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4.2 Character Class Tests: <ctype.h>
For each function, the argument is a character, whose value must be EOF or representable
as an unsigned char, and the return value is an integer.
The functions return non-zero (true) if the argument c satisfies the condition described;
otherwise, zero is returned.
isalnum (c)
isalpha (c)
iscntrl (c)
isdigit (c)
isgraph (c)
islower (c)
isprint (c)
ispunct (c)
isspace (c)
isupper (c)
isxdigit (c)
In addition, there are two functions that convert the case of letters:
int tolower (c)
int toupper (c)
Chapter 4 Standard Library Routine
isalpha (c) or isdigit (c) is true
isupper (c) or islower (c) is true
control character
decimal digit
printing character except space
lower-case letter
printing character including space
printing character except space, letter and digit
space, formfeed, newline, carriage return, tab, vertical tab
upper-case letter
hexadecimal digit
convert c to lower-case
convert c to upper-case
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