Frequency of an element in the vector

[Tanmayee]

To count the no.of times an element occurs in a vector.
For example :
let v = vec![1,1,1,1,2,2,3,3,4];

I would like to know the count of 1,2,3,4 as 4,2,2,1 respectively.

One approach I found was:
v.iter().filter(|&n| *n == 1).count()

If any other ways possible, please suggest me !!!

[H2CO3]

What's wrong with .filter().count()?

[sharshith]

How to get maximum repeated element in the vector using only functional programming or like without using any loops?

[drewkett]

Do you know the max value ahead of time?

[mbrubeck]

If you want to get all the frequencies in a single pass, you can store them in some sort of map, like a HashMap:

    let mut m: HashMap<i32, usize> = HashMap::new();
    for x in v {
        *m.entry(x).or_default() += 1;
    }

Then, iterate over the map to find the key with the highest value:

    let max = m.into_iter().max_by_key(|(_, v)| *v).map(|(k, _)| k);

(Playground)

If you don't like for loops, you can use fold instead, though I don't think this particularly makes it more "functional." It's just a different way of writing the same loop.

    let max = v.into_iter()
        .fold(HashMap::<i32, usize>::new(), |mut m, x| {
            *m.entry(x).or_default() += 1;
            m
        })
        .into_iter()
        .max_by_key(|(_, v)| *v)
        .map(|(k, _)| k);

(Playground)

[sharshith]

No