the function was being summed into sumofCols[0] , which led to sumofCols[1] to equal 0. When I set rowcount within the boundaries of either the first for loop, or the second one, the iteration works just as intended.

for (int rowcount = 0; rowcount < nRows; rowcount++)

This is the code I had to use.

I just wanted to make sure I understand 100% of what I'm doing, as C++ can be quite confusing for a beginner like me.

Oh, and I did change this function to return an int* value, as I'm basically assigning sumofCols to another array, as the exercise demands.

Answer 1

Because after the first iteration rowCount will be equal to nRows. And for the 2nd iteration of the outerloop it doesn't re-initialize to 0 therefore the code inside this for (; rowcount < nRows; rowcount++) loop doesn't work. You have to reset the value of rowCount to 0 after the iteration of the inner loop. If you really want to declare the rowCount outside of the loop you can do the following,

void colSum (int arrayArg[nRows][nCols]) {

    static int sumofCols[nCols] = {};
    int rowcount = 0;

    for (int i = 0; i < nCols; i++) {
        for ( ; rowcount < nRows; rowcount++) {
            sumofCols[i] += arrayArg[rowcount][i];
        }
        cout << sumofCols[i] << endl;
        rowCount = 0; // Here we are resetting the value to 0.
    }
}