< ctype.h >

Judge function
isalnum
Check if character is alphanumeric (function )
判断是不是字母或者数字
isalpha
Check if character is alphabetic (function )
判断是否是字母
isblank
Check if character is blank (function )
判断是否是空白字符
iscntrl
Check if character is a control character (function )
判断是否是控制字符
isdigit
Check if character is decimal digit (function )
判断是否是数字
isgraph
Check if character has graphical representation (function )
除空格以外的任意可打印字符
islower
Check if character is lowercase letter (function )
判断是否是小写字母
isprint
Check if character is printable (function )
判断是否是可打印字符
ispunct
Check if character is a punctuation character (function )
判断是否是标点
isspace
Check if character is a white-space (function )
 判断是空白字符
isupper
Check if character is uppercase letter (function )
判断是否是大写字母
isxdigit
Check if character is hexadecimal digit (function )
判断是十六进制数字

Character conversion functions

Two functions that convert between letter cases:

tolower
 Convert uppercase letter to lowercase (function )
 变成小写
toupper
Convert lowercase letter to uppercase (function )
变成大写