How to print a few 2d arrays with a fuction?

Question

I'm using C and I'd like to print these matrices with the printMatrix function. It's fine when I print A and B. But, I don't know how matrix C is printed. can anyone help?

#include <stdio.h>
void printMatrix(const int A[][3],const int col);

void main()
{
    int A[2][3] = { {1,2,3},{-1,5,-9} };
    int B[2][3] = { {1,2,3},{1,0,4} };
    int C[3][2] = { {-2,1},{1,0},{5,4} };

    printf("A\n");
    printMatrix(A,2,3);
    printf("B\n"); printMatrix(B,2,3);
    printf("\n");
    printf("C\n"); printMatrix(C,3,2);
}


void printMatrix(const int A[][3], const int row,const int col)
{
    int i, j;
    for (i = 0; i < row; i++) {
        for (j = 0; j < col; j++) {
            printf("%d  ", A[i][j]);
        }
        printf("\n");
    }
}

Lundin · Accepted Answer · 2021-03-17T09:50:51.150

You have two problems both related to the wrong array type. They array declaration const int A[][3] when part of a function decays into a pointer to the first element. In this case the first element would be an array of type const int[3] and a pointer to such an element is written as const int(*)[3], so that's what A gets implicitly replaced with.

Now as it happens, C has stricter typing for pointers than it has for regular data. This means that you cannot pass a pointer to an array of a different size than [3], nor can you pass a pointer to an array which doesn't have const int elements.

The size problem can be solved by using a pointer to variable-length array (VLA) instead. The const problem can't be easily solved¹⁾, unfortunately, so the easiest solution is to simply drop const correctness. You could change your function to this and it will work:

void printMatrix(size_t row, size_t col, int A[row][col]);

(size_t is a more correct type than int to use when dealing with array sizes.)

¹⁾ Advanced topic: There are solutions that give both const correctness and type safety, but they are cumbersome. See Const correctness for array pointers?, the _Generic based macro in the answer by chux in particular.

Thanks for your answer, But it doesn't work yet, the error is Parameter not available.(int A[row][col]) , Constant expression is required, Cannot allocate an array of constant size 0. — XXX TELTACIO, Mar 18 '21 at 01:21
@XXXTELTACIO You must use a standard C compiler. It seems like you are using an obsolete non-standard one from the 1990s, such as Visual Studio. Or maybe you compiled the code as C++ by accident. — Lundin, Mar 18 '21 at 09:10

tstanisl · Answer 2 · 2021-03-17T09:55:04.377

The problem is the type of argument A in

void printMatrix(const int A[][3], const int row,const int col);

The second dimension is expected to be 3, but the type of C array is int[3][2]. The second dimension is 2. The type of the argument does not match thus it will not work.

I suggest using Variable Length Arrays:

void printMatrix(const int row,const int col, const int A[row][col])
{
    int i, j;
    for (i = 0; i < row; i++) {
        for (j = 0; j < col; j++) {
            printf("%d  ", A[i][j]);
        }
        printf("\n");
    }
}

now you can print any 2d int array as:

printMatrix(3,2,C);

Edit

As noted in the comments the code will produce a warning. The simple solution is dropping const from A argument:

void printMatrix(const int row,const int col, int A[row][col]);

Alternatively, one can add const qualifiers to arrays A,B, and C.

    const int A[2][3] = { {1,2,3},{-1,5,-9} };
    const int B[2][3] = { {1,2,3},{1,0,4} };
    const int C[3][2] = { {-2,1},{1,0},{5,4} };

This only solves one part of the problem and won't compile. I posted a complete answer. — Lundin, Mar 17 '21 at 09:30
@Lundin, Indeed. It produces warning in pedantic mode. Interesting finding, thanks. — tstanisl, Mar 17 '21 at 09:35
The compiler must produce a diagnostic in any mode, since assigning an array-pointer-to-int to an array-pointer-to-const-int is a constraint violation. — Lundin, Mar 17 '21 at 09:45

How to print a few 2d arrays with a fuction?

2 Answers2

Edit