Day 3 of 30 - Ruby Coding Challenge - Better Prime Algorithm :)

Hey!

Day 3 of 30: Ruby Coding Challenge number 3.

This is the post version of this video on Youtube.

In the previous video, we solved a really common challenge:

How many prime items are in a given array?

The final code was terrible. However, I’m not even sorry. Remembering why:

• How does my brain work when it comes to solving algorithms? Just get the job done! Decorate after that.

When it comes to algorithms, decoration means a few things:

• Better performance.
• Better readability.
• Better maintainability.

Well, better code design. Today, I’ll solve the same problem with (I hope) a better code design:

Below you can see the first ugly version:

#1 - Ugly Algorithm Version

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def count_prime_number_version_1(array)

  prime_count = 0

  for item in array

    next if item == 1

    is_prime = true

    number = item - 1

    while number > 1

      if item % number == 0

        is_prime = false

        break

      else

        number = number - 1

      end

    end

    if is_prime

      prime_count = prime_count + 1

    end

  end

  return prime_count

end

array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

It’s time to organize the kitchen. It would be great to have a method that verifies if a given number is prime.

#2 - Method Organization

We have 2 main responsibilities:

• Iterate over the array, asking questions.
• Validate if a given number is prime or not.

I’m going to split this into 2 methods, which will have those responsibilities above:

def count_prime_number_version_2(array)

  count_prime = 0

  for item in array

    if is_prime(item) # the new method. it's better. It's clearer

      count_prime = count_prime + 1

    end

  end

  return count_prime

end

def is_prime(item)

  return false if item == 1

  count = item - 1

  while count > 1

    if item % count == 0

      return false

    end

    count = count - 1

  end

  return true

end

Cool. It’s time to bring some Ruby power and use Ruby Blocks with the count method, which we did in in the first video.

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def count_prime_number_version_2(array)

  array.count do |item|

    is_prime_number(item)

  end

end

#3 - Ruby Arrays

I don’t know about you, but I hate local variables.

It’s time to get rid of them. In Ruby, it’s possible to indicate which items should be created in an array, given:

• The initial number.
• The pattern to generate the next number.

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def is_prime_number(item)

  return false if item == 1

  (2..(item - 1)).each do |number|

    if item % number == 0

      return false

    end

  end

  return true

end

Amazing, isn’t it?

#4 - Cheating: Ruby Prime Library

Now that we know how to create the Algorithm manually, it usually doesn’t make sense in real life. Ruby has a Prime Library, which can get the job done easily.

We just ask if the number is prime or not. Notice that we’re importing the Prime library

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require 'Prime'

Prime.prime?(item)

Going ahead and replacing the is_prime(item) method:

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def count_prime_version_6(array)

    array.count do |item|

    Prime.prime?(item)

  end

end

And that’s our final version — a little bit different than the first one.

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def count_prime_final_version(array)

    array.count do |item|

    Prime.prime?(item)

  end

end

array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

puts count_prime_final_version(array)

That’s it. I hope that you’ve learned 1 thing or 2 : )

Thinking out loud here, probably the next coding challenge will be rewriting the Algorithm manually using recursion.

Thanks for your visit, let’s keep in touch and see you!

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