Summary - Collections

Combine HashMap and the Entry API to build a word counter.

Collections summary

HashMap: fast key-value, unordered. BTreeMap: sorted, range queries. HashSet/BTreeSet: unique values and set operations. With the entry API, "insert if missing, update if present" happens in one lookup: or_insert, or_default, and_modify. Iteration uses &, &mut or ownership; the output of unordered structures should be sorted before printing.

Grouping

BTreeMap and or_default together give a sorted grouping:

let mut gruplar: BTreeMap<char, Vec<&str>> = BTreeMap::new();
for k in "elma armut erik ayva kiraz".split_whitespace() {
    let ilk = k.chars().next().unwrap();
    gruplar.entry(ilk).or_default().push(k);
}
for (h, liste) in &gruplar {
    println!("{h}: {}", liste.join(", "));
}

Exercise Group by first letter

Group the words of "elma armut erik ayva kiraz" by first letter in a BTreeMap<char, Vec<&str>> (or_default). Print each group as letter: word, word on separate lines.

Show example solution

use std::collections::BTreeMap;

fn main() {
    let mut gruplar: BTreeMap<char, Vec<&str>> = BTreeMap::new();
    for k in "elma armut erik ayva kiraz".split_whitespace() {
        let ilk = k.chars().next().unwrap();
        gruplar.entry(ilk).or_default().push(k);
    }
    for (h, liste) in &gruplar {
        println!("{}: {}", h, liste.join(", "));
    }
}

Test yourself

Which value type best fits grouping words by first letter in a map?
- Vec<&str>
- &str
- usize
- bool
Show answer

Vec<&str> — Several words fall under each key; the entry(..).or_default().push(k) pattern builds this naturally.

Start with the default

gruplar.entry(ilk).____().push(k);

Show answer

gruplar.entry(ilk).or_default().push(k);

Group summary

HashMap

Fast, unordered
BTreeMap

Sorted keys
HashSet

Unique values
entry

Update in one lookup

Next lesson: Command skeleton

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