Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

Chapter 5 ■ Programs with Repetition Logic
and so on. We want to let the user enter as many numbers as he wishes. Since we can have no
idea how many that will be, and the amount could vary from one run of the program to the next,
we must let the user “tell” us when he wishes to stop entering numbers.
How does he “tell” us? Well, the only time the user “talks” to the program is when he types
a number in response to the prompt. If he wishes to stop entering numbers, he can enter some
“agreed-upon” value; when the program reads this value, it will know that the user wishes to stop.
In this example, we can use 0 as the value that tells the program that the user wishes to stop.
When a value is used this way, it is referred to as a sentinel or end-of-data value. It is sometimes
called a rogue value—the value is not to be taken as one of the actual data values.
What can we use as a sentinel value? Any value that cannot be confused with an actual data
value would be okay. For example, if the data values are all positive numbers, we can use 0 or −1
as the sentinel value. When we prompt the user, it is a good idea to remind him what value to use
as the sentinel value.
Assume we want the program to run as follows:
Enter a number (0 to end): 24
Enter a number (0 to end): 13
Enter a number (0 to end): 55
Enter a number (0 to end): 32
Enter a number (0 to end): 19
Enter a number (0 to end): 0
The sum is 143
How do we get the program to run like that? We want to be able to express the following logic
in a form the computer could understand:
As long as the user does not enter 0, keep prompting him for another number and add it
to the sum
It seems obvious that we must, at least, prompt him for the first number. If this number is 0, we
must print the sum (which, of course, would be 0 at this time). If the number is not 0, we must add it
to the sum and prompt for another number. If this number is 0, we must print the sum. If this number
is not 0, we must add it to the sum and prompt for another number. If this number is 0..., and so on.
The process will come to an end when the user enters 0.
This logic is expressed quite neatly using a while construct (also called a while statement or
while loop):
//Algorithm for finding sum
set sum to 0
get a number, num
while num is not 0 do
add num to sum
get another number, num
endwhile
print sum
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