C-Matrix-Elements of Main Diagonal and Anti Diagonal Are Same

Question

Hello! My task is to check if the elements of main and anti diagonal of quadratic matrix are same. If this condition is satisfied 'Yes' should be printed; 'No' in the opposite case. For example, the condition is satisfied for matrix 3x3 with all rows equal to: 1 2 3. My code worked for all the matrices except the one with negative numbers and the one with dimension 5x5 with 4 rows: 1 2 3 4 5 and 5th row: 1 2 3 4 6, and I don't know why. I am beginner. I hope you could help.

      int i,j,M,b=0,m[100][100];
      do{
      printf("Dimension of matrix: ");
      scanf("%d", &M);
      if (M<0 || M>100)
      printf("Wrong input!\n");
      }while (M<0 || M>100);
      printf("Enter the elements: ");
      
      for (i=0;i<M;i++){
      for (j=0;j<M;j++){
      scanf("%d", &m[i][j]);
       }
        }  
      for (i=0;i<M;i++){
      for (j=0;j<M;j++){
      if(m[i][j]!=m[M-(i+1)][j])
      b=0;
      else if ((i==j || i+j==M-1) && (m[i][j]==m[M-(i+1)][j]))
      b=1;
      else b=0;
 }
 }
     if(b==1)
     printf("YES");
     else printf("NO");
     }

Hi Michael and welcome to StackOverflow. Please take the tour, read How to Ask and provide a minimal reproducible example. — Yunnosch
Please apply indentation, it makes structure analysis so much easier. — Yunnosch
Perhaps you need to encapsulate the code into seperate functions to make it easier to read and debug. Something like setMatrix() with the nested for loops and the read from cin, and checkMatrix() to see whether diagonals are equal, maybe even a getDiagonal() and getAntiDiagonal(). — jackw11111

hasref hasref · Accepted Answer · 2020-12-24T00:51:11

As already pointed out by @Bob__, there are a couple of issues with the code:

first, in C, array indices start from 0.
Furthermore, in your inner loop, over j, you only read a single entry but then you try to compare the diagonal entry with the anti-diagonal entry which you haven't read in yet. You probably want to read in the entire matrix first and then, in a separate loop, check whether the diagonal and anti-diagonal are the same.

for (i=0; i < M; i++){
    for (j = 0; j< M; j++){
      scanf("%d", &m[i][j]);
    }
}

Given that array indices in C start at 0, there is a missing pair of parantheses in the matrix index calculation: the index for the anti-diagonal entry should be m[ M - (i+1) ][j]. Once you have read in the entire matrix, something like this ought to work:

for(i = 0; i < M; i++){
    if( m[i][i] != m[M - (i+1)][i] ){
        b = 0;
    }
}

C-Matrix-Elements of Main Diagonal and Anti Diagonal Are Same

2 Answers