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katrinleinweber's solution

to Collatz Conjecture in the R Track

Published at Jul 13 2018 · 0 comments
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code 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.


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

Submitting Incomplete Solutions

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context("collatz conjecture")

test_that("Input of 1 results in 0 steps", {
  expect_equal(collatz_step_counter(1), 0)

test_that("Even input with repeated even steps", {
  expect_equal(collatz_step_counter(16), 4)

test_that("Input which results in small number of even and odd steps", {
  expect_equal(collatz_step_counter(12), 9)

test_that("Input which results in large number of even and odd steps", {
  expect_equal(collatz_step_counter(1000000), 152)

test_that("Input of 0 results in an error", {

test_that("Negative input results in an error", {

message("All tests passed for exercise: collatz-conjecture")
collatz_step_counter <- function(num) {
  # initialise counter
  steps <- 0
  # recurse through cases, after testing for base case
  if (num == 1)
  else if (num %% 2 == 0)
    1 + collatz_step_counter(num / 2)
  else if (num %% 2 != 0)
    1 + collatz_step_counter(num * 3 + 1)
  else  # if (num <= 0)
    stop("Num must be an integer larger than 0.")

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