Nesting

In the last lesson, you saw that sometimes it makes sense to model data using both Lists and Dictionaries -- putting Dictionaries inside Lists, or Lists inside Dictionaries.

"Putting things inside each other" is called nesting, and you've seen it before with nested conditionals (and maybe nested loops!). When Dictionaries and Lists get nested in each other, the syntax can be a bit confusing to read and write.

This lesson is just about code with nested dicts and lists.

Lists in Lists

Lists with lists inside are sometimes called 2D Arrays, since they can be said to have multiple "dimensions".

Here's a list of grayscale pixel brightness values for each row in a 9x9 image:

image = [
  [44, 181, 237, 0, 162, 110, 181, 121, 253],
  [106, 9, 210, 250, 135, 207, 139, 121, 237],
  [217, 36, 161, 224, 173, 183, 143, 109, 252],
  [23, 66, 136, 179, 10, 86, 9, 182, 14],
  [186, 99, 123, 213, 191, 45, 139, 146, 85],
  [214, 197, 214, 226, 39, 129, 249, 86, 215],
  [105, 182, 219, 190, 251, 15, 33, 147, 143],
  [216, 100, 234, 139, 14, 98, 249, 26, 144],
  [223, 141, 92, 194, 12, 158, 254, 48, 85]
]

To access the first row, you'd use image[0]. To access the first pixel in the first row, you'd use image[0][0].

Questions

How would you update the 4th pixel in the 5th row?

How would you loop through all the pixels in the last row?

How would you loop through all the pixels in all the rows?

Lists in Dicts

Dicts are useful for giving structure to data. Sometimes that structure might include Lists! In a previous example, we used a Dictionary to model the attributes for a Pokemon. If we wanted to keep track of the Pokemon's moves, we might keep a list of moves in the dict:

pikachu = {
  "name": "Pikachu",
  "health": 50,
  "attack": 27,
  "moves": [
    "charm",
    "growl",
    "quick attack",
    "thunder shock",
  ]
}

To access the list of moves, you would use pikachu["moves"]. To get the first move, you would use pikachu["moves"][0].

Questions

How would you access the third move?

How would you change the second move to "thunder wave"?

How would you loop through all the moves and print out each one?

Dicts in Lists

Often, there's a similar structure to add to many identical items. You can add the structure using Dictionaries, then keep all the items in a List.

In a previous lesson, we modeled restaurant information using a dict. Let's see what it would look like with a list of multiple restaurants:

restaurants = [
  {
    "name": "Chicken Republic", 
    "cuisine": "Fast food",
    "expense": "$",
    "rating": 3.7,
  },
  {
    "name": "Domino's Pizza", 
    "cuisine": "Pizza",
    "expense": "$$",
    "rating": 4.1,
  },
  {
    "name": "Cold Stone Creamery", 
    "cuisine": "Ice Cream",
    "expense": "$$",
    "rating": 4.5,
  }
]

To access the first restaurant, you'd use restaurants[0]. To access the name of that restaurant, you'd use restaurants[0]["name"].

The access syntax is similar! You have to think about whether the list or the dict comes first.

Questions

How would you access the cuisine of the second restaurant?

How would you update the expense of the last restaurant?

How would you loop through all the restaurants and print their names?

How would you loop through all the restaurants and print their names, but only if the rating is above 4?

Dicts in Dicts

Fairly often, there's structured information inside other structured information. For instance, we might want to know the effects of each of the pokemon's moves. Instead of a List, we could store a dictionary:

pikachu = {
  "name": "Pikachu",
  "health": 50,
  "attack": 27,
  "moves": {
    "charm": "lowers the opponent's attack",
    "growl": "lowers the opponent's attack",
    "quick attack": "deals damage to the opponent, with priority",
    "thunder shock": "deals damage to the opponent, and has a chance to paralyze the target",
  }
}

As before, you would access the moves using pikachu["moves"]. You would get the effect of the move "charm" with pikachu["moves"]["charm"].

Questions

How would you access the effect of the move "growl"?

How would you add the move "thunder wave" and its effect?

How would you remove the move "growl"?

How would you loop through all the moves and print out their names and effects?

If you wanted to keep more information about each move (like the type, level, and power), how would you structure the data?

Deeply Nested Structures

"Dicts in Lists in Dicts in Dicts" sounds like a line straight out of Dr. Seuss, but deeply nested structures show up all the time in real programs!

Let's see it applied to our restaurant example:

restaurants = [
  {
    "name": "Chicken Republic", 
    "cuisine": "Fast food",
    "expense": "$",
    "rating": 3.7,
    "menu": [
      {
        "name": "Express Meal with Chips",
        "price": 1900,
      },
      {
        "name": "Quarter Rotisserie Chicken",
        "price": 1500,
      }
    ]
  },
  {
    "name": "Domino's Pizza", 
    "cuisine": "Pizza",
    "expense": "$$",
    "rating": 4.1,
    "menu": [
      {
        "name": "Any Medium Classic + Free Drink",
        "price": 4000,
      },
      {
        "name": "Any Large Classic Thin Crust Pizza",
        "price": 5000
      }
    ]
  },
]

Accessing the name of the first item on the menu at the first restaurant:

restaurants[0]["menu"][0]['name']
# => "Express Meal with Chips"

It's sometimes helpful in these situations to use intermediate variables, so that you can keep track of what is what:

chicken_republic = restaurants[0]
cr_menu = chicken_republic["menu"]
express_meal = cr_menu[0]
express_meal["name"]
# => "Express Meal with Chips"

Keeping track of deep structure like this is hard! It's often helpful to use the Python repl to navigate step by step towards your target, building up the accessors as you go. Then, you can consolidate into one line of code if you want.

Questions

How would you access the price of the first item on the menu at the second restaurant?

How would you access the rating of the second restaurant?

How would you loop through all the menu items at the first restaurant to find the average price?

How would you loop through all the restaurants and find the average price of all the items on the menu for that restaurant?

Note that some of these are quite tricky!

Programming 1