C++ error when passing 2D array into function and changing elements' values ("error: invalid types ‘int[int]’ for array subscript")

Question

can someone help? i'm trying to create a function for reading a 2D array and then display it in int main(). i get the following error when running the code: "error: invalid types ‘int[int]’ for array subscript" on the line with cin >> v[i][j].

i'm confused because this seems to work with 1D arrays ( f(&v[0]) and void f(int* v, int* n)) so why wouldnt it work in this case as well?

#include <iostream>

using namespace std;

void f(int* v, int* n, int* m)
{
    cin >> *m >> *n;
    int i, j;
    
    for(i=0; i<*m; i++)
        for(j=0; j<*n; j++)
           {
            cin >> v[i][j];
           }

}

int main()
{
    int i, j, m, n, v[10][10];
    
    f(&v[0][0], &n, &m);
    
    for(i=0; i<m; i++)
        { for(j=0; j<n; j++)
            cout << v[i][j] << " ";
            cout << endl;
        }
            return 0;
}

Jo3Jo · Accepted Answer · 2022-10-03 17:48:39Z

1

In your code the type of v in function f is a pointer to int, while it should be a pointer to a fixed-length array of int. Should be:

void f(int* v[10], int* n, int* m)

or equivalently

void f(int v[][10], int* n, int* m)

Also, by passing &v[0][0], you are passing v[0] to the function. You need to pass v (which is the same as &v[0], but you can write just v).

edited Oct 3, 2022 at 17:48

answered Oct 3, 2022 at 12:15

Jo3Jo

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5 Comments

molbdnilo Over a year ago

But a pointer to an array is not a pointer to a pointer. The type of &v[0] is int(*)[10], not int**, and there is no conversion between those two.

Jo3Jo Over a year ago

@molbdnilo Arrays decay to pointers though when passed to a function. 2-dimensional arrays decay to pointers of pointers and so on.

Jo3Jo Over a year ago

Oh, I see now that &v[0][0] is passed. Really, v by itself should be passed (while having the function accept a pointer to a pointer as in my answer) or else you now have a type of 1D array and you can't index v in this fashion - v[i][j] anymore. Edited the answer.

molbdnilo Over a year ago

No, a 2-D array does not decay into a pointer to a pointer; an array decays into a pointer to its first element. If you have int v[10]10], v decays into a int(*)[10] – that is, &v[0] – a pointer to an array of ten ints. There is no conversion from int(*)[10] into int**. (Which you will notice if you attempt your suggestion.)

Jo3Jo Over a year ago

@molbdnilo Oops, you are right. The function has to know how long are the previous dimensions of the array, so that the compiler knows to how many bytes is equivalent each index of the array.

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C++ error when passing 2D array into function and changing elements' values ("error: invalid types ‘int[int]’ for array subscript")

can someone help? i'm trying to create a function for reading a 2D array and then display it in int main(). i get the following error when running the code: "error: invalid types ‘int[int]’ for array subscript" on the line with cin >> v[i][j].

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Collectives™ on Stack Overflow

can someone help? i'm trying to create a function for reading a 2D array and then display it in int main(). i get the following error when running the code: "error: invalid types ‘int[int]’ for array subscript" on the line with cin >> v[i][j].

1 Answer 1

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