function that reads a 2d array in c++

Question

I'm trying to write a function that reads a 2D array of integers where the user is the one who enters the size of the array (it shouldn't be defined before ) . I tried this but it's not working. I appreciate your help!

line 6 is the declaration of the function this is my code:

#include <iostream>

using namespace std;

void Fill_table(int mat[][], int s) {
    for (int i = 0; i<s; i++) {
        for (int j = 0; j<s; j++) {
            cout << "mat[" << i << "][" << j << "]:" << endl;
            cin >> mat[i][j];
        }
    }
}

int main()
{
    int n;

    cout << "Enter the size:  ";
    cin >> n;

    int a[n][n];
    Fill_table(a, n);
    return 0;
}

Does this answer your question? [Passing 2-D array as argument](https://stackoverflow.com/questions/12990568/passing-2-d-array-as-argument) — NutCracker, Jan 23 '20 at 18:47
In the future, it's much better to paste the error as text instead of an image. There's a lot missing. But, your problem should be solved by the above comment. — Lou Franco, Jan 23 '20 at 18:51
Because your size is not known at compile time -- look at this: https://stackoverflow.com/questions/936687/how-do-i-declare-a-2d-array-in-c-using-new — Lou Franco, Jan 23 '20 at 18:52
Another alternative is to use [a simple matrix class.](https://stackoverflow.com/a/2076668/4581301) — user4581301, Jan 23 '20 at 18:54

anastaciu · Answer 1 · 2020-01-23T19:18:39.547

1

You cannot set the size of a multidimensional array at runtime. The error

error: declaration of ‘mat’ as multidimensional array must have bounds for all dimensions except the first void Fill_table(int mat[][], int s)

tells you exactly that.

Consider using C++ data structure instead, in this case vector<vector<int>>, you have a somewhat simple how-to guide in Creating a Matrix using 2D vector in C++ – Vector of Vectors.

edited Jan 23 '20 at 19:18

answered Jan 23 '20 at 18:59

anastaciu

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Note that what's being called a 2D `vector` is actually a `vector` of `vector`s and can be a surprisingly slow solution due to extra pointer chasing and lack of contiguity between the rows. It's not so bad with a large matrix, but with a small matrix, the inability to take full advantage of caching can be murderous. [Here's an alternative that uses one `vector` and a bit of indexing math.](https://stackoverflow.com/a/2076668/4581301) – user4581301 Jan 23 '20 at 21:03

score 1 · Answer 2 · answered Jan 23 '20 at 18:59

Just to give you a simple alternative. Instead of declaring a 2D array with [][], you could use [n*n] and then subscript with i*s + j. The code would be

#include <iostream>

using namespace std;

void Fill_table(int* mat, int s) {
    for (int i = 0; i<s; i++) {
        for (int j = 0; j<s; j++) {
            cout << "mat[" << i << "][" << j << "]:" << endl;
            cin >> mat[i*s+j];
        }
    }
}

int main()
{
    int n;

    cout << "Enter the size:  ";
    cin >> n;

    int a[n * n];
    Fill_table(a, n);
    return 0;
}

1D arrays can be passed as a pointer to a function. You also need to pass the size (as you did). For 2D arrays, you need to know the size at compile time.

Caveats: `int a[n * n];` is a non-Standard Variable Length Array and may not be supported by your compiler. If it is supported, you need to restrict the values of `n` the user can provide since `n*n` gets big fast. An `n` of as little as 500 could exhaust the available Automatic storage on a desktop PC. — user4581301, Jan 23 '20 at 21:02

function that reads a 2d array in c++

2 Answers2