meet it

A hash can grow, change and shrink as you go.

Put a value at a key with =. If the key is new, the pair is added. If the key is already there, its value is replaced. .delete(:bread) takes a pair out, and .key?(:bread) asks whether a key is there.

Square brackets quietly give nil for a missing key, which can hide mistakes. looks a key up too, but stops with a if it is missing. Give fetch a second argument, a , and it hands that back instead: .fetch(:jam, 0).

stock = { milk: 4, bread: 2 }
stock[:eggs] = 12
stock[:milk] = 5
stock.delete(:bread)
p stock
p stock.key?(:bread)
puts stock.fetch(:jam, 0)

Use .fetch when a missing key would be a bug you want to hear about.

guess first

Add one, change one. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

scores = { amy: 3, ben: 5 }
scores[:amy] = scores[:amy] + 2
scores[:cal] = 1
p scores

your turn

Your turn. Restock the corner shop.

Add a new fruit and how many there are. Then fetch a fruit the shop does not stock, with a default of 0.

Your code

press run to see what happens

fix it

The till crashed looking for jam. Can you fix it?

Broken code to fix

press run to see what happens

Read the error. It is a KeyError on line 2. Which key is missing?
There is no :jam in the hash, so .fetch stops. You can give it a default to hand back instead.
Change line 2 to jam = stock.fetch(:jam, 0), then press run.

nailed it

your hashes can change their minds. :D

stock[:eggs] = 12 adds a pair or replaces a value.
.delete removes a pair and .key? checks for one.
.fetch(:jam, 0) gives a default when the key is missing.

new words in your collection

fetch
default
KeyError

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