How to collect reference values from hashmap?

Question

I have written a code snippet to use HashMap<String, Vec<&String>> as the main data structure, and finally, I want to return a collected Vec<Vec<String>>, I can archive it by a for-loop, it there any pure iterator way to do this? (I have tried several times, but got tons of confused compiler errors)

pub fn group_anagrams(strs: Vec<String>) -> Vec<Vec<String>> {
    let mut map: HashMap<String, Vec<&String>> = HashMap::new();
    for s in &strs {
        let mut key = s.clone();
        unsafe {
            key.as_bytes_mut().sort();
        }
        (*map.entry(key).or_insert(vec![])).push(s);
    }
    let mut ans: Vec<Vec<String>> = Vec::new();
    // I don't like these 2 lines
    for v in map.values() {
        ans.push(v.iter().cloned().cloned().collect::<Vec<String>>())
    }
    ans
}

I also tried with this code:

    pub fn group_anagrams(strs: Vec<String>) -> Vec<Vec<String>> {
        let mut map: HashMap<String, Vec<String>> = HashMap::new();
        for s in strs {
            let mut key = s.clone();
            unsafe {
                key.as_bytes_mut().sort();
            }
            (*map.entry(key).or_insert(vec![])).push(s);
        }
        map.values().cloned().collect::<Vec<Vec<String>>>()
    }

In the last line, I want to move all the values from hashmap into result vector but also failed because of the lifetime error(then I gave up with a cloned vector), I am wondering how to make it.

Winston Ewert · Accepted Answer · 2021-04-11 04:00:29Z

    unsafe {
        key.as_bytes_mut().sort();
    }

This works as long as you are working in Ascii, but will have undefined behavior if there is any unicode in the string.

 (*map.entry(key).or_insert(vec![])).push(s);

You don't need the dereference *, calling a method will automatically take care of that.

Its a bit tricky to help with your main question since I don't know what exactly you tried and why that didn't work.

A pretty direct translation of what you wrote into iterator syntax works:

map.values()
    .map(|v| v.iter().cloned().cloned().collect::<Vec<String>>())
    .collect()

I would however suggest doing something more like your second code block, and avoid cloning the strings:

    map.values().cloned().collect::<Vec<Vec<String>>>()

I'd do this:

 map.into_iter()
    .map(|(_key, values)| values)
    .collect()

By consuming the iterator we avoid cloning the vectors.

Alternatively, take the strings as input by reference and return them by reference:

pub fn group_anagrams(strs: &[String]) -> Vec<Vec<&String>> {
    let mut map: HashMap<String, Vec<&String>> = HashMap::new();
    for s in strs {
        let mut key = s.clone();
        unsafe {
            key.as_bytes_mut().sort();
        }
        (*map.entry(key).or_insert(vec![])).push(s);
    }

    map.into_iter()
        .map(|(_key, values)| values)
        .collect()
}

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