How to convert a String of strings to an Array/Vector of Strings in Rust

Question

I have a string from a CSV file that contains multiple lines:

Edit. Rust was not in the actual string.

header1,header2,header3
r1v1,r1v2,r1v3
r2v1,r2v2,r2v3

I am trying to push these into a Vec<Vec<String>>:

// the csv is raw_string
let lines = raw_string.lines();
let mut mainVec = Vec::new();

for row in lines {
    let mut childVec = Vec::new();
    let trimmed = row.trim();
    let values = trimmed.split(',').collect();
    childVec.push(values);
    mainVec.push(childVec.clone());
}

But I get the error

error[E0282]: unable to infer enough type information about `_`
 --> src/main.rs:9:13
  |
9 |         let values = trimmed.split(',').collect();
  |             ^^^^^^ cannot infer type for `_`
  |
  = note: type annotations or generic parameter binding required

@ildjarn Since the OP is "trying to push to a Vec<Vec<String>>", it seems they want a Vec<Vec<String>> (two Vecs, not three). — user4815162342
– user4815162342, Commented Mar 16, 2017 at 9:53
@user4815162342 : Ah, somehow I'd missed that bit. That said, getting rid of childVec should be part of the answer then. — ildjarn
– ildjarn, Commented Mar 16, 2017 at 9:56
@ildjarn No, you are completely right, sorry about that. The OP really does want a Vec<Vec<String>>, but their code is creating a Vec<Vec<Vec<String>>> with an unnecessary one-element vector in the middle. — user4815162342
– user4815162342, Commented Mar 16, 2017 at 13:06
The true answer is to use the csv crate because parsing CSV is harder than everyone thinks it is. — Shepmaster
– Shepmaster, Commented Mar 16, 2017 at 14:10

VP. · Accepted Answer · 2017-03-16 15:00:04Z

5

Another solution, based on iterators:

fn main() {
    let raw_string = r"rust
header1,header2,header3
r1v1,r1v2,r1v3
r2v1,r2v2,r2v3";

    let main_vec = raw_string.lines()
                             .map(|s| s.trim().split(',').map(String::from).collect::<Vec<String>>())
                             .collect::<Vec<Vec<String>>>();

    print!("{:?}", main_vec);
}

As @Simon Whitehead has already said, the the only thing you need to do with collect() is to specify the type because this is a generic method. But it may also be deduced by the compiler itself in some circumstances.

The code above is pretty verbose about type specification: actually you may leave the Vec's value type unspecified and let it be deduced by the compiler like this:

let main_vec = raw_string.lines()
                         .map(|s| s.trim().split(',').map(String::from).collect::<Vec<_>>())
                         .collect::<Vec<_>>();

For more information, see the definition of collect() method.

edited Mar 16, 2017 at 15:00

answered Mar 16, 2017 at 11:29

VP.

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3 Comments

ildjarn Over a year ago

OP wants a Vec<Vec<String>> rather than a Vec<Vec<&str>> so after the split there should be a .map(String::from) or somesuch.

mjwrazor Over a year ago

@ildjarn True that seems to be my problem, I can't figure out the right spot to drop in either the String::from() or the .to_string() methods.

ildjarn Over a year ago

@mjwrazor : What Victor has at the end, plus a .map(String::from) after the split: .map(|s| s.trim().split(',').map(String::from).collect::<Vec<_>>()). (Other options are available e.g. .map(|s| s.trim().split(',').map(ToString::to_string).collect::<Vec<_>>()), but I think using From reads best.)

Shepmaster · Accepted Answer · 2017-03-16 03:04:12Z

3

You need to give the compiler a little hint as to what you want values to be.

You can say you want it to be a vector of something:

let values: Vec<_> = trimmed.split(',').collect();

Working example in the playground

A few other notes about the code, if you're interested.

The variable names should be snake_case. So the vectors should be called main_vec and child_vec.
The child_vec does not need to be cloned when pushing it to the main_vec. Pushing it to main_vec transfers ownership and the loop redeclares the child_vec and so the compiler can guarantee nothing has been violated here.

edited Mar 16, 2017 at 3:04

Shepmaster

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answered Mar 16, 2017 at 3:00

Simon Whitehead

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1 Comment

mjwrazor Over a year ago

This solution also works. gist

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