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

Chapter 7 ■ Functions
7.11 Some Character Functions
In this section, we write several functions relating to characters.
Perhaps the simplest is a function that takes a character as argument; it returns 1 if the
character is a digit and 0, if it is not. (Recall that, in C, a zero value is interpreted as false and a
nonzero value is interpreted as true.) This description suggests that we must write a function that
takes a char argument and returns an int value. We will call it isDigit. Here it is:
int isDigit(char ch) {
return ch >= '0' && ch <= '9';
} //end isDigit
The Boolean expression (ch >= '0' && ch <= '9') is true if ch lies between '0' and '9',
inclusive; that is, if ch contains a digit. Hence, if ch contains a digit, the function returns 1 (for true);
if ch does not contain a digit, it returns 0 (for false).
We could have written the body of the function as
if (ch >= '0' && ch <= '9') return 1;
return 0;
but the single return statement used above is the preferred way.
Similarly, we can write the function isUpperCase, which returns 1 if its argument is an
uppercase letter and 0 if it’s not, thus:
int isUpperCase(char ch) {
return ch >= 'A' && ch <= 'Z';
} //end isUpperCase
Next we have the function isLowerCase, which returns 1 if its argument is a lowercase letter
and 0 if it’s not.
int isLowerCase(char ch) {
return ch >= 'a' && ch <= 'z';
} //end isLowerCase
If we wish to know if the character is a letter (either uppercase or lowercase), we can write
isLetter, which uses isUpperCase and isLowerCase.
int isLetter(char ch) {
int isUpperCase(char), isLowerCase(char);
return isUpperCase(ch) || isLowerCase(ch);
} //end isLetter
Note that we need to include the function prototypes for isUpperCase and isLowerCase.
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