Here is a program to convert the diagonal elements to zero.

It cheaks if the element i==j and converts it to zero. And it cheaks if the element i+j+1 == to the number of rows and converts to zero.

Here is the code.

#include<stdio.h>
#include<math.h>
int main(){
    printf("C program to initialize/fill all the\n diagonal elements of a matrix with zero and print.\n");
    int r,c;
    printf("Enter the number of rows in the matrix such that rows=columns:");
    scanf("%d",&r);
    printf("Enter the number of columns in the matrix:");
    scanf("%d",&c);
    int arr[r][c];
    printf("Enter the elements in the array\n");
    for(int i=0;i<r;i++){
        for(int j=0;j<c;j++){
            printf("Row %d column %d:",i,j);
            scanf("%d",&arr[i][j]);
        }
    }
    for(int i=0;i<r;i++){
        for(int j=0;j<c;j++){
            printf("%d\t",arr[i][j]);
        }
        printf("\n");
    }
    for(int i=0;i<r;i++){
        for(int j=0;j<c;j++){
            if(i==j||(i+j+1)==r){
                arr[i][j]=0;
            }
        }
    }
    printf("The new matrix is :\n");
    for(int i=0;i<r;i++){
        for(int j=0;j<c;j++){
            printf("%d\t",arr[i][j]);
        }
        printf("\n");
    }
    return 0;
}

The main part of the code is.

 for(int i=0;i<r;i++){
        for(int j=0;j<c;j++){
            if(i==j||(i+j+1)==r){
                arr[i][j]=0;
            }
        }
    }

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Thankyou...