Group characters by number of occurrences in a string with Itertools - rust

Question

I have two approaches to grouping characters by the number of occurrences in a string. One of them is using std::collections::HashMap and the second one is using itertools::Itertools::group_by. Unfortunately, grouping with Itertools gives me undesirable results.
Example input word: "Barbara"
Using std::collections::HashMap
let map1 = word.to_lowercase()
.chars()
.fold(HashMap::new(), |mut acc, c| {
*acc.entry(c).or_insert(0) += 1;
acc
});
Result {'a': 3, 'b': 2, 'r': 2}
And using itertools::Itertools::group_by
let map2: HashMap<char, u32> = word.to_lowercase()
.chars()
.group_by(|&x| x)
.into_iter()
.map(|(k, v)| (k, v.count() as u32))
.collect();
Result {'r': 1, 'a': 1, 'b': 1}
Oddly enough, when the input string has identical characters in succession, Itertools takes those characters into account.
The question is, what makes it return different results?
Playground

Answer 1

You're looking for into_group_map_by. group_by only groups consecutive elements according to the docs.
use itertools::Itertools;
use std::collections::HashMap;
fn main() {
let word = "Barbara";
let map1 = word
.to_lowercase()
.chars()
.fold(HashMap::new(), |mut acc, c| {
*acc.entry(c).or_insert(0) += 1;
acc
});
println!("{:?}", map1);
let map2: HashMap<char, u32> = word
.to_lowercase()
.chars()
.into_group_map_by(|&x| x)
.into_iter()
.map(|(k, v)| (k, v.len() as u32))
.collect();
println!("{:?}", map2);
}
Output:
{'b': 2, 'a': 3, 'r': 2}
{'b': 2, 'r': 2, 'a': 3}
Playground
There's also into_grouping_map_by, which can be used for this like:
let map2: HashMap<char, u32> = word
.to_lowercase()
.chars()
.into_grouping_map_by(|&x| x)
.fold(0, |acc, _key, _value| acc + 1);

Answer 2

The documentation says (emphasis added):
fn group_by<K, F>(self, key: F) -> GroupBy<K, Self, F>
where
Self: Sized,
F: FnMut(&Self::Item) -> K,
K: PartialEq,
Return an iterable that can group iterator elements. Consecutive elements that map to the same key (“runs”), are assigned to the same group.
It only groups consecutive elements. You'll need to sort the characters before calling group_by.
let map2: HashMap<char, u32> = word.to_lowercase()
.chars()
.sorted()
.group_by(|&x| x)
...
Output:
{'a': 3, 'r': 2, 'b': 2}
{'b': 2, 'a': 3, 'r': 2}
Playground