Collatz Conjecture

According to Wikipedia

The Collatz conjecture is a conjecture in mathematics named after Lothar Collatz. It concerns a sequence defined as follows: start with any positive integer n. Then each term is obtained from the previous term as follows: if the previous term is even, the next term is one half the previous term. Otherwise, the next term is 3 times the previous term plus 1. The conjecture is that no matter what value of n, the sequence will always reach 1.

Recursive Implementation

Mathematically, this problem is naturally recursive and this is one way you could implement it as such in Python.

def collatz(n, previous=None):
if previous is None:
previous = []
if n == 1:
return previous + [n]

next_number = n//2 if n % 2 == 0 else n*3 + 1
return collatz(next_number, previous+[n])

for n in range(1, 10):
print(collatz(n))

[2, 1]
[3, 10, 5, 16, 8, 4, 2, 1]
[4, 2, 1]
[5, 16, 8, 4, 2, 1]
[6, 3, 10, 5, 16, 8, 4, 2, 1]
[7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]
[8, 4, 2, 1]
[9, 28, 14, 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]

Curried Recursive Implementation

This shows an example of currying, where you take a function that takes multiple arguments, and break it up into two or more functions where each function acts on only one argument in isolation. In functional languages like F#, variadic functions (those that take multiple arguments) are automatically curried, but this is how we would go about doing it in Python.

What I like about this is that we no longer have to assign a default value to the previous parameter and check its value on each call to our recursive function (if previous is None) like in your previous recursive example.

def collatz(n):
def get(previous):
nonlocal n
if n == 1:
return previous + [n]
else:
previous += [n]
n = n//2 if n % 2 == 0 else n*3+1
return get(previous)
return get([])

for n in range(1, 10):
print(collatz(n))

[2, 1]
[3, 10, 5, 16, 8, 4, 2, 1]
[4, 2, 1]
[5, 16, 8, 4, 2, 1]
[6, 3, 10, 5, 16, 8, 4, 2, 1]
[7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]
[8, 4, 2, 1]
[9, 28, 14, 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]

Imperative Generator Implementation

Another way we can solve this problem is using Python generator functions. This example uses a while loop to evaluate new values.

from functools import partial

# this decorator will turn a generator function to one that returns a list
listify = partial(lambda generator: lambda arg: list(generator(arg)))
@listify
def collatz(n):
while n != 1:
yield n
n = n//2 if n % 2 == 0 else n*3 + 1

yield n

if n == 1:
return

for n in range(1, 10):
print(collatz(n))

[2, 1]
[3, 10, 5, 16, 8, 4, 2, 1]
[4, 2, 1]
[5, 16, 8, 4, 2, 1]
[6, 3, 10, 5, 16, 8, 4, 2, 1]
[7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]
[8, 4, 2, 1]
[9, 28, 14, 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]

Recursive Generator Implementation

Finally, this solution (my favorite) uses lazy evaluation and recursion to yield from itself until the base condition is met, that n == 1.

@listify
def collatz(n):
yield n

if n == 1:
return

next_number = n//2 if n % 2 == 0 else n*3 + 1

yield from collatz(next_number)

for n in range(1, 10):
print(collatz(n))

[2, 1]
[3, 10, 5, 16, 8, 4, 2, 1]
[4, 2, 1]
[5, 16, 8, 4, 2, 1]
[6, 3, 10, 5, 16, 8, 4, 2, 1]
[7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]
[8, 4, 2, 1]
[9, 28, 14, 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]