How to do it…

We first define a matrix in R using the following matrix() function:

mat= matrix(1:25, 5,5)

Now, we use the following code to output only those elements where I = J:

for (i in 1:5){
  for (j in 1:5){
    if (i ==j){
      print(mat[i,j])
    }
   }
}

The if statement is nested inside two for loop statements. As we have a matrix, we have to use two for loops instead of just one. The output of the matrix would be values such as 1, 7, 13, and 19.