Convert string slice to int in Rust

Question

Note The code in this question pertains to a version of Rust before 1.0 but the answers have been updated for Rust 1.0.

I have trouble converting a string to an integer.

fn main() {
    let input_text = io::stdin()
        .read_line()
        .ok()
        .expect("failed to read line");

    let input: Option<int> = from_str(input_text.as_slice());

    println!("{}", input);
}

I enter a number on the console (42 for example) and my program prints None.

The documentation says that that is a normal situation when the string is ill-formatted, but what's wrong with my 42?

Answer 1

You can call str::parse(), but you need to make sure that read_line is working. We need a reader:

use std::io;

fn main() {
    let reader = io::stdin();
}

stdin reads the global buffer that handles the input stream and also implements the BufRead trait which has the read_line method method. This takes a mutable String as an input buffer and reads all bytes from the stream until a newline byte is reached and appends them to the buffer. The #expect() method unwraps the Result; if it is an Err it will panic with the message and the cause.

use std::io;

fn main() {
    let reader = io::stdin();
    let mut input_text = String::new();
    reader.read_line(&mut input_text).expect("failed to read line");
}

We now have the input text that we want to convert into an i32. This is where str::parse() will work for us, as long as we give it a type to parse to. str::trim() is necessary because read_line includes the newline byte the buffer

use std::io;

fn main() {
    let reader = io::stdin();
    let mut input_text = String::new();
    reader.read_line(&mut input_text).expect("failed to read line");
    let input = input_text.trim().parse::<i32>();
}

We're not done yet, we still need to ensure that we successfully parsed the input using pattern matching. All the code you need to convert your original input buffer into a usable integer is:

use std::io;

fn main() {
    let reader = io::stdin();
    let mut input_text = String::new();

    reader.read_line(&mut input_text).expect("failed to read line");

    let input_opt = input_text.trim().parse::<i32>();

    let input_int = match input_opt {
        Ok(input_int) => input_int,
        Err(e) => {
            println!("please input a number ({})", e);
            return;
        }
    };

    println!("{}", input_int);
}

This compiles without errors or warnings.