The ctype library are functions related to operations on the character data type. Most functions in ctype library are is**something** type functions. These functions allow you to check if a character is a particular has a particular property. For example suppose you want to test if a variable named x was an stored an alphabetic (a to z in any casing) character you could use: isalpha(x). The function returns a truthy (non-zero) value if x is alphabetic, falsey (zero) if it is alphabetic. When testing always test against 0.

	isalpha(x) == 0
	isalpha(x) != 0

Aside from the functions that test for whether the character is a certain type of character, the ctype library also tests allows has two functions that returns the upper/lower case version of an alphabetic character. These are the toupper() and tolower() functions.

The interesting thing about toupper() and tolower() is that it is safe to use it with any characters even non-alphabetic ones. This is because the function returns the original characters if the characters are not applicable.

For example, the in the following code snippet, we have a string where we will convert all alphabetic characters to upper case while leaving numeric characters alone.

int i;
char robot[5] = "r2d2";

for(i = 0;robot[i]!='\0';i++){
	robot[i] = toupper(robot[i]);
}

//this will print:
//R2D2
printf("%s\n",robot);

:::caution(toupper() and tolower() returns, they do not convert!)

It is really important to note how the pass-by-value nature of functions work with these library functions. toupper() and tolower() accept a character as an argument. C's parameter passing mechanism is pass-by-value. This means that the characters passed to the function are duplicated for the function as its parameters. Any modification to those variables are never reflected back to the calling variables. Thus, the variable that is passed to the function will never change. Therefore, it is important to capture the return value to store what it is that you need.

char lowerA = 'a';

//upperA will hold 'A' after this line runs
char upperA = toupper(lowerA);

//this next line will print:
//lowerA is still a, but upperA is A\n
printf("lowerA is still %c, but upperA is %c\n", lowerA, upperA);

:::

Here is a table showing the ctype library functions and what they check.

ctype.h Library Functions in C

Function	Purpose	Expected Behavior	Example
`isalpha()`	Check if alphabetic	Returns truthy (non-zero) if character is a letter (A-Z, a-z)	`isalpha('A')` returns non-zero (truthy)
`isdigit()`	Check if digit	Returns non-zero (truthy) if character is a digit (0-9)	`isdigit('5')` returns non-zero (truthy)
`isalnum()`	Check if alphanumeric	Returns non-zero (truthy) if character is a letter or digit	`isalnum('A')` returns non-zero (truthy)
`isspace()`	Check if whitespace	Returns non-zero (truthy) if character is whitespace (space, tab, newline, etc.)	`isspace(' ')` returns non-zero (truthy)
`isupper()`	Check if uppercase	Returns non-zero (truthy) if character is uppercase letter	`isupper('A')` returns non-zero (truthy)
`islower()`	Check if lowercase	Returns non-zero (truthy) if character is lowercase letter	`islower('a')` returns non-zero (truthy)
`isprint()`	Check if printable	Returns non-zero (truthy) if character is printable (including space)	`isprint('A')` returns non-zero (truthy)
`isgraph()`	Check if graphical	Returns non-zero (truthy) if character is printable but not space	`isgraph('A')` returns non-zero (truthy)
`ispunct()`	Check if punctuation	Returns non-zero (truthy) if character is punctuation	`ispunct('!')` returns non-zero (truthy)
`iscntrl()`	Check if control	Returns non-zero (truthy) if character is a control character	`iscntrl('\n')` returns non-zero (truthy)
`isxdigit()`	Check if hex digit	Returns non-zero (truthy) if character is hexadecimal digit (0-9, A-F, a-f)	`isxdigit('F')` returns non-zero (truthy)
`toupper()`	Convert to uppercase	Returns uppercase equivalent of character, or unchanged if not lowercase	`toupper('a')` returns `'A'`
`tolower()`	Convert to lowercase	Returns lowercase equivalent of character, or unchanged if not uppercase	`tolower('A')` returns `'a'`