This is the code and here is the image of output , i need these output to be only n's and 0 but i have these numbers for some reason. How to fix that and replace these numbers by 0's?

output

#include <iostream>

using namespace std;

int main()
{
    int n = 9;

    int matrix[n][n];
    int x = 1;
// output array element's value

    for (int i = 0; i < 9; i  )
    {
        for (int j = 0; j < 9; j  )
        {
            matrix[i][0] = x;
            matrix[i][n - 1] = x;
            matrix[i][i] = x;
        }
    }
    for (int i = 0; i < 9; i  )
    {
        for (int j = 0; j < 9; j  )
        {

            std::cout << matrix[i][j] << " ";
        }
        std: cout << "\n";
    }

    return 0;
}

CodePudding user response：

The code does not initialise the values of the matrix, apart from a few places. You need to write zeros everywhere else, otherwise you get whatever data was in the memory beforehand (strictly this is 'undefined behaviour', and the program could do anything, but in practice you will print out whatever happened to be in the memory the program uses before it ran)

CodePudding user response：

i need these output to be only n's and 0

• For 0 to be guaranteed to appear, you need to initialize matrix (or explicitly assign 0 to the rest of the elements). This is simply done with int matrix[n][n]{}; (zero initialization). Reading uninitialized values, like you currently do, makes the program have undefined behavior and printing "garbage" values is one possible outcome.
• For ns to appear, you need to assign the elements the value held by n. You currently assign the value of x to the elements in matrix.
• The inner assigning loop, j, doesn't do anything but repeating the same thing 9 times. Just remove it.

#include <iostream>

int main() {
    constexpr int n = 9;           // int n = 9; makes `matrix` a non-standard VLA

    int matrix[n][n]{};            // zero initialization

    for (int i = 0; i < 9; i  ) {
        matrix[i][0] = n;      // n
        matrix[i][n - 1] = n;  // n
        matrix[i][i] = n;      // n
    }

    for (int i = 0; i < 9; i  ) {
        for (int j = 0; j < 9; j  ) {
            std::cout << matrix[i][j] << ' ';
        }
        std::cout << '\n';
    }
}

Output:

9 0 0 0 0 0 0 0 9 
9 9 0 0 0 0 0 0 9 
9 0 9 0 0 0 0 0 9 
9 0 0 9 0 0 0 0 9 
9 0 0 0 9 0 0 0 9 
9 0 0 0 0 9 0 0 9 
9 0 0 0 0 0 9 0 9 
9 0 0 0 0 0 0 9 9 
9 0 0 0 0 0 0 0 9 

CodePudding user response：

As already pointed out, this is undefined behavior, C doesn't have variable-length arrays, and the program is displaying whatever junk is left in memory. I'd suggest defining the matrix in Globals, outside main.

#include <iostream>

using namespace std;
int matrix[9][9];

int main(){
int n = 9;

//int matrix[n][n];
int x = 1;
// output array element's value




for (int i = 0 ; i < 9 ; i  )
{
    for(int j = 0 ; j < 9 ; j  )
        {
matrix[i][0] = x;
matrix[i][n-1] = x;
matrix[i][i] = x;
    }
}
    for (int i = 0 ; i < 9 ; i  )
{
    for(int j = 0 ; j < 9 ; j  )
    {

    std::cout << matrix[i][j]<< " ";
    }
    std:cout << "\n";
}


    return 0;
    }