C Language Introduction


Example

The header ctype.h is a part of the standard C library. It provides functions for classifying and converting characters.

All of these functions take one parameter, an int that must be either EOF or representable as an unsigned char.

The names of the classifying functions are prefixed with 'is'. Each returns an integer non-zero value (TRUE) if the character passed to it satisfies the related condition. If the condition is not satisfied then the function returns a zero value (FALSE).

These classifying functions operate as shown, assuming the default C locale:

int a;
int c = 'A';
a = isalpha(c); /* Checks if c is alphabetic (A-Z, a-z), returns non-zero here. */
a = isalnum(c); /* Checks if c  is alphanumeric (A-Z, a-z, 0-9), returns non-zero here. */
a = iscntrl(c); /* Checks is c is a control character (0x00-0x1F, 0x7F), returns zero here. */
a = isdigit(c); /* Checks if c is a digit (0-9), returns zero here. */
a = isgraph(c); /* Checks if c has a graphical representation (any printing character except space), returns non-zero here. */
a = islower(c); /* Checks if c is a lower-case letter (a-z), returns zero here. */
a = isprint(c); /* Checks if c is any printable character (including space), returns non-zero here. */
a = isupper(c); /* Checks if c is a upper-case letter (a-z), returns zero here. */
a = ispunct(c); /* Checks if c is a punctuation character, returns zero here. */
a = isspace(c); /* Checks if c is a white-space character, returns zero here. */
a = isupper(c); /* Checks if c is an upper-case letter (A-Z), returns non-zero here. */
a = isxdigit(c); /* Checks if c is a hexadecimal digit (A-F, a-f, 0-9), returns non-zero here. */
C99
a = isblank(c); /* Checks if c is a blank character (space or tab), returns non-zero here. */

There are two conversion functions. These are named using the prefix 'to'. These functions take the same argument as those above. However the return value is not a simple zero or non-zero but the passed argument changed in some manner.

These conversion functions operate as shown, assuming the default C locale:

int a;
int c = 'A';

/* Converts c to a lower-case letter (a-z). 
 * If conversion is not possible the unchanged value is returned. 
 * Returns 'a' here. 
 */
a = tolower(c);

/* Converts c to an upper-case letter (A-Z). 
 * If conversion is not possible the unchanged value is returned. 
 * Returns 'A' here. 
 */
a = toupper(c);

The below information is quoted from cplusplus.com mapping how the original 127-character ASCII set is considered by each of the classifying type functions (a • indicates that the function returns non-zero for that character)

ASCII valuescharactersiscntrlisblankisspaceisupperislowerisalphaisdigitisxdigitisalnumispunctisgraphisprint
0x00 .. 0x08NUL, (other control codes)
0x09tab ('\t')
0x0A .. 0x0D(white-space control codes: '\f','\v','\n','\r')
0x0E .. 0x1F(other control codes)
0x20space (' ')
0x21 .. 0x2F!"#$%&'()*+,-./
0x30 .. 0x390123456789
0x3a .. 0x40:;<=>?@
0x41 .. 0x46ABCDEF
0x47 .. 0x5AGHIJKLMNOPQRSTUVWXYZ
0x5B .. 0x60[]^_`
0x61 .. 0x66abcdef
0x67 .. 0x7Aghijklmnopqrstuvwxyz
0x7B .. 0x7E{}~bar
0x7F(DEL)