Help me in can't do sum of diagonal elements of a matrix

Question

C language
https://code.sololearn.com/cG6I4MVXbPHh/?ref=app

Answer 1

//read comments and try to correct mistakes..
// hope it helps..

#include <stdio.h>

int main() {
int N ,M;
int mat[N][M],i,j; //N,M have undefined still so first take values then declare array
scanf("N=%d and M=%d",&N,&M); //dont put anything except farmat specifiers in scanf.remove N= and ..

for(i=0;i<N;i++){
for(j=0;j<M;j++){
printf ("enter matrix [%d][%d] \t",i,j);
scanf("%d %d",mat[N][M]); //must be mat[i][j] , i,j. not N,M
}
}
int sum=0;
for(i=0;i<N;i++){
for(j=0;j<M;j++){
if(i==j){
sum=sum+mat[N][M];// here also mat[i][j], not mat[N][M]
}
else{ // dont need else here just output after loop next one.
printf("the req sum of a diagonal elemensts of a matrix is %d",sum);
}
}
}
return 0;
}