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brunnock's solution

to Collatz Conjecture in the JavaScript Track

Published at Aug 24 2018 · 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.

Setup

Go through the setup instructions for ECMAScript to install the necessary dependencies:

http://exercism.io/languages/ecmascript

Requirements

Install assignment dependencies:

$ npm install

Making the test suite pass

Execute the tests with:

$ npm test

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by changing xtest to test.

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.

collatz-conjecture.spec.js

import { steps } from './collatz-conjecture';

describe('steps()', () => {
  test('zero steps for one', () => {
    expect(steps(1)).toEqual(0);
  });

  xtest('divide if even', () => {
    expect(steps(16)).toEqual(4);
  });

  xtest('even and odd steps', () => {
    expect(steps(12)).toEqual(9);
  });

  xtest('Large number of even and odd steps', () => {
    expect(steps(1000000)).toEqual(152);
  });

  xtest('zero is an error', () => {
    expect(() => {
      steps(0);
    }).toThrow(new Error('Only positive numbers are allowed'));
  });

  xtest('negative value is an error', () => {
    expect(() => {
      steps(-15);
    }).toThrow(new Error('Only positive numbers are allowed'));
  });
});
function steps(n,nSteps=0) {
  if (n<1) {
    throw('Only positive numbers are allowed');
  } else if (n===1) {
    return(nSteps);
  } else if (n%2===0) {
    n/=2;
  } else {
    n = n*3 +1;
  }
  return steps(n, ++nSteps);
}

module.exports={steps}

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