OOP Java P1 Wk2 cont.

NOTE:

I used Eclipse for week2, until realized had to use Netbeans to release third week exercises. I transferred codes from Eclipse to Netbeans. It’s recommended to use Netbeans for MOOC.

The 2nd part of the Week 2!

The first part of the Week 2 is all about looping and using basic mathematics. Now this second part of the Week 2 is going to be about using looping and basic mathematics on strings. Plus we will get to learn how to do methods properly!

EXERCISE #37 & #38

In this exercise, it enables us to understand how method works. We create a method then apply to the main method. Method is pretty much a function… well it can be used as a field attribute, but mostly used as a function. Take a look below!

package week2;

import java.util.Scanner;

public class week2_ex37_38{
    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);

        System.out.print("How many? ");
        int i = 0;
        int num = Integer.parseInt(reader.nextLine());
        num = num - 1;

        while( i <= num) {
            printText();
            i++;
        }
    }

    public static void printText(){
        System.out.println("In the beginning there were the swamp, the hoe, and Java");
    }
}

The assignment is to create a method printText() that only prints a string. A very simple and cute method. That is the exercise #37. It isn’t the end of that exercise, I am to include method printText() inside the main(String[] args) method. It shows me that the “inside” method pops out its function in the main() method. It only can be done that way. Class file must have a main method, because it is the entry point to execute functions out. If a package has mutilple class files, can use just one class file’s main method to execute (don’t worry about that now).

In Exercise #38, it wants me to carry out printText() mutiple times at the number of user’s preference. Since main() method is the only executable method, add the while() loop inside main() method. We know how to do loop with an increment, no need to explain how, or go back to previous post. Insert printText() inside the loop in main(). Da-ta! The method is executed.

EXERCISE #30 & #40

This exercise will teach more about methods involving with printing stars and them into shapes! Exciting, isn’t it?

Let’s do a simple programming…

• create a method that prints star and allow user to decide how many stars should be printed

public static void printStars(int amount){
    int i = 0;
    while (i < amount) {
        system.out.print("*");
        i++;
    } system.out.println();
}

public static void main(String[] args) {
    printStars(5);
    printStars(3);
    printStars(9);
}

// Result:
// *****
// ***
// *********

A real simple programming, now let’s do all other bits of this Exercise #39 altogether! I am going to be doing several methods. The list below are methods it wants me to create:

• printing stars into a shape square aka method printSquare(int sideSize)
• printing stars into a rectangle aka method printRectangle(int width, int height)
• printing stars into a left-aligned triangle aka method printTriangle(int size)

public class Printing {

    public static void printStars(int amount) {
        // 39.1
        int i = 0;
        while(i < amount) {
            System.out.print("*");                          
            i++;
        } System.out.println();
    }

    public static void printSquare(int sideSize) {
        // 39.2
        int i = 0;
        while (i < sideSize) {
            printStars(sideSize);
            i++;
        } System.out.println();
    }

    public static void printRectangle(int width, int height) {
        // 39.3
        int i = 0;
        while (i < height) {
            printStars(width);
            i++;
        } System.out.println();
    }

    public static void printTriangle(int size) {
        int i = 1;
        while (i <= size) {
            printStars(i);
            i++;
        }
    }

    public static void main(String[] args) {
        printStars(3);
        System.out.println("\n---");
        printSquare(4);
        System.out.println("\n---");
        printRectangle(5, 6);
        System.out.println("\n---");
        printTriangle(3);
        System.out.println("\n---");
    }

}

Each method does a task, but is not executed until it’s in method main(). I think it’s better to split tasks into methods and then put altogether under one method. This is very basic. We will get a better understanding on “methods” in next post Guessing A Number. Let’s go over to Exercise #40, similar with Exercise #39. I might need to cover one method that is kind of troublesome.

What Exercise #40 asks for:

• create a method printWhitespaces(int size), giving amount of whitespaces
• create a method printTriangle(int size), this time right-aligned
• create a christmas tree using all previous methods

The top is easy to do, just use printStars method from Exercise #39 and replace star with a whitespace “ “. Be sure to take System.out.println(); off!

