Matrix-Swapped diagonals

#include <stdio.h>

int main(){
    int matt[100][100];
    int n,i,j,k;
    scanf("%d", &n);
    for(i=0; i<n; i++){
        for(j=0; j<n; j++){
            scanf("%d", &matt[i][j]);
        }
    }
    //Main diagonal
    int main_diag[100];
    for(i=0; i<n; i++){
        for(j=0; j<n; j++){
            if(i==j)
            main_diag[j] = matt[i][j];
        }
    }

    //Opposite diagonal
    int opposite_diag[100];
     for(i=0; i<n; i++){
        for(j=0; j<n; j++){
            if(i+j == n-1)
            opposite_diag[j] = matt[i][j];
        }
    }

    //Printing the original matrix before making any changes...
    printf("The original matrix: \n");
    for(i=0; i<n; i++){
        for(j=0; j<n; j++){
            printf("%d\t", matt[i][j]);
        }
        printf("\n");
    }

    //Adding the opposite diagonal on the main diagonals position
    for(i=0; i<n; i++){
        for(j=0, k=n-1; j<n, k>=0; j++, k--){
        if(i==j)//Main diagonal
            matt[i][j] = opposite_diag[k];
        }
    }

    //Adding the main diagonal on the opposite diagonals position
    for(i=0; i<n; i++){
        for(j=0, k=n-1; j<n, k>=0; j++, k--){
        if(i+j == n-1)//Opposite diagonal
            matt[i][j] = main_diag[k];

        }
    }

    printf("\nThe main diagonal elements are: \n");
    for(i=0; i<n; i++){
        printf("%d\t", main_diag[i]);
    }

    printf("\n\nThe opposite diagonal elements are: \n");
    for(i=n-1; i>=0; i--){
        printf("%d\t", opposite_diag[i]);
    }

    //Printing the new matrix
    printf("\n\nThe final matrix with swapped diagonals: \n");
    for(i=0; i<n; i++){
        for(j=0; j<n; j++){
            printf("%d\t", matt[i][j]);
        }
        printf("\n");
    }

    return 0;
}