What's wrong with the matrix?

Question

What's wrong with this code? My task is: Create a square matrix of integers with a size of 9x9. Fill the matrix with random numbers. Display the main and side diagonal symmetrically with respect to the vertical axis. The example of expected result is here: matrix

Matrix :

20      20      76      65      93      76      16      2       85

6       87      78      43      48      81      71      90      38

10      12      35      77      48      88      24      53      7 

12      66      51      35      74      7       30      22      49

58      14      71      46      68      68      10      81      51

98      16      74      47      64      25      17      30      37

2       44      44      74      34      54      86      73      28

85      4       57      75      18      28      51      76      2 

35      17      53      76      15      91      83      85      72

The main and side diagonal:

85      20      76      65      93      76      16      2       20

6       90      78      43      48      81      71      87      38

10      12      24      77      48      88      35      53      7 

12      66      51      7       74      35      30      22      49

58      14      71      46      68      68      10      81      51

98      16      74      25      64      47      17      30      37

2       44      86      74      34      54      44      73      28

85      76      57      75      18      29      51      4       2 

72      17      53      76      15      91      83      85      35

But in fact the program prints only the main matrix with random numbers and after that stops.

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <Windows.h>
int main()
{
    int a = 9;
    int matrix[a][a];
    int temp;
    int i, j, n;
    srand((unsigned)time(NULL));

    printf("Matrix : \n\n");
    for (i = 0; i < a; ++i) {
        for (j = 0; j < a; ++j) {
            matrix[i][j] = rand() % 100;
            printf("%d\t", matrix[i][j]);
        }
        printf("\n\n");
    }
    printf("The main and side diagonal:\n\n");
    for (i = 0; i < a; ++i) {
        temp = matrix[i][i];
        matrix[i][i] = matrix[i][(a - 1) - i];
        matrix[i][(a - 1) - i] = temp;
    }
    for (i = 0; i < a; ++i) {
        for (j = 0; j < a; ++j) {
            printf("Result:", matrix[i][j]);
            printf("\n\n\n");
            system("pause");
            return 0;
        }
    }
}

Answer 1

Use functions. You print the matrix twice; you should have a function to print the matrix which you call twice.

With such a function, you'd not make the mistakes in the tail end of your code. For example, you could use this:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

static void print_matrix(const char *tag, int size, int matrix[size][size])
{
    printf("%s (%dx%d):\n\n", tag, size, size);
    for (int i = 0; i < size; ++i)
    {
        const char *pad = "";
        for (int j = 0; j < size; ++j)
        {
            printf("%s%-2d", pad, matrix[i][j]);
            pad = "      ";
        }
        printf("\n\n");
    }
}

int main(int argc, char **argv)
{
    unsigned seed = time(0);
    int a = 9;
    int matrix[a][a];

    if (argc == 2)
        seed = atoi(argv[1]);
    srand(seed);
    printf("Seed: %u\n", seed);

    for (int i = 0; i < a; ++i)
    {
        for (int j = 0; j < a; ++j)
            matrix[i][j] = rand() % 100;
    }

    print_matrix("Matrix", a, matrix);

    for (int i = 0, j = a - 1; i < a; ++i, --j)
    {
        int temp = matrix[i][i];
        matrix[i][i] = matrix[i][j];
        matrix[i][j] = temp;
    }

    print_matrix("The main and side diagonal", a, matrix);

    return 0;
}

The code reports the seed it uses; that allows you to reproduce any run by specifying the seed to use as a command line argument.

Example output:

Seed: 1511470282
Matrix (9x9):

11      39      3       88      98      63      75      81      76

93      9       60      22      45      50      46      58      65

13      99      25      43      14      57      44      70      65

30      57      55      0       37      84      47      49      40

60      28      46      1       96      78      33      20      9 

93      61      11      38      84      16      91      26      15

43      85      66      72      85      39      96      45      45

45      25      33      3       78      90      61      65      62

88      84      56      34      74      8       78      57      74

The main and side diagonal (9x9):

76      39      3       88      98      63      75      81      11

93      58      60      22      45      50      46      9       65

13      99      44      43      14      57      25      70      65

30      57      55      84      37      0       47      49      40

60      28      46      1       96      78      33      20      9 

93      61      11      16      84      38      91      26      15

43      85      96      72      85      39      66      45      45

45      65      33      3       78      90      61      25      62

74      84      56      34      74      8       78      57      88

The swapping process, in case it isn't obvious, swaps the first and last elements of the first row, the second and last but one element in the second row, and so on, forming an X of swapped elements.

Answer 2

You are returning where you are not supposed to. (in middle of the calculation). You should return after you end up working on the for loops.

for (i = 0; i < a; ++i) {
        for (j = 0; j < a; ++j) {
            printf("Result:", matrix[i][j]); <--- Not printing element
            printf("\n\n\n");
            system("pause");
            return 0; <-----
        }
    }

It should be

for (i = 0; i < a; ++i) {
        for (j = 0; j < a; ++j) {
            printf("Result: %d ", matrix[i][j]); <----You forgot the
                                                //format specifier
            printf("\n\n\n");
            system("pause");

        }
    }
  return 0;<-----

Readability is hampered when the indentation is like this. You implemented wrong logic out of it.

---

• OP asks that it stops after printing "Result" that is because you forgot to put the format specifier in the code. That's why none of the element is printed.

---

Op wants to print the main and side diagonal symmetrically with respect to the vertical axis.

Now this is everything to with the print part.

Now we have to find a way that will let us distinguish which one is diagonal element and which one is not.

Suprisingly the answer should be known to someone who is writing the previous swapping logic. (Though it is not clear why OP swapped it).

Now all element matrix[p][q] will be from either diagonal if p=q or p+q=a-1. (Note that matrix is a square matrix).

---

But OP meant to print the matrix

---

for (i = 0; i < a; ++i) {
        if( i == 0) printf("The main and side diagonal : \n");
        for (j = 0; j < a; ++j) {
            printf("%d\t", matrix[i][j]);
          }
        printf("\n");
    }
}