CODEWITHSUNDEEP
ubuntucompilationrust
Amandasaurus
Amandasaurus

Reputation: 60759

Why can't rust compiler deduce from splitting stdin lines

I am very new to rust, and I'm trying to make a programme that reads stdin, splits each line based on the , character and prints the first item (a simple CSV-like / unix programme).

This is the programme and the compiler error is below.

use std::io;

fn main() {
    let mut stdin = io::stdin();

    for line in stdin.lines() {
        let fields = line.unwrap().as_slice().split_str(",").collect();
        print!("{}", fields[0]);
    }
}

And I get this error:

groupby.rs:12:22: 12:31 error: the type of this value must be known in this context
groupby.rs:12         print!("{}", fields[0]);
                               ^~~~~~~~~

I have no idea what's going on, nor why this doesn't work. Shouldn't it be obvious (to the compiler) that fields is now a Vec of strings(-type things)?

This is rustc 0.12.0-pre from the Ubuntu nightly PPA (201409260407~bb66281 )

Upvotes: 1

Views: 199

Answers (1)

Francis Gagné
Francis Gagné

Reputation: 65822

The problem comes from collect(). Iterator::collect()'s return type is generic: it can be any type that implements FromIterator<T>. Vec<T> is such a type. You simply need to add a type annotation:

use std::io;

fn main() {
    let mut stdin = io::stdin();

    for line in stdin.lines() {
        let line = line.unwrap(); // must bind the result of unwrap()
                                  // to fix a lifetime error
        let fields: Vec<&str> = line.as_slice().split_str(",").collect();
        print!("{}", fields[0]);
    }
}

Upvotes: 3

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