Which line of code should replace the missing code to set the main diagonal elements to 1?

public class Diagonal {
    public static void setDiagonal(int[][] matrix) {
        for (int i = 0; i < matrix.length; i++) {
            /* missing code */
        }
    }
    public static void main(String[] args) {
        int[][] square = new int[3][3];
        setDiagonal(square);
        System.out.println(square[1][1]);
    }
}