While & Do While Loops

While Loop
The while loop is another way of looping a snippet of code while something meets a criteria. For example, while lowOnStock is false then keep checking stock. Once the criteria is not met then we carry on with the rest of the code.

Below is a really basic example showing how while loops work.

• Your first asked if you want to enter the loop.
• Upon entering the loop your then asked if you want to stay in the loop.
• Answering No to any of the questions will then carry on with the code.

Try it out for a better understanding of how it works.
Notice how we use code from the previous String Equality tutorial.

[hide=15][java]
import java.util.*;
public class whiledo {
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
String ans;

System.out.println("Do you want to go into the loop? [Yes/No]");
ans = sc.next();

while(ans.equalsIgnoreCase("Yes"))
{
System.out.println("Do you want to stay in the loop? [Yes/No]");
ans = sc.next();
}

System.out.print("Program Finished");
}
}
[/java]

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Do While Loop
The do while loop is a way of looping a specific section of code depending on a criteria just like the while loop. The difference being is that in a while loop the code will only be executed if the criteria is met, if its not it will just skip it.
A Do While loop will always execute the loop code once and then it will check the criteria. An easy way to remember this is that in a do while loop, the 'while' comes after the do, so it does the code first then checks. In a while loop, the 'while' comes first meaning it checks before it does the code.

The example below is really basic and kind of pointless, but still shows how a do while works.
Notice how it asks for a value before it actually checks what the value is.

All this example does is;

• Ask for a value
• If the value is between 1 and 100 then finish the program
• If the value is outside the criteria then loop the 'ask for value' code

[java]
import java.util.*;
public class dowhile {
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
int guess;

do
{
System.out.println("I'm thinking of a number between 1 and 100");
System.out.println("What do you think it is?");
guess = sc.nextInt();
} while((guess<1) || (guess>100)); //Process guess value

System.out.println("Program finished");
}
}
[/java][/hide]