C language standard library (1)-# include (ctype. h)

C language standard library (1)-# include (ctype. h)
C Standard Library-# include By jxlijunhao in this header file contains a function for character testing, such as checking whether a character in a string is a number or a letter, and also contains a character ing function, for example, you can map uppercase letters to lowercase letters. The following describes the usage of some functions in the Standard Manual for future use.
1. isalpha: determines whether it is an isdigit letter or a number.

#include
 
  #include
  
   int main(){int i=0;char str[]="c++11";while (str[i]){if (isalpha(str[i]))printf("character %c is alpha\n",str[i]);else if (isdigit(str[i]))printf("character %c is digit\n",str[i]);else printf("character %c is not alpha or digit\n",str[i]);i++;}}
  
 

Output result:

character c is alphacharacter + is not alphacharacter + is not alphacharacter 1 is digitcharacter 1 is digit

Isxdigit: determines whether it is ~ F, ~ F


2. isalnum: it is a letter or number.

# Include
 
  
# Include
  
   
Int main () {int I = 0; int count = 0; char str [] = "c ++ 11 "; // count the number of letters or numbers in a string. while (str [I]) {if (isalnum (str [I]) count ++; I ++ ;} printf ("there are % d alpha or digit is % s", count, str); return 0 ;}
  
 

Output result:

3

3. islower, whether it is lower case, isupper, whether it is upper case tolower, convert it to lower case touuper to upper case

# Include
 
  
# Include
  
   
Int main () {char str [] = "I love ShangHai"; // converts lowercase letters in the string to uppercase letters int I = 0; char c; while (str [I]) {c = str [I]; if (islower (c) str [I] = toupper (c); I ++ ;} printf ("% s \ n", str );}
  
 

I LOVE SHANGHAI

4. isspace: determines whether a character is a space.

#include 
 
  #include 
  
   int main (){  char c;  int i=0;  char str[]="Example sentence to test isspace\n";  while (str[i])  {    c=str[i];    if (isspace(c)) c='\n';    putchar (c);    i++;  }  return 0;
  
 

Output:

Examplesentencetotestisspace

One application: Contains 0 ~ 9, ~ The string (hexadecimal) of F is converted to an integer:

# Include
 
  
# Include
  
   
# Include
   
    
Long atox (char * s) {char xdigs [] = "0123456789 ABCDEF"; long sum; while (isspace (* s) ++ s; for (sum = 0L; isxdigit (* s); ++ s) {int digit = strchr (xdigs, toupper (* s)-xdigs; // The address minus sum = sum * 16 + digit ;} return sum;} int main () {char * str = "21"; long l = atox (str); printf ("% d \ n", l ); return 0 ;}
   
  
 

The strchr function is contained. The prototype of a function is

const char * strchr ( const char * str, int character );

Returns the position of the character to be searched in the string for the first time. The returned value is a pointer to the character.