Java Beginner: Lesson 6

do while loop

Same as the while loop, shown in the lesson 5, there are other loops which make possible to repeat an action or a set of actions. In this lesson we will cover the do while loop.

In the main method which in the previous lesson was as follow:

Example code: public static void main(String[] args) { // TODO code application logic here Scanner scanner = new Scanner(System.in); double nr1=0; double nr2=0; String operation; double result =0; while( true ){ System.out.println("**********NEW OPERATION*********"); System.out.println("Give the first number: "); nr1= scanner.nextDouble(); System.out.println("Give the operation: "); operation = scanner.next(); System.out.println("Give the second number: "); nr2= scanner.nextDouble(); if(operation.equalsIgnoreCase("+")){ result = nr1+nr2; }else if(operation.equalsIgnoreCase("-")){ result = nr1-nr2; }else if(operation.equalsIgnoreCase("*")){ result = nr1*nr2; }else if(operation.equalsIgnoreCase("/")){ if(nr2!=0){ result = nr1/nr2; }else{ System.out.println("Can not devide by 0\n"); return; } } System.out.println("The result of nr1 and nr2 is "+ result); } }

We will make the changes as shown bellow.

public static void main(String[] args) { // TODO code application logic here Scanner scanner = new Scanner(System.in); double nr1=0; double nr2=0; String operation; double result =0; do{ System.out.println("**********NEW OPERATION*********"); System.out.println("Give the first number: "); nr1= scanner.nextDouble(); System.out.println("Give the operation: "); operation = scanner.next(); System.out.println("Give the second number: "); nr2= scanner.nextDouble(); if(operation.equalsIgnoreCase("+")){ result = nr1+nr2; }else if(operation.equalsIgnoreCase("-")){ result = nr1-nr2; }else if(operation.equalsIgnoreCase("*")){ result = nr1*nr2; }else if(operation.equalsIgnoreCase("/")){ if(nr2!=0){ result = nr1/nr2; }else{ System.out.println("Can not devide by 0\n"); return; } } System.out.println("The result of nr1 and nr2 is "+ result); }while( true ); }

In this case, with do while loop, we will have the same result with while loop. The difference would be if insted of true we would put false in the while condition. In the while case if we would put false within the rounded brackets the code inside the currly brackets we would get a compilatin errore because wouldn't have been executed at all because the condition is tested before the start of the execution, meanwhile in the case of do while loop even though we put false in the while condition the code gets executed one time and than exits the loop.

In the figure 1 bellow is shown the case when within the while condition is puted true so we can repeat the action infinite times.

Figure 1: do while true

In the figure 2 is shown the execution when in the while condition is false, which means that the code between the do and while keyword will be executed only once.

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