Returning a function's static array to another array in the main() function

Question

I figured out how to make this one work already but can't explain in details what is so different in these two codes here.

Incorrect code:

const int nRows = 2;
const int nCols = 2;
int * colSum (int [nRows][nCols]);
int * rowSum (int [nRows][nRows]);

int main() {

    int my2Darray[nRows][nCols] = {{10, 20}, {30, 40}};
    int totalsByColumn[nCols] = {};
    *totalsByColumn = *(colSum(my2Darray));
    
    for (int i = 0; i < nCols; i++) {
        cout << totalsByColumn[i] << endl;
    } 
}

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

    static int arr[nRows] = {};

    for (int i = 0; i < nCols; i++) {
        for (int rowcount = 0; rowcount < nRows; rowcount++) {
            arr[i] += arrayArg[rowcount][i];
        }
    }
    return arr;
}

I was getting 40 0 as the output.

Then I fixed it by doing this:

int main() {

    int my2Darray[nRows][nCols] = {{10, 20}, {30, 40}};
    int *totalsByColumn = colSum(my2Darray);
    
    for (int i = 0; i < nCols; i++) {
        cout << totalsByColumn[i] << endl;
    } 
}

Output is 40 60, just what I wanted.

Was I just decaying to the first element of totalsByColumn by using the dereference operator on my first block of code? I feel like there might be a quicker way of adding the columns and rows together and assigning them to arrays in the main function, but as long as it does what I want I'm okay with that for the moment.

*totalsByColumn = *(colSum(my2Darray)) is equivalent to totalsByColumn[0] = colSum(my2Darray)[0]. You are only assigning one element of totalsByColumn, the other preserves whatever value it had previously. — Igor Tandetnik
– Igor Tandetnik, Commented Dec 14, 2020 at 15:55
I have no idea why you are making your life difficult with raw arrays, use std::vector. — Quimby
– Quimby, Commented Dec 14, 2020 at 15:55
@Quimby Or std::array if the dimension of each array is a compile-time constant as it is here. Passing arrays to and from functions is fiddly and annoying, the standard library containers are a lot nicer. — Nathan Pierson
– Nathan Pierson, Commented Dec 14, 2020 at 15:56
@Quimby Newbies love pointers. It's a fact of life (or bad teaching). — john
– john, Commented Dec 14, 2020 at 15:57

eerorika · Accepted Answer · 2020-12-14 16:13:21Z

1

Was I just decaying to the first element of totalsByColumn by using the dereference operator on my first block of code?

Yes.

I feel like there might be a quicker way of adding the columns and rows together

Certainly. Also ways that are thread safe unlike your solution. A simple way is to use an output iterator to write directly to the array where you want the results:

int* colSum (int arrayArg[][nCols], int out[]) {
    for (int i = 0; i < nCols; i++) {
        out[i] = 0;
        for (int rowcount = 0; rowcount < nRows; rowcount++) {
            out[i] += arrayArg[rowcount][i];
    // ...

int totalsByColumn[nCols];
colSum(my2Darray, totalsByColumn);

edited Dec 14, 2020 at 16:13

answered Dec 14, 2020 at 16:01

eerorika

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1 Comment

João Viscaino Over a year ago

Thank you so much, that's much better!

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