How to read an integer input from the user in Rust 1.0?

Question

Existing answers I\'ve found are all based on from_str (such as Reading in user input from console once efficiently), but apparently from_str(x) h

Answer 1

If you are looking for a way to read input for the purpose of competitive programming in websites like codechef or codeforces where you do not have access to text_io.

This is not a new way to read rather one mentioned in the above answers, I just modified it to suit my needs.

I use the following macro to read from stdin the different values:

use std::io;

#[allow(unused_macros)]
macro_rules! read {
    ($out:ident as $type:ty) => {
        let mut inner = String::new();
        io::stdin().read_line(&mut inner).expect("A String");
        let $out = inner.trim().parse::<$type>().expect("Parseble");
    };
}

#[allow(unused_macros)]
macro_rules! read_str {
    ($out:ident) => {
        let mut inner = String::new();
        io::stdin().read_line(&mut inner).expect("A String");
        let $out = inner.trim();
    };
}

#[allow(unused_macros)]
macro_rules! read_vec {
    ($out:ident as $type:ty) => {
        let mut inner = String::new();
        io::stdin().read_line(&mut inner).unwrap();
        let $out = inner
            .trim()
            .split_whitespace()
            .map(|s| s.parse::<$type>().unwrap())
            .collect::<Vec<$type>>();
    };
}

In main

fn main(){
   read!(x as u32);
   read!(y as f64);
   read!(z as char);
   println!("{} {} {}", x, y, z);

   read_vec!(v as u32); // Reads space separated integers and stops when newline is encountered.
   println!("{:?}", v);
}

NOTE: I am no Rust Expert, if you think there is a way to improve it, please let me know. It will help me, Thanks.

Answer 2

Probably the easiest part would be to use the text_io crate and write:

#[macro_use]
extern crate text_io;

fn main() {
    // read until a whitespace and try to convert what was read into an i32
    let i: i32 = read!();
    println!("Read in: {}", i);
}

If you need to read more than one value simultaneously, you might need to use Rust nightly.

See also:

• Is there a way to check that the user entered an integer with text_io's read!() macro?

Answer 3

Here are a few possibilities (Rust 1.7):

use std::io;

fn main() {
    let mut n = String::new();
    io::stdin()
        .read_line(&mut n)
        .expect("failed to read input.");
    let n: i32 = n.trim().parse().expect("invalid input");
    println!("{:?}", n);

    let mut n = String::new();
    io::stdin()
        .read_line(&mut n)
        .expect("failed to read input.");
    let n = n.trim().parse::<i32>().expect("invalid input");
    println!("{:?}", n);

    let mut n = String::new();
    io::stdin()
        .read_line(&mut n)
        .expect("failed to read input.");
    if let Ok(n) = n.trim().parse::<i32>() {
        println!("{:?}", n);
    }
}

These spare you the ceremony of pattern matching without depending on extra libraries.

Answer 4

parse is more or less the same; it’s read_line that’s unpleasant now.

use std::io;

fn main() {
    let mut s = String::new();
    io::stdin().read_line(&mut s).unwrap();

    match s.trim_right().parse::<i32>() {
        Ok(i) => println!("{} + 5 = {}", i, i + 5),
        Err(_) => println!("Invalid number."),
    }
}

Answer 5

You can create an extension method if you want a simple syntax:

use std::error::Error;
use std::io;
use std::str::FromStr;

trait Input {
    fn my_read<T>(&mut self) -> io::Result<T>
    where
        T: FromStr,
        T::Err: Error + Send + Sync + 'static;
}

impl<R> Input for R where R: io::Read {
    fn my_read<T>(&mut self) -> io::Result<T>
    where
        T: FromStr,
        T::Err: Error + Send + Sync + 'static,
    {
        let mut buff = String::new();
        self.read_to_string(&mut buff)?;

        buff.trim()
            .parse()
            .map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))
    }
}

// Usage:

fn main() -> io::Result<()> {
    let input: i32 = io::stdin().my_read()?;

    println!("{}", input);

    Ok(())
}

Answer 6

Here is a version with all optional type annotations and error handling which may be useful for beginners like me:

use std::io;

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

    let trimmed = input_text.trim();
    match trimmed.parse::<u32>() {
        Ok(i) => println!("your integer input: {}", i),
        Err(..) => println!("this was not an integer: {}", trimmed),
    };
}