Integer stdin reading code

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#1

Hi everyone! I am quite new to Rust. I have been reading The Book, completing the rustlings exercises and would like to train with some 'codewars' and 'open.kattis' challenges.

Sometimes the code is required to read a given input from terminal. And when it is a number, usually I see this method which parses a string:

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

After this I can extract the numeric value, if there is one, and make calculations with it.

I wonder, though, if it is possible to not have this conversion step. But to expect and integer and store it directly, like it is possible with C, with only standard libraries?

#2

I usually start my kattis submissions with

let mut input = String::new();
io::stdin().read_to_string(&mut input).unwrap();
let mut input = input.split_ascii_whitespace();

then to read an integer, you just copy paste this:

let v: i32 = input.next().unwrap().parse().unwrap();

Note that the above does not care about newline vs space.

#3

To make fast solutios to kattis problems, I generally read the entire input into a Vec<u8> with stdin().read_to_end(...), and then parse numbers (or other symbols) directly from the buffer, one byte at a time.

Handling one byte at a time (if its in (b'0'..=b'9') do result = result * 10 + (byte - b'0'), if it's a whitespace (or whatever) return result) is gonna be a lot faster than first finding out how long subslice of the data is a number and then do subslice.parse(). Splitting the input in lines is also often wasted, as well as checking that the input is valid utf-8.

(Yes, my kattis solutions are a lot different than my other rust code. Still, I rarely use any unsafe code or raw pointers, so I still think my solutions benefit from being writen in Rust rather than c++).

#4

Hey @kaj what a nice performance you managed!

Those are some interesting ideas. Thank you for the quick response.