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Rust II: data, choices and confidence · Lesson 07 / 12
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A map for each name

Counting equipment by name differs from looking it up by position. A HashMap associates each key with a value.

Learning paths12 min

Let's discover together

  1. 1

    use std::collections::HashMap brings the type name into scope. HashMap<String, u16> relates owned names to quantities. get looks up a key and may return None.

  2. 2

    entry(key).or_insert(0) returns a mutable reference to the existing value or inserts zero if absent. *count += 1 changes the value that reference points to.

  3. 3

    insert on an existing key replaces its value; it does not add automatically. Here entry accumulates deliberately. This small fixed dataset cannot reach the u16 limit.

  4. 4

    HashMap iteration order is not guaranteed. An interface or test needing stable order must sort keys, use BTreeMap or preserve a separate sequence. Here we only look up sensor.

FROM LESSON TO PRACTICE

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Read the steps before using the reference below. Commands run in the indicated system’s terminal; signup happens on the official site. Rullst Academy exercises are practice and do not verify your installation or external account.

use std::collections::HashMap;
fn main() {
    let mut counts = HashMap::new();
    for name in ["sensor", "sensor", "motor", "sensor"] {
        let count = counts.entry(String::from(name)).or_insert(0_u16);
        *count += 1;
    }
    println!("{}", counts["sensor"]);
}

Official links and next step

✦
YOUR MISSION

Accumulate counts with HashMap without relying on iteration order.

Now it's your turn

Discovery workshop

Read, predict, investigate

Predict before revealing. This panel shows prepared results; it does not compile your code. Compiler errors are summarised; exact wording can vary with the compiler version.
Mini experiment

Learning simulation: compare cards and check your predictions. The browser is not compiling Rust or starting a server.

use std::collections::HashMap;
fn main() {
    let mut counts = HashMap::new();
    for name in ["sensor", "sensor", "motor", "sensor"] {
        let count = counts.entry(String::from(name)).or_insert(0_u16);
        *count += 1;
    }
    println!("{}", counts["sensor"]);
}
Build your answers and check when you are ready.
A moment to understand

How can we obtain a predictably ordered list?

I need a hint

An observed order is not a HashMap contract.

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Take it with you

Add motor and count it separately. Then replace the accumulator with insert(name, 1), observe the defect and write a test that detects it.

RUST II · .rs

Your Rust II workbench

Read, predict and answer here. To actually compile, extract the activity ZIP into a new folder, open a terminal beside Cargo.toml and use the commands below. Rust and your operating system tools must be installed.

cargo check --offline
cargo test --offline
cargo run --offline
My files and GitHub

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