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Comments from last section

• Do I play valorant
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• Recursion practice! (With helper methods)
• Efficiency (CS 61A's stance on efficiency)
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Temperature Check

• Recursion
• Tree Recursion
• map, filter, reduce
• for loops
• Lists
  • List Comprehensions
• Dictionaries

Tree Recursion

• Tree recursion is recursion but with two (or more!) recursive calls
• Useful when you need to break down a problem in more than 1 way
• Useful when there are multiple choices to deal with at one function call
• The recursive call diagram will expand similar to the roots of a tree

Example 1: Recursive Fibonacci

• Notice how this still follows the rules of recursion
  • We have base case(s)
  • We reduce our problem (fib(n - 1) and fib(n - 2))
  • We connect it together (with +)
• Often you combine things with +, -, *, / or some other function (max, min, etc).

Example 1: Recursive Fibonacci

You can also write down

Lists

• So far, we've only really been able to store one piece of data at a time
• A list allows you to store multiple pieces of data in 1 variable
• Lists can store any data type, and can be a mix of different data types
  • For example: [1, "hello", [2, "hi"]]
• Very useful for storing data/information

Creating Lists

In general, to create a list, you wrap something in square brackets

• For example: [1, 2, 3] creates a list.

(List) Slicing

Syntax is lst[<start index>:<end index>:<step>]; this creates a copy of all or part of your list (will be important later)

• start index is inclusive (if not included, it defaults to the first value)
• end index is exclusive (if not included, it defaults to the end of the list)
• step can be negative!

For loops

Example:

List Comprehensions

Very similar to for loops, but can be done in 1 line!

map, filter, reduce

• Used to manipulate sequences
• Makes copies of lists
• map applies a function to each element in a sequence
• filter chooses elements in a sequence based on a predicate
• reduce combines elements together based on a function

Dictionaries

• Maps keys to values
• Doesn't really have an order
• Access elements using keys rather than indices

Dictionaries

• Maps keys to values
• Doesn't really have an order
• Access elements using keys rather than indices
• Defined with curly braces ({})
  • {key: value}

Demo:

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