In this post I’ll try to solve the Collatz challenge using Marco.
The Hailstone Sequence and The Collatz Conjecture
Take any natural number n. If n is even, divide it by 2 to get n / 2. If n is odd, multiply it by 3 and add 1 to obtain 3n + 1. Repeat the process indefinitely. The Collatz conjecture is that no matter what number you start with, you shall always eventually reach 1.
For example, for n = 6:
6, 3, 10, 5, 16, 8, 4, 2, 1
Find the longest hailstone sequence for
n between 1 and 1000000 (1 million).
A hailstone sequence can be easily computer with a recursive function:
'(6 3 10 5 16 8 4 2 1)
Here we define
collatz to be a function that takes a number
n and generates the sequence for
n. It’s a fairly standard recursive function that constructs a list.
Let’s set the max value of 100 for now (1 million comes much later):
We can now generate all numbers from 1 to max:
And generate the sequence for each number:
Then we can calculate the size of each sequence:
And print the max:
There! Solved! Let’s go home! Or actually…
Not Just Yet
What if we increase the value of max-n ?
It works up until 900. 901 causes a stack overflow error. We are still very far from 1 million.
The reason for this is that most of these functions are recursive. They will keep stacking frames until it blows up. How do we solve this problem? I’d love to see some comments on this.
Extra: New Features
This sample code uses the following new features of Marco:
map: takes a function
fand a list
land returns a new list where each element is
fapplied to the corresponding element of
length: gets the length of a list
list-max: finds the maximum element in a number list
cons: takes two arguments
v2and creates a new pair.
Lists in Marco are built on top of pairs and nil: A list is either nil (empty list), or a pair where the second element is a list.
(cons 1 2) returns a pair, but
(cons 1 nil) is a one element list. To make a bigger list, you can use
(cons 1 (cons 2 (cons 3 nil))), which is what is used in the recursive call in