# <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;
}
```