# 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

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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