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brunnock's solution

to Prime Factors in the JavaScript Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

Note:

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.

Compute the prime factors of a given natural number.

A prime number is only evenly divisible by itself and 1.

Note that 1 is not a prime number.

Example

What are the prime factors of 60?

  • Our first divisor is 2. 2 goes into 60, leaving 30.
  • 2 goes into 30, leaving 15.
    • 2 doesn't go cleanly into 15. So let's move on to our next divisor, 3.
  • 3 goes cleanly into 15, leaving 5.
    • 3 does not go cleanly into 5. The next possible factor is 4.
    • 4 does not go cleanly into 5. The next possible factor is 5.
  • 5 does go cleanly into 5.
  • We're left only with 1, so now, we're done.

Our successful divisors in that computation represent the list of prime factors of 60: 2, 2, 3, and 5.

You can check this yourself:

  • 2 * 2 * 3 * 5
  • = 4 * 15
  • = 60
  • Success!

Setup

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

http://exercism.io/languages/javascript/installation

Running the test suite

The provided test suite uses Jasmine. You can install it by opening a terminal window and running the following command:

npm install -g jasmine

Run the test suite from the exercise directory with:

jasmine prime-factors.spec.js

In many test suites all but the first test have been marked "pending". Once you get a test passing, activate the next one by changing xit to it.

Source

The Prime Factors Kata by Uncle Bob http://butunclebob.com/ArticleS.UncleBob.ThePrimeFactorsKata

Submitting Incomplete Solutions

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

prime-factors.spec.js

var primeFactors = require('./prime-factors');

describe('primeFactors', function () {
  it('returns an empty array for 1', function () {
    expect(primeFactors.for(1)).toEqual([]);
  });

  xit('factors 2', function () {
    expect(primeFactors.for(2)).toEqual([2]);
  });

  xit('factors 3', function () {
    expect(primeFactors.for(3)).toEqual([3]);
  });

  xit('factors 4', function () {
    expect(primeFactors.for(4)).toEqual([2, 2]);
  });

  xit('factors 6', function () {
    expect(primeFactors.for(6)).toEqual([2, 3]);
  });

  xit('factors 8', function () {
    expect(primeFactors.for(8)).toEqual([2, 2, 2]);
  });

  xit('factors 9', function () {
    expect(primeFactors.for(9)).toEqual([3, 3]);
  });

  xit('factors 27', function () {
    expect(primeFactors.for(27)).toEqual([3, 3, 3]);
  });

  xit('factors 625', function () {
    expect(primeFactors.for(625)).toEqual([5, 5, 5, 5]);
  });

  xit('factors 901255', function () {
    expect(primeFactors.for(901255)).toEqual([5, 17, 23, 461]);
  });

  xit('factors 93819012551', function () {
    expect(primeFactors.for(93819012551)).toEqual([11, 9539, 894119]);
  });
});
module.exports.for = function(num) {
    var factors=[];
    for (var factor=2; factor<=num/2; ++factor) {
      while (!(num % factor)) {
	factors.push(factor);
	num /= factor;
      }
      if (num==1) break;
    }
    if (num>1) factors.push(num);
    return factors;
}

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