# Chapter 3: Recursion

* **every recursive function has two parts:**

  * base case
  * recursive case

  ex:

  ```
  def countdown(i):
    print i
    if i <=0:  //Base case
      return
    else: //recursive case
      countdown(i-1)
  ​
  print(countdown(5))
  ```
* **The call stack**

  ```
  ​
  def factorial(num):
    if num ==1:
      return 1
    else:
      return num * factorial(num-1)
  ​
  print(factorial(4))
  ```
* **Recap**
  * Recursion is when a function calls itself.
  * Every recursive function has two cases: the base case and the recursive case.
  * A stack has two operations: push and pop.
  * All function calls go onto the call stack.
  * The call stack can get very large, which takes up a lot of memory.
* **Stack**
  * Push(Add a new item to the top)
  * Pop(remove the topmost item and read it)