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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.

class Test
{
  static void Main()
  {
      System.Console.WriteLine("Enter the number: ");
      int a = int.Parse(System.Console.ReadLine()); // Taking input in variable a

      while (a > 0) // This loop will terminate when the value is not greater than 0
      {
          System.Console.Write(a % 10);
          a /= 10; // Dividing a by 10 and assigning the result to variable a
      }
      System.Console.WriteLine("");
  }
}
Reverse of an integer value

In the program above:

  • In line 6, we take input from the user, and then convert it into an integer value using int.Parse().

  • The loop statement, while, followed by a conditional expression.

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

  • The next four statements after the while loop, line 9 to 12, are in the body of the loop indicated by the indentation.

  • The body ...