在循环中插入哈希图

Question

I'm opening a CSV file and reading it using BufReader and splitting each line into a vector. Then I try to insert or update the count in a HashMap using a specific column as key.

let mut map: HashMap<&str, i32> = HashMap::new();

let reader = BufReader::new(input_file);
for line in reader.lines() {
    let s = line.unwrap().to_string();
    let tokens: Vec<&str> = s.split(&d).collect(); // <-- `s` does not live long enough
    if tokens.len() > c {
        println!("{}", tokens[c]);
        let count = map.entry(tokens[c].to_string()).or_insert(0);
        *count += 1;
    }
}

The compiler kindly tells me s is shortlived. Storing from inside a loop a borrowed value to container in outer scope? suggests "owning" the string, so I tried to change

let count = map.entry(tokens[c]).or_insert(0);

to

let count = map.entry(tokens[c].to_string()).or_insert(0);

but I get the error

expected `&str`, found struct `std::string::String`
help: consider borrowing here: `&tokens[c].to_string()`

When I prepend ampersand ( & ) the error is

creates a temporary which is freed while still in use
note: consider using a `let` binding to create a longer lived

There is some deficiency in my Rust knowledge about borrowing. How can I make the hashmap own the string passed as key?

Answer 1

The easiest way for this to work is for your map to own the keys. This means that you must change its type from HasMap<&str, i32> (which borrows the keys) to HashMap<String, i32> . At which point you can call to_string to convert your tokens into owned strings:

let mut map: HashMap<String, i32> = HashMap::new();

let reader = BufReader::new(input_file);
for line in reader.lines() {
    let s = line.unwrap().to_string();
    let tokens:Vec<&str> = s.split(&d).collect();
    if tokens.len() > c {
        println!("{}", tokens[c]);
        let count = map.entry(tokens[c].to_string()).or_insert(0);
        *count += 1;
    }
}

Note however that this means that tokens[c] will be duplicated even if it was already present in the map. You can avoid the extra duplication by trying to modify the counter with get_mut first, but this requires two lookups when the key is missing:

let mut map: HashMap<String, i32> = HashMap::new();

let reader = BufReader::new(input_file);
for line in reader.lines() {
    let s = line.unwrap().to_string();
    let tokens:Vec<&str> = s.split(&d).collect();
    if tokens.len() > c {
        println!("{}", tokens[c]);
        if let Some (count) = map.get_mut (tokens[c]) {
            *count += 1;
        } else {
            map.insert (tokens[c].to_string(), 1);
        }
    }
}

I don't know of a solution that would only copy the key when there was no previous entry but still do a single lookup.