Error: subscripted value is not an array, pointer, or vector

Question

When I compile the following C code with Clang, I get this error: "Error: subscripted value is not an array, pointer, or vector" at the line collect[i][j] = result.

double * Matrix_Add(double *X, double *Y, double *input_length)
{
    int rows = (int)input_length[0];
    int cols = (int)input_length[1];
    int output_size = rows * cols;

    double *collect = (double *)calloc(rows * cols, sizeof(double));
    double result; 

    int i = 0, j = 0;

    // __________

    while (i <= rows){ 
        j = 1;
        while (j <= cols){

            result = X[i] + Y[j];
            collect[i][j] = result;
            j = j + 1; }

        i = i + 1;
    }

    return 0;
}

So I added a int*

    int i = 0, j = 0;
    int * i_ptr =  &i;
    int * j_ptr =  &j;

    // __________

    while (i <= rows){ 
    j = 1;
    while (j <= cols){

        result = X[i] + Y[j];
        collect[i_ptr][j_ptr] = result;
        //collect[i][j] = result;
        j = j + 1; }

    i = i + 1;
}

but when I compile, I get "error: array subscript is not an integer."

I've read about a dozen Stack Overflow posts on this and I still don't know how to fix it.

Thanks.

collect is a pointer to a "one-dimensional" array - you are trying to use it like a "two-dimensional" array — UnholySheep
– UnholySheep, Commented Sep 16, 2020 at 19:57
And I have no idea what you did in that second example, but randomly guessing definitely does not work in C — UnholySheep
– UnholySheep, Commented Sep 16, 2020 at 19:57

John Bollinger · Accepted Answer · 2020-09-16 20:00:01Z

1

Given that collect is declared as a double *, it follows that collect[i] is a double. You cannot subscript that.

Perhaps you want collect[i * cols + j] instead.

Or if your compiler supports the variable-length array option, then perhaps you wanted to declare collect differently:

double (*collect)[cols] = calloc(rows * sizeof(*collect));

..., in which case collect[i][j] would do what you expect.

answered Sep 16, 2020 at 20:00

John Bollinger

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