Maps

A map is like drawers with labels.

Each label is a key. Inside the drawer is a value.

You open a drawer by its label and get the value fast.

Keys must be types you can compare, like string or int.

Creating a map

Make a map with a literal or with make.

Write map[KeyType]ValueType.

Never write into a nil map. Create it first.

Writing and reading

Set a value with m[key] = value.

Read a value with m[key].

If the key is missing, you get the starter value for that value type.

Checking if a key exists

Use the two-value form: value, ok := m[key].

If ok is false, that drawer was empty. The key was missing.

This stops you from confusing a real 0 with a missing key.

delete

delete removes a key and its value.

It is safe even if the key was already gone.

Ranging over a map

range walks each key and value.

The order is not promised. Do not depend on a fixed order.

Create the map with make or a literal before you write into it.