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to Prime Factors in the C# Track

Published at Feb 11 2019 · 0 comments
Instructions
Test suite
Solution

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!

Running the tests

To run the tests, run the command dotnet test from within the exercise directory.

Initially, only the first test will be enabled. This is to encourage you to solve the exercise one step at a time. Once you get the first test passing, remove the Skip property from the next test and work on getting that test passing. Once none of the tests are skipped and they are all passing, you can submit your solution using exercism submit PrimeFactors.cs

Further information

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

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.

PrimeFactorsTest.cs

// This file was auto-generated based on version 1.1.0 of the canonical data.

using Xunit;

public class PrimeFactorsTest
{
    [Fact]
    public void No_factors()
    {
        Assert.Empty(PrimeFactors.Factors(1));
    }

    [Fact(Skip = "Remove to run test")]
    public void Prime_number()
    {
        Assert.Equal(new[] { 2 }, PrimeFactors.Factors(2));
    }

    [Fact(Skip = "Remove to run test")]
    public void Square_of_a_prime()
    {
        Assert.Equal(new[] { 3, 3 }, PrimeFactors.Factors(9));
    }

    [Fact(Skip = "Remove to run test")]
    public void Cube_of_a_prime()
    {
        Assert.Equal(new[] { 2, 2, 2 }, PrimeFactors.Factors(8));
    }

    [Fact(Skip = "Remove to run test")]
    public void Product_of_primes_and_non_primes()
    {
        Assert.Equal(new[] { 2, 2, 3 }, PrimeFactors.Factors(12));
    }

    [Fact(Skip = "Remove to run test")]
    public void Product_of_primes()
    {
        Assert.Equal(new[] { 5, 17, 23, 461 }, PrimeFactors.Factors(901255));
    }

    [Fact(Skip = "Remove to run test")]
    public void Factors_include_a_large_prime()
    {
        Assert.Equal(new[] { 11, 9539, 894119 }, PrimeFactors.Factors(93819012551));
    }
}
using System.Collections.Generic;

public static class PrimeFactors
{
    public static int[] Factors(long number)
    {
        var result = new List<int>();
        var i = 2;
        while (number != 1)
        {
            if (number % i == 0)
            {
                result.Add(i);
                number = number / i;
            }
            else
            {
                i++;
            }
        }
        return result.ToArray();
    }
}

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