CODEWITHSUNDEEP
javafor-loop
Tarek Saied
Tarek Saied

Reputation: 6616

help to understand this for loop code

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);
            }
        }
    }
}

Upvotes: 1

Views: 260

Answers (6)

scott
scott

Reputation: 974

the outer and inner loop will create an NRows*NColumns grid and the modulus operator (%) will color every other square since there are only two values with MOD 2.

Upvotes: 0

Larry K
Larry K

Reputation: 49104

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:

Upvotes: 1

Patrick87
Patrick87

Reputation: 28292

This is an example of a nested loop - one loop inside another. The outer loop causes the code it contains to be executed for values of i between 0 and nrows - 1, inclusive. The inner loop causes the code it contains to be executed for values of j between 0 and ncols - 1, inclusive. Since the code in the inner loop is executed for n values of i, and for each of these values of i, m values of j, the total number of executions is nm.

Upvotes: 0

Kyle
Kyle

Reputation: 4278

While both the posters above are correct consider that the second loop runs 8 times for each one time the first loop runs. To demonstrate put two different System.out.println statements in each loop and view the order that they are printed in.

Upvotes: 1

Brian Kelly
Brian Kelly

Reputation: 19295

It's because there are actually two for loops, not just one. This is called "nesting" loops, and it's the typical approach used to create row x column grids such as a chessboard.

Upvotes: 0

Bohemian
Bohemian

Reputation: 424983

There's a loop within a loop:

for (int i = 0; i < Nrows; i++) {
    for (int j = 0; j < Ncolumns; j++)

Each loop has 8 iterations:

/* number of columns */
private static final int Ncolumns = 8;
/* number of rows*/
private static final int Nrows = 8;

That makes 64 total iterations (8 x 8) through the inner loop - one for each chess square.

Upvotes: 2

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