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8. Collections and iteration

Incan has Python-like collections and a familiar for loop.

Coming from Rust?

Incan’s built-in collections compile to the standard Rust collections:

  • list[T] / List[T] → Vec<T>
  • dict[K, V] / Dict[K, V] → HashMap<K, V>
  • set[T] / Set[T] → HashSet<T>

(See the prelude type mapping in the imports/modules reference for details.)

Lists

def main() -> None:
    xs = [1, 2, 3]
    println(f"first={xs[0]}")

Dicts

def main() -> None:
    scores = dict([("alice", 10), ("bob", 7)])
    println(f"alice={scores['alice']}")

Sets

Use a set to deduplicate values:

def main() -> None:
    names = ["Alice", "Bob", "Alice"]
    unique = set(names)
    println(f"unique_count={len(unique)}")

Iteration with for

def main() -> None:
    names = ["Alice", "Bob", "Cara"]
    for name in names:
        println(name)

Comprehensions (quick transforms)

Use comprehensions to build a new list/dict from an existing collection:

def main() -> None:
    names = [" Alice ", "Bob", " Cara "]

    normalized = [name.strip().lower() for name in names]
    counts = {name: 1 for name in normalized}

    println(f"normalized={normalized:?}")
    println(f"counts={counts:?}")

Try it

  1. Read a list of names, normalize (strip().lower()), and print them.
  2. Create a dict[str, int] of counts for how often each name appears.
  3. Use a set to print only unique names.
One possible solution 
def main() -> None:
    names = ["Alice", "Alice", "Bob", "Bob", "Cara"]

    # 1) Normalize + print
    normalized = [name.strip().lower() for name in names]
    for name in normalized:
        println(name)

    # 2) Count occurrences
    name_counts: Dict[str, int] = {}
    for name in normalized:
        current_count = name_counts.get(name).unwrap_or(0)
        name_counts[name] = current_count + 1

    # 3) Deduplicate + print counts (set iteration order is not guaranteed)
    unique = set(normalized)
    for name in unique:
        println(f"{name}: {name_counts.get(name).unwrap()}")

Where to learn more

Next

Back: 7. Strings and formatting

Next chapter: 9. Enums and better match