C标准库之ctype.h

The ctype header is used for testing and converting characters. A control character refersto a character that is not part of the normal printing set. In the ASCII character set, the controlcharacters are the characters from 0 (NUL) through 0x1F (US), and the character 0x7F (DEL). Printable characters are those from 0x20 (space) to 0x7E (tilde).

Functions:

isalnum();
isalpha();
iscntrl();
isdigit();
isgraph();
islower();
isprint();
ispunct();
isspace();
isupper();
isxdigit();
tolower();
toupper();


2.2.1 is... Functions

Declarations:

int isalnum(int character );
int isalpha(int character );
int iscntrl(int character );
int isdigit(int character );
int isgraph(int character );
int islower(int character );
int isprint(int character );
int ispunct(int character );
int isspace(int character );
int isupper(int character );
int isxdigit(int character );


The is... functions test the given character and return a nonzero (true) result if it satisfiesthe following conditions. If not, then 0 (false) is returned.

Conditions:

isalnum	a letter (A to Z or a to z) or a digit (0 to 9)
isalpha	a letter (A to Z or a to z)
iscntrl	any control character (0x00 to 0x1F or 0x7F)
isdigit	a digit (0 to 9)
isgraph	any printing character except for the space character (0x21 to 0x7E)
islower	a lowercase letter (a to z)
isprint	any printing character (0x20 to 0x7E)
ispunct	any punctuation character (any printing character except for spacecharacter or isalnum)
isspace	a whitespace character (space, tab, carriage return, new line, vertical tab,or formfeed)
isupper	an uppercase letter (A to Z)
isxdigit	a hexadecimal digit (0 to 9, A to F, or a to f)

2.2.2 to... Functions

Declarations:

int tolower(int character );
int toupper(int character );


The to... functions provide a means to convert a single character. If the charactermatches the appropriate condition, then it is converted. Otherwise the character is returnedunchanged.

Conditions:

tolower	If the character is an uppercase character (A to Z), then it is converted tolowercase (a to z)
toupper	If the character is a lowercase character (a to z), then it is converted touppercase (A to Z)

Example:

 

#include<ctype.h>
#include<stdio.h>
#include<string.h>
 
int main(void)
{
 int loop;
 char string[]="THIS IS A TEST";
 for(loop=0;loop<strlen(string);loop++)
 string[loop]=tolower(string[loop]);
 printf("%s\n",string);
 return 0;
}