Step 6 of 7

Reading all the input at once

Sometimes it's simplest to read the whole input into one String first, then take it apart. io::read_to_string does that:

use std::io;

fn main() {
    let input = io::read_to_string(io::stdin()).expect("failed to read input");
    let mut total = 0;
    let mut count = 0;
    for word in input.split_whitespace() {
        let n: i32 = word.parse().expect("not a number");
        total += n;
        count += 1;
    }
    println!("{count} numbers, total {total}");
}

3 4
5
  6 7

5 numbers, total 25

• io::read_to_string(io::stdin()) reads everything up to the end of the input, newlines included, and returns it as a String (wrapped in a Result, so .expect again).
• split_whitespace on the whole input treats newlines like spaces, so you get every value no matter how the lines are laid out. Perfect when only the values matter.
• When the lines matter, input.lines() splits the String into lines (without the newlines), just like io::stdin().lines() but with no Result to unwrap:

use std::io;

fn main() {
    let input = io::read_to_string(io::stdin()).expect("failed to read input");
    for line in input.lines() {
        println!("{} -> {} words", line, line.split_whitespace().count());
    }
}

the quick brown fox
jumps
over the dog

the quick brown fox -> 4 words
jumps -> 1 words
over the dog -> 3 words

You may also meet the older form io::stdin().read_to_string(&mut input), which needs use std::io::Read; (another trait, like BufRead).

Which way to read?

read_to_string keeps the whole input in memory. That's fine for kilobytes or megabytes; for a 10 GB log file, go line by line.

Common mistakes

• Parsing the whole input as one number. The input ends with a newline (and may hold several values), so input.parse() fails. Use input.trim() for a single value, or split_whitespace() for many.
• Forgetting that the input might be empty. With no input, read_to_string gives an empty String; a loop over it simply runs zero times. Make sure your program still prints something sensible.

Your task

Read the whole input: whole numbers (possibly negative) spread over any number of lines. Print their average with two decimals, or no numbers if there are none:

average: 5.00

Challenge 2

Write a tiny version of the Unix wc tool: read the whole input and print the number of lines, words and characters (count every character of the input, including the newlines):

lines: 2
words: 6
characters: 29

Challenge 3

Read the whole input and print its lines in reverse order (last line first):

third
second
first

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