Lesson 6 — Lists and hashes

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What are we learning?

A list is an ordered row of values: first, second, third, …

A hash is a collection of labeled values: each item has a name (a key) and a value.

Lists

nums: list = [1, 2, 3];
say(nums[0]);
nums.push(4);
say(nums);

Output

1
[1, 2, 3, 4]

What this does

Line	Meaning
nums: list = [1, 2, 3];	Create a list with three numbers
nums[0]	The first item (counting starts at 0)
nums.push(4);	Add 4 at the end
say(nums);	Print the whole list

Hashes

user: hash = { name: "Ada", active: true };
say(user["name"]);
user["role"] = "admin";
say(user);

Output

Ada
{"name": "Ada", "active": true, "role": "admin"}

• { name: "Ada", active: true } — labels name and active.
• user["name"] — read the value for label name.
• user["role"] = "admin"; — add or change a label.

Try it

Make a list of three favorite foods (as text). Print the first one, then push a fourth food and print the list.

Predict it

xs: list = ["a", "b"];
say(xs[1]);

Show answer

b

Fix it — wrong index

Lists start at 0. For [10, 20, 30], the last item is index 2, not 3. Asking for a missing index stops the program with an error.

Deeper (later)

Full method lists (map, filter, ensure, …) live in Lists (methods) and Hashes (methods).

Next

→ Lesson 7 — Strings