Python Sentinel Controlled Loop Example Program

By | March 19, 2020

Topic: Python Sentinel Controlled Loop Example Program

What is a Sentinel Controlled Loop inPython?

A Sentinel Controlled Loop is a loop whose execution depends upon a special value. This sentinal controlled loop is terminated if a special value called sentinel value is encountered.

Python Sentinel Controlled Loop Example Program

Python Sentinel Controlled Loop Example Program

The sentinel controlled loops are conditional loops. For example we can use a while loop as a sentinl controlled as follows:

Exapmple Of a Sentinel Controlled Loop Situation

Suppose that we have to take n nubers from the user at run time and to show their sum. We do not know the value of n in advance. It depends on the will of the user if he / she enters 10 numbers, 20 numbers or 31 numbers.

Solution to this Problem by Sentinel Controlled Loop

Suppose we choose -999 as sentinel value. Therefore we will use a while loop to take a number from user repeatedly and sum it as long as it is not -999. The loop will terminated if the user enters -999.

Here is the complete program to solve this problem using a while loop as sentinel controlled loop.

# An example Python program 
# to show the use of Sentinel controlled loops 
# The user will be allowed to enter numbers for adding 
# until he enter -999 to stop.
# the program then will show the sum of all
# numbers entered exxcept -999 sentinel value.
# Author : www.EasyCodebook.com (c) 


sum = 0     # sum of numbers


num = input( "Enter a number to add, (Enter -999 to STOP): " )
num = int(num)   # convert string to an integer number

while num != -999: # -999 is used as a sentinel value to stop
    sum = sum + num
    
    num = input( "Enter a number to add, (Enter -999 to STOP): " )  
    num = int( num )
   
print ("The Sum of all entered numbers is:", sum)

Output

Enter a number to add, (Enter -999 to STOP): 2
Enter a number to add, (Enter -999 to STOP): 4
Enter a number to add, (Enter -999 to STOP): 5
Enter a number to add, (Enter -999 to STOP): 9
Enter a number to add, (Enter -999 to STOP): -999
The Sum of all entered numbers is: 20

How this sentinel controlled loop works?

First of all the user input a number 2 before entering in loop.

The loop condition verifies num=2 is not equal to -999, so the control will enter into loop body. Note that sum is initialized to 0 before loop.

The number 2 is added to sum, sum=2 now.

The program asks the user again, to enter number or -999 to stop . The user enters 4. The loop body ends. So loop condition is checked again. 4 is not equal to -999. The loop continues. 4 is added to sum. So sum is 6 now.

The user when asked, enters a 5. 5 is not equal to -999. So 5 isadded to sum. The sum is 11 now.

The user next time, enters a 9. 9 is not equal to -999. So 9 isadded to sum. The sum is 20 now.

At last the user enters -999 when asked to enter a number or -999 o stop.

Now -999!=-999 condition becomes false. Hence loop is terminated with sentinel value -999.

Topic: Python Sentinel Controlled Loop Example Program

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