How to iterate and extract values out of a for loop in Rust

Question

I'm new to Rust and looking to understand concepts like borrowing. I'm trying to create a simple two dimensional array using standard input. The code:

use std::io;

fn main() {
    let mut values = [["0"; 6]; 6]; // 6 * 6 array

    // iterate 6 times for user input
    for i in 0..6 {
        let mut outputs = String::new();
        io::stdin().read_line(&mut outputs).expect(
            "failed to read line",
        );

        // read space separated list 6 numbers. Eg: 5 7 8 4 3 9
        let values_itr = outputs.trim().split(' ');
        let mut j = 0;
        for (_, value) in values_itr.enumerate() {
            values[i][j] = value;
            j += 1;
        }
    }
}

This won't compile because the outputs variable lifetime is not long enough:

error[E0597]: `outputs` does not live long enough
  --> src/main.rs:20:5
   |
14 |         let values_itr = outputs.trim().split(' ');
   |                          ------- borrow occurs here
...
20 |     }
   |     ^ `outputs` dropped here while still borrowed
21 | }
   | - borrowed value needs to live until here

How can I get the iterated values out of the block into values array?

bluss · Accepted Answer · 2016-01-07 15:37:41Z

7

split() gives you substrings (string slices) borrowed from the original string, and the original string is outputs from line 6.

The string slices can't outlive the scope of outputs: when a loop iteration ends, outputs is deallocated. Since values is longer lived, the slices can't be stored there.
We can't borrow slices of outputs across a modification of outputs. So even if the String outputs itself was defined before values, we couldn't easily put the string slices from .split() into values; modifying the string (reading into it) invalidates the slices.

A solution needs to either

Use a nested array of String, and when you assign an element from the split iterator, make a String from the &str using .to_string(). I would recommend this solution. (However an array of String is not at as easy to work with, maybe already this requires using Vec instead.) ¹
Read all input before constructing a nested array of &str that borrows from the input String. This is good if the nested array is something that you only need temporarily.

¹: You can use something like vec![vec![String::new(); 6]; 6] instead

edited Jan 7, 2016 at 15:37

answered Jan 7, 2016 at 10:34

bluss

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2 Comments

vim Over a year ago

It would be ideal if I can solve this using arrays. But seems there is no way to create an array of Strings in Rust. Isn't it? First method works fine with the Vectors. Thanks for the answer.. (including your last remarks on IRC) "value is a &str, a substring from outputs. so .to_string() allocates space for a String and copies the substring into that. so value was a &str (a reference) but is turned into a String. since it's not a reference it's not restricted by the borrow checker or the life of outputs."

bluss Over a year ago

There's no nice way to create an array of strings.

[[String::new(), String::new(), String::new(), String::new(), String::new(), String::new()], [String::new(), String::new(), String::new(), String::new(), String::new(), String::new()], ...]

certainly isn't nice.

Shepmaster · Accepted Answer · 2016-01-10 14:49:41Z

1

This answer was moved from the question, where it solved the OPs needs.

use std::io;

fn main() {
    let mut values = vec![vec![String::new(); 6]; 6];
    for i in 0..6 {
        let mut outputs = String::new();
        io::stdin().read_line(&mut outputs)
                .expect("failed to read line");

        let values_itr = outputs.trim().split(' ');
        let mut j = 0;
        for (_, value) in values_itr.enumerate() {
            values[i][j] = value.to_string();
            j += 1;
        }
    }
}

answered Jan 10, 2016 at 14:49

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