Hash maps and entry-based aggregation

The idea

HashMap stores values under hashable keys. get returns an Option because a key may be absent; a stored zero is still a present value. The entry API combines lookup with insertion when necessary, avoiding a separate search-and-update pattern. or_insert returns a mutable reference to the existing or newly inserted value, so incrementing through that reference updates the map entry.

Hash-map iteration does not provide a sorted output contract. Collect and sort keys or use an ordered data structure when presentation order matters. Ownership still applies to inserted keys and values: moving a String key into a map transfers ownership, while borrowed lookups can often avoid another allocation. State whether an aggregation keeps empty groups, negative totals, and zero totals so the algorithm does not discard meaningful results.

use std::collections::HashMap;
fn main(){ let mut counts=HashMap::new(); for word in ["red","blue","red"] {*counts.entry(word).or_insert(0)+=1;} println!("{}",counts["red"]); println!("{}",counts.contains_key("green")); }

2
false

The first red inserts zero before incrementing; the second red updates the existing count.