Avatar of artemkorsakov

artemkorsakov's solution

to Collatz Conjecture in the Go Track

Published at Jan 31 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

To run the tests run the command go test from within the exercise directory.

If the test suite contains benchmarks, you can run these with the --bench and --benchmem flags:

go test -v --bench . --benchmem

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

Further information

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.

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.

cases_test.go

package collatzconjecture

// Source: exercism/problem-specifications
// Commit: d94e348 collatz-conjecture: fix capitalized description (#1292)
// Problem Specifications Version: 1.2.1

var testCases = []struct {
	description string
	input       int
	expectError bool
	expected    int
}{
	{
		description: "zero steps for one",
		input:       1,
		expected:    0,
	},
	{
		description: "divide if even",
		input:       16,
		expected:    4,
	},
	{
		description: "even and odd steps",
		input:       12,
		expected:    9,
	},
	{
		description: "large number of even and odd steps",
		input:       1000000,
		expected:    152,
	},
	{
		description: "zero is an error",
		input:       0,
		expectError: true,
	},
	{
		description: "negative value is an error",
		input:       -15,
		expectError: true,
	},
}

collatz_conjecture_test.go

package collatzconjecture

import (
	"testing"
)

func TestCollatzConjecture(t *testing.T) {
	for _, testCase := range testCases {
		steps, err := CollatzConjecture(testCase.input)
		if testCase.expectError {
			if err == nil {
				t.Fatalf("FAIL: %s\n\tCollatzConjecture(%v) expected an error, got %v",
					testCase.description, testCase.input, steps)
			}
		} else {
			if err != nil {
				t.Fatalf("FAIL: %s\n\tCollatzConjecture(%v) returns unexpected error %s",
					testCase.description, testCase.input, err.Error())
			}
			if steps != testCase.expected {
				t.Fatalf("FAIL: %s\n\tCollatzConjecture(%v) expected %v, got %v",
					testCase.description, testCase.input, testCase.expected, steps)
			}
		}
		t.Logf("PASS: %s", testCase.description)
	}
}

func BenchmarkCollatzConjecture(b *testing.B) {
	for i := 0; i < b.N; i++ {
		for _, testCase := range testCases {
			CollatzConjecture(testCase.input)
		}
	}
}
package collatzconjecture

import "errors"

func CollatzConjecture(n int) (int, error) {
	if n < 1 {
		return 0, errors.New("Invalid input")
	}
	if n == 1 {
		return 0, nil
	}
	if n%2 == 0 {
		res, _ := CollatzConjecture(n / 2);
		return res + 1, nil
	}
	res, _ := CollatzConjecture(3*n + 1);
	return res + 1, nil
}

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?