Now this method printTriangle() right-aligned is where I got stuck for a bit. Oui rusty. Take a look:

public static void printTriangle(int size) {
    int i = 1;
    while(i <= size) {
        printWhitespaces(size - i);
        printStars(i);
        i++;
    }
}

//    *
//   **
//  ***
// ****

How do I make a right-aligned triangle? Earlier, this exercise asked me to create a printWhitespaces(), so that’s a hint. Adding printWhitespaces() method inside this method. Let’s say if size is given a 4 and it goes in both printStars(size) and to printWhitespaces(size). Basically, there’s 4 characters made of either stars, or whitespaces. There will be 4 stars on the bottom for sure, and there won’t be any whitespace. Now about top, we want one star on top, and meaning need three whitespaces. We have no problem printing desired number of star/s, but how do we make whitespaces come first? Place printWhitespaces() first inside the loop. We want three whitespaces and size in this case means 4 characters. 4 characters - 1 star = 3 whitespaces. Ah-ha! printWhitespaces(size-1);

Loop rolls on, size reduce its number, but still subtracting 1 inside printWhitespaces(), giving the right amount of whitespaces needed. printStars() method on the next, creating right-aligned triangle.

xmasTree(int height) Method

The last part of this Exercise #40 wants me to create a xmas tree using all previous methods, and maybe small tweaks. Don’t worry about xmas trunk or the bottom part. The challenge is making right-aligned triangle and left-aligned triangle go together…. or is it?

Using printWhitespaces() and printStars() again and this time height as an argument. printWhitespaces() placed first in the loop and subtracts by 1 same as with right-aligned triangle method. Now next step is the tricky part. It might look like left-aligned triangle and right-aligned triangle is thrown together to make a xmas tree. Actually it isn’t!

public static void xmasTree(int height) {
int i = 1;
    while (i <= height) {
        printWhitespaces(height - i);
        printStars(2 * i - 1);
        i++;
    }
}

Insert printStars() beneath printWhitepsaces(). If height is appointed to a number 4, for xmas tree, we want one star on the top always. On the second row, we wouldn’t want two stars because top star wouldn’t be in the middle above two stars. We will need three stars on the second row.

• each row should have odd numbers of stars
• able to manipuate the number of stars in printStars() through each loop
• printWhitespaces()’s true function is to make a slope angle
• printStars()’s true function is to print a number of stars

The trick

we used modulo % 2 = 0 to find even number. Well we could multiply by 2 to make any number an even number too. 2 is a magical number! But if we substract the product by 1, it will always be an odd number. We will use i to manipulate printStars().

if height = 4

printStars(2 * i - 1 ); = 2 * 1 - 1 = 1 star
printStars(2 * i - 1 ); = 2 * 2 - 1 = 3 stars
printStars(2 * i - 1 ); = 2 * 3 - 1 = 5 stars
printStars(2 * i - 1 ); = 2 * 4 - 1 = 7 stars

See how that works?

If printed, we get a triangle. Stumped, not a xmas tree. Time to make a stump!

I at first thought stump was going to be easy to create, since it’s just a square or rectangle… oops! Forgot about whitespaces, how to keep stump in the middle. What if user decided to give xmasTree(height) a height of 10 or 20? Let’s try again.

We want to manipualate printWhitespaces() based on xmasTree(). Stump only needed to be 2 stars tall. We could use i again, because the first while() loop is completed, and then second while() loop may continue. So set i to 2 and have it decrements down to 0. How do we manipulate printWhitespaces() according to xmasTree()’s height?

i = 2;      
while (--i >= 0) {
    printWhitespaces(height - 2);
    printStars(3);
}

Yep! Using height to correlate with printWhitespaces(). Stump will always be aligned in the middle based on it height. All small tasks are turned into methods, and thrown altogether into main() method to make it work. That’s fantastic!

public static void xmasTree(int height) {
    int i = 1;
    while (i <= height) {
        printWhitespaces(height - i);
        printStars(2 * i - 1);
        i++;
    }

    i = 2;      
    while (--i >= 0) {
    printWhitespaces(height - 2);
    printStars(3);
    }
}

public static void main(String[] args) {
    System.out.println("---");
    xmasTree(4);
    System.out.println("---");
    xmasTree(10);
}