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angelikatyborska's solution

to Collatz Conjecture in the Erlang Track

Published at Jan 19 2020 · 0 comments
Test suite


This exercise has changed since this solution was written.

The Collatz Conjecture or 3x+1 problem can be summarized as follows:

Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.

Given a number n, return the number of steps required to reach 1.


Starting with n = 12, the steps would be as follows:

  1. 12
  2. 6
  3. 3
  4. 10
  5. 5
  6. 16
  7. 8
  8. 4
  9. 2
  10. 1

Resulting in 9 steps. So for input n = 12, the return value would be 9.

Running tests

In order to run the tests, issue the following command from the exercise directory:

For running the tests provided, rebar3 is used as it is the official build and dependency management tool for erlang now. Please refer to the tracks installation instructions on how to do that.

In order to run the tests, you can issue the following command from the exercise directory.

$ rebar3 eunit


For detailed information about the Erlang track, please refer to the help page on the Exercism site. This covers the basic information on setting up the development environment expected by the exercises.


An unsolved problem in mathematics named after mathematician Lothar Collatz https://en.wikipedia.org/wiki/3x_%2B_1_problem

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


%% Based on canonical data version
%% Generated with 'testgen v0.1.0'
%% https://github.com/exercism/problem-specifications/raw/d94e348371af1d3efd8728a5b4e71d1b3d95c37e/exercises/collatz-conjecture/canonical-data.json
%% This file is automatically generated from the exercises canonical data.



'1_zero_steps_for_one_test_'() ->
    {"zero steps for one",
     ?_assertMatch(0, collatz_conjecture:steps(1))}.

'2_divide_if_even_test_'() ->
    {"divide if even",
     ?_assertMatch(4, collatz_conjecture:steps(16))}.

'3_even_and_odd_steps_test_'() ->
    {"even and odd steps",
     ?_assertMatch(9, collatz_conjecture:steps(12))}.

'4_large_number_of_even_and_odd_steps_test_'() ->
    {"large number of even and odd steps",
     ?_assertMatch(152, collatz_conjecture:steps(1000000))}.

'5_zero_is_an_error_test_'() ->
    {"zero is an error",
     ?_assertError(badarg, collatz_conjecture:steps(0))}.

'6_negative_value_is_an_error_test_'() ->
    {"negative value is an error",
     ?_assertError(badarg, collatz_conjecture:steps(-15))}.


steps(N) when is_integer(N), N > 0 -> steps(N, 0);
steps(_) -> erlang:error(badarg).

steps(1, Acc) -> Acc;
steps(N, Acc) when N rem 2 == 0 -> steps(N div 2, Acc + 1);
steps(N, Acc) -> steps(3 * N + 1, Acc + 1).

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