Day 23: Scope and Recursion

Quick Answer

What You Will Learn

• global count is needed inside a function to change a global variable; without it you get UnboundLocalError.
• fact(n) has base case n <= 1, where it stops. Without a base case recursion never ends.
• fact(5) = 5 * 4 * 3 * 2 * 1 = 120.
• @lru_cache remembers past answers (memoization), which makes fib fast.
• Recursion suits tree-like and nested problems; for simple repetition a loop is enough.

Code

count = 0

def inc():
    global count
    count += 1
    return count

print(inc())

def fact(n):
    if n <= 1:
        return 1
    return n * fact(n - 1)

print(fact(5))

from functools import lru_cache

@lru_cache
def fib(n):
    if n < 2:
        return n
    return fib(n - 1) + fib(n - 2)

print(fib(10))

Output

1
120
55

Code Explained, Line by Line

1. global count is needed inside a function to change a global variable; without it you get UnboundLocalError.
2. fact(n) has base case n <= 1, where it stops. Without a base case recursion never ends.
3. fact(5) = 5 * 4 * 3 * 2 * 1 = 120.
4. @lru_cache remembers past answers (memoization), which makes fib fast.
5. Recursion suits tree-like and nested problems; for simple repetition a loop is enough.

Real-Life Use

File trees, JSON data and algorithms like binary search use recursion.

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