Output of C Program | Set 23

Predict the output of following C Program.

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#include <stdio.h>
#define R 4
#define C 4
  
void modifyMatrix(int mat[][C])
{
   mat++;
   mat[1][1] = 100;
   mat++;
   mat[1][1] = 200;
}
  
void printMatrix(int mat[][C])
{
    int i, j;
    for (i = 0; i < R; i++)
    {
        for (j = 0; j < C; j++)
            printf("%3d ", mat[i][j]);
        printf("\n");
    }
}
  
int main()
{
    int mat[R][C] = { {1, 2, 3, 4},
        {5, 6, 7, 8},
        {9, 10, 11, 12},
        {13, 14, 15, 16}
    };
    printf("Original Matrix \n");
    printMatrix(mat);
  
    modifyMatrix(mat);
  
    printf("Matrix after modification \n");
    printMatrix(mat);
  
    return 0;
}

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Output: The program compiles fine and produces following output:

Original Matrix
  1   2   3   4
  5   6   7   8
  9  10  11  12
 13  14  15  16
Matrix after modification
  1   2   3   4
  5   6   7   8
  9 100  11  12
 13 200  15  16

At first look, the line “mat++;” in modifyMatrix() seems invalid. But this is a valid C line as array parameters are always pointers (see this and this for details). In modifyMatrix(), mat is just a pointer that points to block of size C*sizeof(int). So following function prototype is same as “void modifyMatrix(int mat[][C])”

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void modifyMatrix(int (*mat)[C]);

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When we do mat++, mat starts pointing to next row, and mat[1][1] starts referring to value 10. mat[1][1] (value 10) is changed to 100 by the statement “mat[1][1] = 100;”. mat is again incremented and mat[1][1] (now value 14) is changed to 200 by next couple of statements in modifyMatrix().

The line “mat[1][1] = 100;” is valid as pointer arithmetic and array indexing are equivalent in C.

On a side note, we can’t do mat++ in main() as mat is 2 D array in main(), not a pointer.

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