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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.

#include <iostream>
using namespace std;

int main()
{
  int a;
  cout << "Input a number: "; // Taking an input in a variable a
  cin >> a;
  while (a > 0) // This loop will terminate when the value of a is not greater than 0.
  { 
    cout << a % 10;
    a = a / 10; // Dividing by 10 and assigning the value back to a
  }
  cout << endl;
  return 0;
}
The reverse of an integer value

In the program above:

  • Line 6: We declare an integer variable a.
  • Line 7: We prompt the user to input a number.
  • Line 8: We take the input number from the user and store it in a.
  • Line 9: This is the loop statement, while, followed by a conditional expression and then an opening brace, { on line 10.

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

  • Line 11–12: These two statements are in the body of the loop indicated by the braces. The loop’s body executes if the conditional