I have a function that reads a 2-column CSV file (using the csv crate) and loads each row into a BTreeMap. To do this, I need to move the Strings out of the Vec<String> that the reader gives me, and into the map. After an initial solution using into_iter, I settled on just pulling them out with mem:…

The most basic way to take ownership of a vector element is to remove it from the Vec. For that reason, this will be fine: let key = row.remove(0); let val = row.remove(0); This time of course adjusting the index of the second remove after how they shifted due to the first. It's also good …

IntoIterator + collect() is the proper way to do it but you'll need a custom iterator after into_iter() to group your elements 2 by 2. Have a look at the itertools crate. I think something like Tuples might be what you want.

Here's a quick and dirty version of the idea: use std::collections::HashMap; fn main() { let list: Vec<Vec<String>> = vec![vec!["foo".into(), "bar".into()], vec!["batz".into(), "bla".into()]]; let map = list.into_iter().map(|mut row| { (row.remove(0), row.remove(1)) }).collect::<HashM…

Yes, that works in my example, but that makes the rather specific assumption that we have a vector at hand. Not all indexable things implement it.

Well, @dimfeld specifically asked about Vec: [image] dimfeld: is there a more idiomatic way to take ownership of all the items in a Vec?

My initial solution used into_iter on the Vec to accomplish pretty much the same thing as drain. It just felt a little weird since for this really simple case I ended up just calling next twice instead of actually using the more functional-style Iterator functions. Using remove seems obvious now th…

Be careful with remove() though, it will shift all the elements after the one removed by one to the left. Repeated calls to remove(0) is a big code smell. drain() and into_iter() are a lot more efficient here.

Oh, for sure. My case here is somewhat special since the Vec will always have exactly 2 items in it. I was planning to use vec.remove(1); vec.remove(0); to avoid the shift, but into_iter definitely makes more sense for more general cases or larger vectors.

For your use case, swap_remove(0) is going to be what you want. It doesn't allocate two pointless strings as does mem::replace and it doesn't shift as with remove(). It should give you the same memcpy as doing remove in reverse, but offers the additional benefit of your code not looking like it is d…

Idiomatic way to take ownership of all items in a Vec<String>?

help

birkenfeld November 2, 2016, 6:02am 12

Small correction: String::new() doesn't allocate.

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Idiomatic way to take ownership of all items in a Vec<String>?

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