Learn to Code: Python 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, a 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.

a = int(input("Enter the number:")) # Taking input in variable a
while a > 0: # This loop will terminate when the value of a is not greater than 0
  print(a % 10,end="" )
  a //=10 # Dividing a by 10 and assigning the result to variable a
print()
Reverse of an integer value

In the program above:

  • In the first line, we take input from the user.
  • The loop statement, while, is followed by a conditional expression and then a colon :.
  • The next two statements are in the body of the loop indicated by the indentation.
  • The body of the loop executes if the condition expression a > 0 is true. This statement tests the condition before entering the loop. Such a loop is called a pretest loop.
  • The loop terminates when the condition evaluates to false.
  • In the body of the loop, the first statement displays the value of the units position
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