Any C programmer eager to mess with characters or strings knows about the handy ctype functions. I use this name because these functions, which include a few macros, are defined in the ctype.h header file. Their job is to manipulate and examine characters.

I divide the ctype functions into the “to” and “is” categories.

The “to” functions manipulate characters. Only two of them are available: toupper() and tolower(), which convert a character from lowercase to uppercase and vice-versa, respectively. Both functions start with “to.”

The “is” functions return TRUE or FALSE based on the character’s attributes. For example, isalpha() returns TRUE when the character examined is alphabetic, upper- or lowercase. The function starts with “is,” which is how I define this category. Lotsa “is” ctype functions are available:

isalnum()
isalpha()
isascii()
isblank()
iscntrl()
isdigit()
isgraph()
islower()
isprint()
ispunct()
isspace()
isupper()
isxdigit())

All of these functions are defined in the ctype.h header file. They all have a similar man page format. For example:

int isspace(int c)

The argument c is specified as an integer, though it must have the value of an unsigned char or EOF. (The EOF is why the prototype is an integer, which allows this function to work with standard I/O.)

The return value is non-zero for a TRUE or positive result, zero otherwise. For example, isspace(' ') returns TRUE when character c is a whitespace character.

These functions work reliably on standard ASCII characters. Supposedly, they can also function in other languages when the locale is set, and variations on the functions are available to handle different alphabets. I’ve been unable to verify whether this feature works. So, my exploration of these functions is limited to standard ASCII, the Latin alphabet.

It’s easy to guess what each of the function does based on the name, though some are kinda weird. Here are brief descriptions:

Over the next few weeks, I’ll cover these functions and how they work. I’ll also present code that emulates the functions just because it’s a fun thing to do!

These are “ctype” functions.