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

Chapter 7 ■ Functions
else if (d == 7) printf("Saturday\n");
else printf("Invalid day\n");
} //end printDay
Now that we have delegated the printing to a function, notice how main is much less
cluttered. However, we do have to write the function prototype for printDay in main so that
printDay can be called from main. Here is the prototype:
void printDay(int);
As with all C programs, execution begins with the first statement in main. This prompts the user
for a number, and the program goes on to print the name of the day by calling the function printDay.
A sample run is:
Enter a day from 1 to 7: 4
Wednesday
In main, suppose n has the value 4. The call printDay(n) is executed as follows:
• The value of the argument n is determined. It is 4.
• The value 4 is copied to a temporary memory location. This location is
passed to the function printDay where it is labeled with the name of the
parameter, d. In effect, d is set to the value of the argument.
• The body of the function is executed. In this case, since d is 4, the statement
printf("Wednesday\n") will be executed.
• After printing Wednesday, the function is finished. The location containing
the argument is discarded and control returns to main to the statement
following the call printDay(n). In this case, there are no more statements so
the program ends.
7.6 Highest Common Factor
In Chapter 5, we wrote Program P5.2, which read two numbers and found their highest common
factor (HCF). You should refresh your memory by taking a look at the program.
It would be nice if whenever we want to find the HCF of two numbers (m and n, say), we could
make a function call hcf(m, n) to get the answer. For instance, the call hcf(42, 24) would return
the answer 6. To be able to do this, we write the function as follows:
//returns the hcf of m and n
int hcf(int m, int n) {
while (n != 0) {
int r = m % n;
m = n;
n = r;
}
return m;
} //end hcf
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