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

Chapter 5 ■ Programs with Repetition Logic
happens, execution continues with the statement, if any, after <statement>. If <condition> is
false the first time, <statement> is not executed and execution continues with the following
statement, if any.
In Program P5.1, <condition> is num != 0 and <statement> is the block
{
}
sum = sum + num;
printf("Enter a number (0 to end): ");
scanf("%d", &num);
Whenever we want to execute several statements if <condition> is true, we must enclose the
statements by { and }. Effectively, this makes them into one statement, a compound statement,
satisfying C’s syntax rule that requires one statement as the body.
5.2.1 Highest Common Factor
Let us write a program to find the highest common factor, HCF (also called the greatest common
divisor, GCD), of two numbers. The program will run as follows:
Enter two numbers: 42 24
Their HCF is 6
We will use Euclid’s algorithm for finding the HCF of two integers, m and n. The algorithm is as
follows:
1. if n is 0, the HCF is m and stop
2. set r to the remainder when m is divided by n
3. set m to n
4. set n to r
5. go to step 1
Using m as 42 and n as 24, step through the algorithm and verify that it gives the correct
answer, 6.
Steps 2, 3, and 4 are executed as long as n is not 0. Hence, this algorithm can be expressed
using a while loop as follows:
while n is not 0 do
set r to m % n
set m to n
set n to r
endwhile
HCF is m
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