Reading an integer from input and assigning it to a variable

Question

I've been trying to find an easy way to read variables in Rust, but haven't had any luck so far. All the examples in the Rust Book deal with strings AFAIK, I couldn't find anything concerning integers or floats that would work.

Answer 1

I don't have a Rust compiler on this machine, but based in part on this answer that comes close, you want something like...

let user_val = match input_string.parse::<i32>() {
    Ok(x) => x,
    Err(_) => -1,
};

Or, as pointed out in the comments,

let user_val = input_string.parse::<i32>().unwrap_or(-1);

...though your choice in integer size and default value might obviously be different, and you don't always need that type qualifier (::<i32>) for parse() where the type can be inferred from the assignment.

Answer 2

To read user input, you always read a set of bytes. Sometimes, you can interpret those bytes as a UTF-8 string. You can then further interpret the string as an integral or floating point number (or lots of other things, like an IP address).

Here's a complete example of reading a single line of input and parsing it as a 32-bit signed integer:

use std::io;

fn main() {
    let mut input = String::new();
    io::stdin().read_line(&mut input).expect("Not a valid string");
    let input_num: i32 = input.trim().parse().expect("Not a valid number");
    println!("Your number plus one is {}", input_num + 1);
}

Note that no user-friendly error handling is taking place. The program simply panics if reading input or parsing fails. Running the program produces:

$ ./input
41
Your number plus one is 42

Answer 3

A set of bytes comprises an input. In Rust, you accept the input as a UTF-8 String. Then you parse the string to an integer or floating point number. In simple ways you accept the string and parse it, then write an expect`` statement for both, to display a message to the user what went wrong when the program panics during runtime.

fn main() {
    let mut x = String::new();
    std::io::stdin().read_line(&mut x)
        .expect("Failed to read input.");
    let x: u32 = x.trim().parse()
        .expect("Enter a number not a string.");
    println!("{:?}", x);
}

If the program fails to parse the input string then it panics and displays an error message. Notice that the program still panics and we are not handling an error perfectly. One more thing to notice is that we can use the same variable name x and not some x_int because of the variable shadowing feature. To handle the error better we can use the match construct.

fn main() {
    let mut x = String::new();
    match std::io::stdin().read_line(&mut x) {
        Ok(_) => println!("String has been taken in."),
        Err(_) => {
            println!("Failed to read input.");
            return;
        },
    };
    let x: u32 = match x.trim().parse() {
        Ok(n) => {
            println!("Converted string to int.");
            n
        },
        Err(_) => {
            println!("Failed to parse.");
            return;
        },
    };
    println!("{:?}", x);
}

This is longer way but a nicer way to handle errors and input and parse a number.