CODEWITHSUNDEEP
javaloopswhile-loopcomparedo-while
IAmTheWalrus
IAmTheWalrus

Reputation: 23

Difference in output between similar while loop and do while loop?

For my Java class, I was asked to create a while loop and then convert it to a do-while loop and then compare the output. This is the question:

  1. Replace the while loop in question 4 with a do while loop. What is the possible difference between the output from question 4 and this question?

I can't find a difference in the output, possible or not. Here is the code and output for both below.

while loop

package fordemo;
import java.util.Scanner;

public class ForDemo {


public static void main(String[] args) {

    System.out.println("Input a number:");
    Scanner user_input = new Scanner (System.in);
    int number = user_input.nextInt();
    int n = -1;
    while (n < number){
        n=n+2;
        System.out.print(n + " ");

    }
  }
}

run:

Input a number:
15
1 3 5 7 9 11 13 15 BUILD SUCCESSFUL (total time: 1 second)

do-while loop

package fordemo;
import java.util.Scanner;

public class ForDemo {


public static void main(String[] args) {

    Scanner user_input = new Scanner (System.in);
    System.out.println("Input a number:");
    int number = user_input.nextInt();
    int n = -1; 
    do {
        n+=2;
        System.out.print(n + " ");
    }
    while (n < number);            

  }
}

run:

Input a number:
11
1 3 5 7 9 11 BUILD SUCCESSFUL (total time: 1 second)

Upvotes: 0

Views: 676

Answers (5)

paxdiablo
paxdiablo

Reputation: 882566

The difference between while (condition) {something} and do {something} while (condition) is that the latter always executes at least once.

The former checks the condition before the iteration, the latter checks it afterwards.

If you enter -1 or less, the former will give you nothing and the latter will give you 1.

In its simplest form, you can look at the difference between the two lines:

while (false) { System.out.println("x"); }
do { System.out.println("x"); } while (false);

The former complains bitterly about unreachable code because it never enters the loop. The latter does not complain because it runs the loop once.

Upvotes: 5

Rustam
Rustam

Reputation: 6515

first test the condition then execute the body

  while (n < number){   // Top tested loop or (pre test loop)
            n=n+2;
            System.out.print(n + " ");

        }

first execute the body of the loop and then test the condition;

do {
        n+=2;
        System.out.print(n + " ");
    }
    while (n < number);   // Bottom tested loop (post test loop)

Upvotes: 0

Wessel van der Linden
Wessel van der Linden

Reputation: 2622

The first time running the while loop, n is set to -1, then it checks if n

In the do-while loop your code sets n to -1, then raises it with 2. Prints the line and after that it checks if n

So if you input is -1 the while loop wont print anything (because -1 < -1 evaluates to false). But your do-while loop will print 1

Upvotes: 0

sol4me
sol4me

Reputation: 15708

The while statement continually executes a block of statements while a particular condition is true. The difference between do-while and while is that do-while evaluates its expression at the bottom of the loop instead of the top. Therefore, in your case if you input value less then -1 while loop will executes 0 times whereas do-while loop will executes 1 times

Upvotes: 0

sid smith
sid smith

Reputation: 533

A do while loop will execute at least once whereas a while loop may not even execute even once if its conditions are not met. The do while starts checking its condition after the first iteration.

Upvotes: 1

Related Questions