Summation Over Lines in a File (Advent of Code 2018 Day 1)

Question

Preface: I am learning Rust through the Advent of Code

The task is to read lines from a file, parse each as an integer, then provide a summation of all the numbers. My solution looks like this:

use std::fs::File;
use std::io::{self, prelude::*, BufReader};
fn main() -> io::Result<()> {
 let file = File::open("day01.txt")?;
 let reader = BufReader::new(file);
 let mut sum: i32 = 0;
 for line in reader.lines() {
 let value: i32 = line.unwrap().parse().unwrap();
 sum += value
 }
 println!("{}", sum);
 Ok(())
}

I'd like to use the iterator methods map and sum in my solution, but I haven't figured out how to do so with the type checker. Suggestions?

Answer 1

That's how you use Iterator::sum:

use std::fs::File;
use std::io::{self, prelude::*, BufReader};
fn main() -> io::Result<()> {
 let file = File::open("day01.txt")?;
 let reader = BufReader::new(file);
 let sum: i32 = reader
 .lines()
 .map(|line| line.unwrap().parse::<i32>().unwrap())
 .sum();
 println!("{}", sum);
 Ok(())
}

You must first say to what type the line must be parsed: parse::<i32>() and then, you must give the sum a type because the output of the Add trait is not the same as self and other.