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Learn to Code: Java for Absolute Beginners

Sentinel Loops

Explore how sentinel values are used to terminate while loops in Java when the number of repetitions is unknown. Understand pretest loops and common mistakes in loop programming. Practice writing programs that use sentinel loops to solve problems like summing inputs, generating geometric sequences, and finding the greatest common divisor.

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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.

  • ...