Learn to Code: Java for Absolute Beginners/

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Sentinel Loops

Sentinel Loops

Learn and practice loops controlled by a sentinel value.

We'll cover the following...

The sentinel value

Sometimes, the loop doesn’t have a fixed number of repetitions. Instead, an indicator value stops the loop. This special value is called the sentinel value. For example, we don’t know how much data is in a file without reading it all. However, we know that every file ends with an end-of-file (EOF) mark. So, the EOF mark is the sentinel value in this case.

Note: We should select a sentinel value that’s not expected in the normal input.

The while loop

We use the while loop when the termination of the loop depends on the sentinel value instead of a definite number of iterations. As a simple example, we want to display the reverse sequence of digits in a positive integer value input by the user.

import java.util.Scanner;
class Test
{
  public static void main(String[] args) 
  {
    Scanner myObj = new Scanner(System.in);
    System.out.println("Enter a number: ");
    int a = myObj.nextInt();  // Taking input in variable a
    while (a > 0)// This loop will terminate when the value is not greater than 0
    {
      System.out.println(a % 10);
      a /= 10; //Dividing a by 10 and assigning the result to variable a
    }
  }
}
Reverse of an integer value

In the program above:

  • Line 1: We import the Scanner class.

  • Line 6: We create a Scanner object named myObj.

  • Line 8: We take input from the user and store it in variable a.

  • Line 9: The body of the while loop executes if the condition expression a > 0 is true. This statement tests the condition before entering the loop. Therefore, such a loop is called a pretest loop.

    Note: There is no semicolon (;) at the end of line 9.

  • The loop terminates when the condition evaluates to false.

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