Lists and memory

Are lists atomic values?

Based on what we’ve already learned, they must not be–we can modify them in place! So, what happens when we create a list–say, a list of Coord’s?

> trees = [Coord(1, 3), Coord(3, 6)]

Directory	name	value
	`trees`	`loc 1002`
Memory	location	value
	`loc 1000`	`Coord(x=1, y=3)`
	`loc 1001`	`Coord(x=3, y=6)`
	`loc 1002`	`[loc 1000, loc 1001]`

Indexing into lists

What happens when we evaluate:

> print(trees[1].x)

List methods

> trees.append(Coord(5, 10))

First, we’re calling a data constructor so we add Coord(5, 10) to memory at loc 1003. Then we add it to the list!

Updates inside lists

What’s the difference between:

> trees[1].y = 4

> trees[1] = Coord(1, 4)

The first is updating the field on a Coord instance inside the list; the second is making a new Coord instance and putting it into the list. See the lecture capture for more details.

An example: ecology simulator

Imagine we’re doing a simulation of animal behavior patterns. We have a couple of classes:

@dataclass
class Habitat:
  food: int
  # could have other fields here--location, temperature, etc.

@dataclass
class Animal:
  time_since_food: int
  food_consumption: int
  habitat: Habitat

Each habitat is going to support multiple animals, who will consume the food in that habitat via this function:

def maybe_eat(animal: Animal):
  if animal.time_since_food > 8:
    animal.habitat.food = animal.habitat.food - animal.food_consumption
    animal.time_since_food = 0
  else:
    animal.time_since_food = animal.time_since_food + 1

Let’s say we wanted to create a couple of animals sharing the same habitat, which will start with 50 food. How would we do it?

Option 1:

> animal1 = Animal(0, 10, Habitat(50))
> animal2 = Animal(0, 20, Habitat(50))

Option 2:

> animal1 = Animal(0, 10, Habitat(50))
> animal2 = Animal(0, 20, animal1.habitat)

Option 3:

> habitat = Habitat(50)
> animal1 = Animal(0, 10, habitat)
> animal2 = Animal(0, 20, habitat)

Option 4:

> food = 50
> animal1 = Animal(0, 10, Habitat(food))
> animal2 = Animal(0, 20, Habitat(food))

We’ll talk about the differences between these options next lecture.