HashMap

HashMaps are like Arrays, but where you store data by keys of your choice, instead of by index 0, 1, 2, 3. To create a HashMap, you specify both the key and value of each element. Here's an example:

You can read from a HashMap like this:

You can write to it like this:

You can also insert an element like this:

You can delete an element like this, but it's uncommon:

HashMaps don't have to shift elements over when you insert/delete, unlike the Array. This is because the position of every element in a HashMap is independent from all the other elements - see the summary on how elements in HashMaps are stored. This means all actions on a HashMap are fast, taking $O(1)$ time on average.

Read/Write Complexity $O(1)$

Insert/Delete Complexity $O(1)$

Set

Sets are just HashMaps where you don't specify values. You only specify keys. Behind the scenes, they essentially just create a HashMap where every value is True. They're just for convenience - you could use a HashMap instead if you felt like it.

Here's how you create a Set:

Here's more syntax for Sets:

Read Complexity $O(1)$

Add/Remove Complexity $O(1)$

Important: Python only allows the keys of HashMaps and Sets to be immutable - they can be strings, numbers, or tuples, but they can't be mutable types like Arrays. Python thought it would be too confusing to let you use an object as a key and then change it later.