The header file ctype.h defines various functions for determining what class a character belongs to. These consist of the following functions:
| Function | Character Class | Returns Nonzero for Characters | ||
|---|---|---|---|---|
| isdigit() | – | Decimal digits | – | 0-9 |
| isxdigit() | – | Hexadecimal digits | – | 0-9, a-f, or A-F |
| isalnum() | – | Alphanumerics | – | 0-9, a-z, or A-Z |
| isalpha() | – | Alphabetics | – | a-z or A-Z |
| islower() | – | Lowercase alphabetics | – | a-z |
| isupper() | – | Uppercase alphabetics | – | A-Z |
| isspace() | – | Whitespace | – | Space, tab, vertical tab, newline, form feed, or carriage return |
| isgraph() | – | Nonblank characters | – | Any character that appears nonblank when printed (ASCII 0x21 through 0x7E) |
| isprint() | – | Printable characters | – | All the isgraph() characters, plus space |
| ispunct() | – | Punctuation | – | Any character in isgraph() that is not in isalnum() |
| iscntrl() | – | Control characters | – | Any character not in isprint() (ASCII 0x00 through 0x1F plus 0x7F) |
There are three very good reasons for calling these macros instead of writing your own tests for character classes. They are pretty much the same reasons for using standard library functions in the first place. First, these macros are fast. Because they are generally implemented as a table lookup with some bit-masking magic, even a relatively complicated test can be performed much faster than an actual comparison of the value of the character.
Second, these macros are correct. It’s all too easy to make an error in logic or typing and include a wrong character (or exclude a right one) from a test.
Third, these macros are portable. Believe it or not, not everyone uses the same ASCII character set with PC extensions. You might not care today, but when you discover that your next computer uses Unicode rather than ASCII, you’ll be glad you wrote code that didn’t assume the values of characters in the character set.
The header file ctype.h also defines two functions to convert characters between upper- and lowercase alphabetics. These are toupper() and tolower(). The behavior of toupper() and tolower() is undefined if their arguments are not lower- and uppercase alphabetic characters, respectively, so you must remember to check using islower() and isupper() before calling toupper() and tolower().