Avatar of artemkorsakov

artemkorsakov's solution

to Collatz Conjecture in the Java Track

Published at Mar 16 2019 · 0 comments
Instructions
Test suite
Solution

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.

Examples

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 the tests

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.

Source

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.

CollatzCalculatorTest.java

import org.junit.Before;
import org.junit.Ignore;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

import static org.junit.Assert.assertEquals;

public class CollatzCalculatorTest {

    @Rule
    public ExpectedException expectedException = ExpectedException.none();

    private CollatzCalculator collatzCalculator;

    @Before
    public void setUp() {
        collatzCalculator = new CollatzCalculator();
    }

    @Test
    public void testZeroStepsRequiredWhenStartingFrom1() {
        assertEquals(0, collatzCalculator.computeStepCount(1));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCorrectNumberOfStepsWhenAllStepsAreDivisions() {
        assertEquals(4, collatzCalculator.computeStepCount(16));
    }

    @Ignore("Remove to run test")
    @Test
    public void testCorrectNumberOfStepsWhenBothStepTypesAreNeeded() {
        assertEquals(9, collatzCalculator.computeStepCount(12));
    }

    @Ignore("Remove to run test")
    @Test
    public void testAVeryLargeInput() {
        assertEquals(152, collatzCalculator.computeStepCount(1000000));
    }

    @Ignore("Remove to run test")
    @Test
    public void testZeroIsConsideredInvalidInput() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Only natural numbers are allowed");

        collatzCalculator.computeStepCount(0);
    }

    @Ignore("Remove to run test")
    @Test
    public void testNegativeIntegerIsConsideredInvalidInput() {
        expectedException.expect(IllegalArgumentException.class);
        expectedException.expectMessage("Only natural numbers are allowed");

        collatzCalculator.computeStepCount(-15);
    }

}
class CollatzCalculator {

    int computeStepCount(int start) {
        if (start < 1) {
            throw new IllegalArgumentException("Only natural numbers are allowed");
        }

        int count = 0;
        while (start != 1) {
            start = start % 2 == 0 ? start / 2 : 3 * start + 1;
            count++;
        }
        return count;
    }

}

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?