Iteration / Loops

Iteration / Loops#

Key Ideas#

Do Loop
While Loop
For Loop
For Each Loop
Loop Pattern for Menus / Programming Structure
Loops for grids

Readings#

https://books.trinket.io/thinkjava2/chapter6.html

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html

Lecture#

Definition

Iteration / Loops: the repetition of a sequence of computer instructions a specified number of times or until a condition is met. Merriam Webster

Java Loops#

Iteration has several purposes:

Reusability - loops allow us to repeat code as often as we wish in the form that we can write it once and run it many times.
Loops allow us to traverse the items in a list, array, or data structure.

Types Of Loops#

Java has four main types of loops

Do Loop - This loop will always run once
While Loop - This loop runs while a condition is true
For Loop - This loop runs a specific amount of times
For Each Loop - This loop is a form of the for-loop. It is set up to easily iteration of each item in an array, list, or data structure.

Example Basic Loops#

/*
Loop Lecture Code
Programmer: James Goudy
 */



public class Loops {

    public static void main(String[] args) {
      
        int cntr = 0;
        
        //Do Loop This loop will always run once       
        do
        {
            System.out.println(cntr + " do loop");
            
            //We have to increment a counter 
            //otherwise our while statement will never
            //go to false and we will be in a endless loop.
            cntr++;
        }while(cntr < 2000);
        
        
        //while loop - This loop will always check for true before running
        cntr = 0;
        
        while (cntr < 2000)
        {
            System.out.println(cntr + " while loop");
            cntr++;
            //We have to increment a counter 
            //otherwise our while statement will never
            //go to false and we will be in a endless loop.
        }
        
        //for statments are excellent if we have 
        //a known number of cycles to loop.
        //Also we create a cntr, check for true and increment
        //the counter all on one line.
        for(int cntr2 = 0; cntr2 < 2000; cntr2++)
        {
            System.out.println(cntr2 + " for loop");        
        }
             
    }
    
}

Loop To Control Menu#

Note this is using a while-loop and a switch statement.
This is a programming style that fits 90% of your programs. Note that the functions are not calling the menu() function.

/* Switch / 
Program: Menu Lecture
Programmer: Jim Goudy
This is a demo program to demonstrate a design pattern for creating a menu system in the console environment. 
:)----  Bob Ross Coding ---- :)

 */
package menu_lecture;

import java.util.Scanner;

public class Menu_Lecture {

    //Happy Function 1
    static void HappyFunction1() {
        System.out.println("\n\nThis is happy function 1");
    }
    
    //Happy Function 2
    static void HappyFunction2() {
        System.out.println("\n\nThis is happy function 2");
    }
    
    //Happy Function 3
    static void HappyFunction3() {
        System.out.println("\n\nThis is happy function 3");
    }

    //This function creates a menu system.
    static void menu() {     
        //variables
        int choice = 0;     //hold user choice

        //Scanner for user input
        Scanner myScan = new Scanner(System.in);

        //display the user choices
        System.out.println("\nMENU\n"
                + "\n1. Happy Function 1"
                + "\n2. Happy Function 2"
                + "\n3. Happy Function 3"
                + "\nPlease choose a function 1,2 or 3");
        choice = Integer.parseInt(myScan.nextLine());

        //take the choice stored in choice and use it in the switch
        switch (choice) {
            case 1:
                HappyFunction1();
                break;
            case 2:
                HappyFunction2();
                break;
            case 3:
                HappyFunction3();
                break;
            default:
                //default is used if the user entered any 
                //number that was note 1,2 or 3
                System.out.println("\nThat wasn't a choice");
        }
    }

    public static void main(String[] args) {

        //variables
        String quit = "n";      

        //Scanner for user input
        Scanner myScan = new Scanner(System.in);

        //quit has to be initialized with the value of n
        //so the loop will run at least once. Putting the menu
        //call in a while statement will allow the user to run
        //the program as many times as they wish.
        while (quit.equals("n")) {
            
            //call the menu
            menu();
            //prompt the user if they want to quit
            System.out.println("\nWould you like to quit -  y or n");
            quit = myScan.nextLine().toLowerCase();
        }

        //Inform the user the program is done executing
        System.out.println("Good Bye");
    }
}

Loops For Printing A Grid#

/*
Program: Loop Lecture Grid
Programmer: James Goudy
This program demonstrates how to write a grid
using loops.  The important note is that the 
counters in the loops act as coordinate to the row and columns
of the x's that are being printed out.

 */
package loops_lecture_grid;

public class Loops_Lecture_Grid {

    public static void main(String[] args) {

        //Variables
        String xx = "x";

        int Colcntr = 0;    //This is our column counter
        int ColStop = 5;    //This is the actual number of 
        //columns for the grid

        int Rowcntr = 0;    //This is our row counter
        int RowStop = 5;    //This is the actual number of 
        //rows for the grid  

        //loop to control the number of rows
        while (Rowcntr < RowStop) {
            //loopt to control the number of columns
            while (Colcntr < ColStop) {
                //Note the print. If a println is used
                //all of the starts will print down / "vertical"
                System.out.print("x");

                //Increment the column counter to the next column
                Colcntr++;
            }

            //this represents the end of the row
            //the cursor needs to be put on the next row
            //so a println will be used.
            System.out.println("");

            //Next the Colcntr needs to be reset to zero
            Colcntr = 0;

            //Advance the rowcnter to the next row
            Rowcntr++;
        }

    }

}

End Of Topic