Borrowed value does not live long enough problem in for

help
1

Hi everyone! I have the problem with the next snippet. I am trying to read a line from a text file and count how many times the words are repeated.

use std::collections::HashMap;
use std::fs::File;
use std::io;
use std::io::BufRead;
use std::path::Path;

fn read_lines<P>(filename: P) -> io::Result<io::Lines<io::BufReader<File>>>
    where P: AsRef<Path>, {
    let file = File::open(filename)?;
    Ok(io::BufReader::new(file).lines())
}

fn main(){
    println!("Contador de palabras");

    let lines = read_lines("./palabras.txt");

    match lines {
        Ok(lines) => {
            let  mut words_matches: HashMap<&str, i32> = HashMap::new();
            for line in lines {

                if let Ok(text) = line {
                    let lower_case_text: String = text.to_lowercase();
                    let words = lower_case_text.split(" ");
                    for word in words {
                        //https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.entry
                        let matches = words_matches.entry(word).or_insert(0);
                        *matches += 1;
                    }
                    println!("{:?}", words_matches);
                }
            }
        },
        Err(err) => println!("Error, {}", err)
    }

}

The compiler throw the next error:

                 let words = lower_case_text.split(" ");
   |                                 ^^^^^^^^^^^^^^^ borrowed value does not live long enough
...
28 |                         let matches = words_matches.entry(word).or_insert(0);
   |                                       ------------- borrow later used here
...
32 |                 }
   |                 - `lower_case_text` dropped here while still borrowed

I'm confused about the borrowing here. I don't see how freeing lower_case_text is a problem in the hashmap.

Thanks!

2

Your HashMap takes its keys by reference, instead of owning it, but on every new line iteration the previos references are dropped. If you use String instead of &str, that would be fixed.

1 Like
3

Everything inside this block uses a temporary String which is dropped at the end of the block. words is an iterator over the String, and so each word is a borrowed &str with the lifetime of lower_case_text. These word references cannot outlive the if let block, which means you cannot use them as keys in the hash map.

The easiest fix is to have words_matches own its keys. Without borrows there are no lifetimes, and thus no lifetime issues.

let mut words_matches: HashMap<String, i32> = HashMap::new();

for word in words {
    words_matches.entry(word.to_owned())
        .and_modify(|m| { *m += 1 })
        .or_insert(0);
}
4

Awesome!! thank you for your time, it works!!!

5

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