How can I read user input in Rust?

Question

I intend to make a tokenizer. I need to read every line the user types and stop reading once the user presses Ctrl + D.

I searched around and only found one example on Rust IO which does not even compile. I looked at the io module's documentation and found that the read_line() function is part of the ReaderUtil interface, but stdin() returns a Reader instead.

The code that I would like would essentially look like the following in C++:

vector<string> readLines () {
    vector<string> allLines;
    string line;

    while (cin >> line) {
        allLines.push_back(line);
    }

    return allLines;
}

_{This question refers to parts of Rust that predate Rust 1.0, but the general concept is still valid in Rust 1.0.}

Solution 1

In Rust 0.4, use the ReaderUtil trait to access the read_line function. Note that you need to explicitly cast the value to a trait type, for example, reader as io::ReaderUtil:

fn main() {
    let mut allLines = ~[];
    let reader = io::stdin();

    while !reader.eof() {
            allLines.push((reader as io::ReaderUtil).read_line());
    }

    for allLines.each |line| {
            io::println(fmt!("%s", *line));
    }
}

_{This answer predates Rust 1.0. Please see the other answers for modern solutions.}

Solution 2

Rust 1.x (see documentation):

use std::io;
use std::io::prelude::*;

fn main() {
    let stdin = io::stdin();
    for line in stdin.lock().lines() {
        println!("{}", line.unwrap());
    }
}

Rust 0.10–0.12 (see documentation):

use std::io;

fn main() {
    for line in io::stdin().lines() {
        print!("{}", line.unwrap());
    }
}

Rust 0.9 (see 0.9 documentation):

use std::io;
use std::io::buffered::BufferedReader;

fn main() {
    let mut reader = BufferedReader::new(io::stdin());
    for line in reader.lines() {
        print(line);
    }
}

Rust 0.8:

use std::io;

fn main() {
    let lines = io::stdin().read_lines();
    for line in lines.iter() {
        println(*line);
    }
}

Rust 0.7:

use std::io;

fn main() {
    let lines = io::stdin().read_lines();
    for lines.iter().advance |line| {
        println(*line);
    }
}

Solution 3

As of 17 April 2015, from mdcox on the Firefox Rust IRC channel.

use std::io;

fn main() {
    let mut stdin = io::stdin();
    let input = &mut String::new();

    loop {
        input.clear();
        stdin.read_line(input);
        println!("{}", input);
    }
}

Solution 4

In Rust 1.0 and later, you can use the lines method on anything that implements the std::io::BufRead trait to obtain an iterator over lines in the input. You could also use read_line , but using the iterator is more likely what you'd want. Here is a version of the function in the question using iterators; see below for a more detailed explanation. (playground link)

use std::io;
use std::io::prelude::*;

pub fn read_lines() -> Vec<String> {
    let stdin = io::stdin();
    let stdin_lock = stdin.lock();
    let vec = stdin_lock.lines().filter_map(|l| l.ok()).collect();

    vec
}

And here's a version that is more like the C++ version in the question, but is not really the idiomatic way to do this in Rust (playground):

use std::io;
use std::io::prelude::*;

pub fn read_lines() -> Vec<String> {
    let mut vec = Vec::new();
    let mut string = String::new();

    let stdin = io::stdin();
    let mut stdin_lock = stdin.lock();

    while let Ok(len) = stdin_lock.read_line(&mut string) {
        if len > 0 {
           vec.push(string);
           string = String::new();
        } else {
            break
        }
    }

    vec
}

To obtain something that implements BufRead, which is needed to call lines() or read_line(), you call std::io::stdin() to obtain a handle to standard input, and then call lock() on the result of that to obtain exclusive control of the standard input stream (you must have exclusive control to obtain a BufRead, because otherwise the buffering could produce arbitrary results if two threads were reading from stdin at once).

To collect the result into a Vec<String>, you can use the collect method on an iterator. lines() returns an iterator over Result<String>, so we need to handle error cases in which a line could not be read; for this example, we just ignore errors with a filter_map that just skips any errors.

The C++ like version uses read_line, which appends the read line to a given string, and we then push the string into our Vec. Because we transfer ownership of the string to the Vec when we do that, and because read_line would otherwise keep appending to the string, we need to allocate a new string for each loop (this appears to be a bug in the original C++ version in the question, in which the same string is shared and so will keep accumulating every line). We use while let to continue to read until we hit an error, and we break if we ever read zero bytes which indicates the end of the input.

Solution 5

The question is to read lines from standard input and return a vector. In Rust 1.7:

fn readlines() -> Vec<String> {
    use std::io::prelude::*;
    let stdin = std::io::stdin();
    let v = stdin.lock().lines().map(|x| x.unwrap()).collect();
    v
}

View more solutions

Comments

[Jimmy Lu]

I don't think I quite understand how cast works in Rust. :-( It doesn't seem all that similar to casts I'm used to in C/C++. What does it mean when you cast something to a trait type? Does it simply instruct the compilers to let you bind those methods from the trait type onto the thing you're casting from? (in this case, it's whatever returns from stdin()...)

[Bilal Husain]

I am not well familiar with Rust internals, but I guess the cast is for the type checker because the actual method to call is decided at runtime.

[Jimmy Lu]

hmm okay. This is the kind of solution that I wanted. So your answer is accepted :-) Thanks!

[robinst]

See The future of iterators in Rust on rust-dev for a discussion about how the for loop could become simpler in the future (basically get rid of .iter().advance).

[huon]

This can actually be for io::stdin().each_line |line| { .. }, or using extra::fileinput: for fileinput::input |line| { .. }

[weberc2]

The rust 1.0.0 version (top) no longer compiles (even when std::io is changed to std::old_io). :/

[robinst]

@weberc2 Yes, see this rust issue and RFC PR.

[yagooar]

When I do stdin.read_line(input);, the compiler warns me about this line being not used.

[gsgx]

@BeyondSora, it would be nice if you could accept the more upvoted answer below so it appears at the top.

[gsgx]

@yagooar, it should warn about the result not being used, not the line. That means reading from stdin might have resulted in an error but you're not checking for it.

[Peter Mortensen]

Rustfmt doesn't like it: "error: expected type, found ~ ... pub fn read () -> ~[int] {"