Question

I can not understand how this code draws a complete chess board not only 8 squares

Especially : How for-loop it works.

import acm.program.*;
import acm.graphics.*;

public class chbord extends GraphicsProgram {
    /* number of columns */
    private static final int Ncolumns = 8;
    /* number of rows*/
    private static final int Nrows = 8;

    public void run() {
        int sqSize = getHeight() / Nrows;
        for (int i = 0; i < Nrows; i++) {
            for (int j = 0; j < Ncolumns; j++) {
                int x = j * sqSize;
                int y = i * sqSize;

                GRect sq = new GRect(x, y, sqSize, sqSize);
                sq.setFilled(((i + j) % 2) != 0);
                add(sq);
            }
        }
    }
}

Answer 1

The first time the outer loop runs, the inner loop runs completely (8 times). Then the outer loop runs a second time and the inner loop is then run completely once again (another 8 times).

This continues through the eighth row.

So you get 8 rows drawn, but each row is drawn as 8 columns.

Result: all 64 squares are drawn.

Especially : How for-loop it works.

A For-loop is a key part of programming. Here are some explanation articles:

• An article explaining javascript loops. — The syntax may be a bit different but the idea is the same.
• Wikipedia article