Exercism v3 launches on Sept 1st 2021. Learn more! ๐๐๐

Published at Sep 04 2020
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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.

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!

In order to run the tests for this track, you will need to install DUnitX. Please see the installation instructions for more information.

If Delphi is properly installed, and `*.dpr`

file types have been associated with Delphi, then double clicking the supplied `*.dpr`

file will start Delphi and load the exercise/project. `control + F9`

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will compile and run the project.

Alternatively you may opt to start Delphi and load your project via. the `File`

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We monitor the Pascal-Delphi support room on gitter.im to help you with any questions that might arise.

Note that, when trying to submit an exercise, make sure the exercise file you're submitting is in the `exercism/delphi/<exerciseName>`

directory.

For example, if you're submitting `ubob.pas`

for the Bob exercise, the submit command would be something like `exercism submit <path_to_exercism_dir>/delphi/bob/ubob.pas`

.

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

It's possible to submit an incomplete solution so you may request help from a mentor.

```
unit uPrimeFactorsTests;
interface
uses
DUnitX.TestFramework;
const
CanonicalVersion = '1.1.0.1';
type
[TestFixture]
TPrimeFactorsTest = class(TObject)
private
procedure CompareArrays(Array1, Array2: TArray<Int64>);
public
[Test]
// [Ignore('Comment the "[Ignore]" statement to run the test')]
procedure no_factors;
[Test]
[Ignore]
procedure prime_number;
[Test]
[Ignore]
procedure square_of_a_prime;
[Test]
[Ignore]
procedure cube_of_a_prime;
[Test]
[Ignore]
procedure product_of_primes_and_non_primes;
[Test]
[Ignore]
procedure product_of_primes;
[Test]
[Ignore]
procedure factors_include_a_large_prime;
end;
implementation
uses
System.SysUtils, uPrimeFactors;
procedure TPrimeFactorsTest.CompareArrays(Array1, Array2: TArray<Int64>);
var
i: integer;
begin
Assert.AreEqual(Length(Array1), Length(Array2), ' - Array lengths must be equal');
for i := Low(Array1) to High(Array1) do
Assert.AreEqual(Array1[i], Array2[i], format('Expecting element %d to = %d, Actual = %d',
[i, Array1[i], Array2[i]]));
end;
procedure TPrimeFactorsTest.cube_of_a_prime;
begin
CompareArrays([2, 2, 2], TPrimeFactors.factors(8));
end;
procedure TPrimeFactorsTest.factors_include_a_large_prime;
begin
CompareArrays([11, 9539, 894119], TPrimeFactors.factors(93819012551));
end;
procedure TPrimeFactorsTest.no_factors;
begin
CompareArrays([], TPrimeFactors.factors(1));
end;
procedure TPrimeFactorsTest.prime_number;
begin
CompareArrays([2], TPrimeFactors.factors(2));
end;
procedure TPrimeFactorsTest.product_of_primes;
begin
CompareArrays([5, 17, 23, 461], TPrimeFactors.factors(901255));
end;
procedure TPrimeFactorsTest.product_of_primes_and_non_primes;
begin
CompareArrays([2, 2, 3], TPrimeFactors.factors(12));
end;
procedure TPrimeFactorsTest.square_of_a_prime;
begin
CompareArrays([3, 3], TPrimeFactors.factors(9));
end;
initialization
TDUnitX.RegisterTestFixture(TPrimeFactorsTest);
end.
```

```
unit uPrimeFactors;
interface
uses
System.Generics.Collections, System.Math;
type
TPrimeFactors = class
private
class var FFactors: TList<Int64>;
class procedure sieve(Number: Int64);
public
class function factors(Number: Int64): TArray<Int64>;
end;
implementation
{ TPrimeFactors }
class function TPrimeFactors.factors(Number: Int64): TArray<Int64>;
var
Index: Integer;
begin
// First, we'll create a list to temporarily store the factors
FFactors := TList<Int64>.Create;
// Use the Sieve of Eratosthenes to factor the number
sieve(Number);
// Set the length of the result array to the number of factors found
SetLength(Result, FFactors.Count);
// Now, we copy the factors into the result array
for Index := 0 to FFactors.Count - 1 do
Result[Index] := FFactors.Items[Index];
// Clean up
FFactors.Destroy;
end;
class procedure TPrimeFactors.sieve(Number: Int64);
var
Factor: Int64;
begin
// The number one isn't prime, so we can leave early
if Number = 1 then
Exit;
// We start at the first prime, 2
Factor := 2;
// We only need to loop until we reach the square root of Number
while Factor <= Trunc(Number * 1.0) do
begin
// Is this factor a prime factor?
if Number mod Factor = 0 then
begin
// Yes, it is, so we add it to the list of factors
FFactors.Add(Factor);
// Add now we test the other factor
sieve(Number div Factor);
// Set the factor to be great than the start number to force
// exit of the loop
Factor := Number + 1;
end;
// Lastly, we need to move to the next possible factor
if Factor = 2 then
// The second prime is 3, so we only need to add one to Factor
Inc(Factor)
else
// All remaining primes are odd number, so we add two to Factor
Inc(Factor, 2);
end;
end;
end.
```

A huge amount can be learned from reading other peopleโs 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?
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