Maps, sets and keyed records

A register often needs a mark by student ID rather than by position. A HashMap stores key-value associations, while a HashSet stores unique values. Keys must support equality and hashing so the collection can locate and distinguish them.

Insert associates a key with a value and replaces the previous value for the same key. Get returns an Option containing a reference. The entry API supports creating or updating a value without a separate lookup. Hash collections have no promised iteration order; sort a report explicitly or use an ordered collection such as BTreeMap.

Connect this idea with enums and optional values and result and recoverable errors.

Save the complete source as main.rs in the Rust environment. No standard input is needed.

Rust 2024 · complete program

use std::collections::{HashMap, HashSet};
fn main() {
  let mut marks = HashMap::new();
  marks.insert("S01", 80);
  *marks.entry("S01").or_insert(0) += 5;
  println!("S01: {}", marks["S01"]);
  println!("Missing: {:?}", marks.get("S02"));
  let attendees: HashSet<_> = ["S01", "S01", "S02"].into_iter().collect();
  println!("Unique attendees: {}", attendees.len());
}

Expected output

S01: 85
Missing: None
Unique attendees: 2

Follow the execution

The existing mark 80 is found through entry and updated to 85. S02 has no map entry, so get returns None. The set removes duplicate IDs according to equality, leaving two unique attendees. We print its length rather than an unstable iteration order.

Try the complete program in the Rust compiler. For tests, Cargo commands and local files, use the specified local environment.

Your task: Does inserting S01 again create a second separate mark under that key?

Rust 2024 · complete program

use std::collections::{HashMap, HashSet};
fn main() {
  let mut marks = HashMap::new();
  marks.insert("S01", 80);
  *marks.entry("S01").or_insert(0) += 5;
  println!("S01: {}", marks["S01"]);
  println!("Missing: {:?}", marks.get("S02"));
  let attendees: HashSet<_> = ["S01", "S01", "S02"].into_iter().collect();
  println!("Unique attendees: {}", attendees.len());
}

Worked answer

No. A map keeps one value per equal key. A new insert replaces that key’s previous value; entry can instead update the existing one.

Indexing a missing map key panics. The demonstrated S01 indexing is safe only because the program inserted it; use get for uncertain input. Hashing does not sort records and does not itself protect sensitive student data.

No. A map keeps one value per equal key. A new insert replaces that key’s previous value; entry can instead update the existing one. Indexing a missing map key panics.

Further reading: the official Rust Book on maps, sets and keyed records.