how do i define a function in C that writes an int or a char from a given string pointer
To create a function in C that writes an int or a char from a given string pointer, you need to determine how to distinguish between an int and a char within the string. In this example, let's assume that you have a string that contains only digits and letters (no spaces or other characters). We'll create a function that takes a pointer to a string and an index, and prints the int or char at the specified index.

Here's a simple example:
``` 
#include <stdio.h>
#include <ctype.h>

void print_int_or_char(const char* str, int index) {
    char c = str[index];
    
    if (isdigit(c)) {
        int num = c - '0'; // Convert the char digit to an integer value
        printf(Integer: %d\n, num);
    } else if (isalpha(c)) {
        printf(Character: %c\n, c);
    } else {
        printf(Invalid input\n);
    }
}

int main() {
    const char* input_string = a1b2c3;
    int index = 2; // Specify the index of the character you want to print
    
    print_int_or_char(input_string, index);

    return 0;
}
``` 
In this example, the print_int_or_char function takes a string pointer and an index as its arguments. The function checks if the character at the specified index is a digit or a letter. If it's a digit, it converts the character to an integer value and prints it. If it's a letter, it prints the character directly. If the input is neither a digit nor a letter, the function prints an Invalid input message.
Can you write the equivalent of the above code in Rust please? Is the use of pointers still advisable for the Rust code?
Sure! Here's an equivalent implementation of the function in Rust:

```rust
fn print_int_or_char(input_string: &str, index: usize) {
    let c = input_string.chars().nth(index);

    match c {
        Some(digit) if digit.is_ascii_digit() => {
            let num = digit.to_digit(10).unwrap();
            println!(Integer: {}, num);
        }
        Some(letter) if letter.is_ascii_alphabetic() => {
            println!(Character: {}, letter);
        }
        _ => {
            println!(Invalid input);
        }
    }
}

fn main() {
    let input_string = a1b2c3;
    let index = 2;

    print_int_or_char(input_string, index);
}
```

In Rust, we can use the `&str` type to represent a string slice, which is a reference to a portion of a string. We can access a specific character in the string slice using the chars method, which returns an iterator over the characters in the string. We can then use the nth method to get the character at the specified index.

Note that Rust has its own memory management system based on ownership and borrowing, so the use of pointers is not necessary in most cases. In this example, we can pass the string slice as a reference to the function instead of a pointer, which ensures that the ownership of the string is not transferred to the function.