Published at Mar 16 2019
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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.

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

- 12
- 6
- 3
- 10
- 5
- 16
- 8
- 4
- 2
- 1

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

You can run all the tests for an exercise by entering

```
$ gradle test
```

in your terminal.

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

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

```
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;
}
}
```

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?

## Community comments