# <ctype.h>

**[<ctype.h>](https://en.cppreference.com/w/c/header/ctype)** provides character classification (`isalpha`, `isdigit`, `isspace`) and conversion (`toupper`, `tolower`). These functions handle the full range of `unsigned char` correctly and respect locale.

Always cast to `unsigned char` when passing chars to avoid undefined behavior with negative signed values.

## Example

This example classifies characters in a string as letters, digits, or spaces.

```c
// compile: gcc -o ctypeexample ctypeexample.c
// run: ./ctypeexample
// description: count character types in a string

#include <ctype.h>
#include <stdio.h>
#include <string.h>

int main() {
    const char* str = "Hello World 123";
    int letters = 0, digits = 0, spaces = 0;
    
    for (size_t i = 0; str[i]; i++) {
        unsigned char c = (unsigned char)str[i];
        if (isalpha(c)) letters++;
        else if (isdigit(c)) digits++;
        else if (isspace(c)) spaces++;
    }
    
    printf("letters=%d digits=%d spaces=%d\n", letters, digits, spaces);
    return 0;
}
```

## Common functions

**Character classification** (return non-zero if true, zero if false):
- `isalpha(c)`: alphabetic letter (A-Z, a-z)
- `isdigit(c)`: decimal digit (0-9)
- `isalnum(c)`: alphanumeric (letter or digit)
- `isspace(c)`: whitespace (space, tab, newline, etc.)
- `isblank(c)`: space or tab (C99)
- `isupper(c)`: uppercase letter
- `islower(c)`: lowercase letter
- `isxdigit(c)`: hexadecimal digit (0-9, a-f, A-F)
- `ispunct(c)`: punctuation
- `isprint(c)`: printable (not control character)
- `isgraph(c)`: visible character (print except space)
- `iscntrl(c)`: control character (ASCII 0-31, 127)
- `isascii(c)`: ASCII character (0-127, non-standard)

**Case conversion**:
- `toupper(c)`: convert to uppercase (returns character unchanged if not lowercase)
- `tolower(c)`: convert to lowercase (returns character unchanged if not uppercase)

**Notes**:
- All functions take `int` (though typically passed `char`, which is promoted)
- Behavior depends on locale (LC_CTYPE category)
- For safe use with char arrays, use `isxxx((unsigned char)c)` to avoid negative char UB