Day 5 of 30 Coding Challenge: Sum Prime Numbers Functionally

Hey!

Day 5 of 30 of this Ruby Coding Challenge series, and this is the post version of the Youtube video.

Today, we’ll explore the following problem:

Generate a Sum of Prime Numbers in a Given an Array

The previous Ruby Coding Challenge was about counting how many items are prime in a given array.

This one is quite similar and as usual, we’ll:

• Solve the problem manually.
• Solve the problem in a more lovable Ruby way.

Let’s get into it

#1 - Let’s Revisit the Previous Ruby Coding Challenge

Below, we have the final version of our algorithm in Ruby, which has the method count_prime_numbers()

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def count_prime_numbers(array)

  array.count do |item|

    is_prime(item)

  end

end

def is_prime(item)

  return false if item == 1

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

    if item % number == 0

      return false

    end

  end

  return true

end

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

puts count_prime_numbers(array)

#2 - Creating the Sum Method

Let’s replace the count_prime_numbers() method by a method that sum the prime numbers, which will be creatively called sum_prime_numbers():

The new method should:

• Iterate over the array.
• Check if the current item is prime.
• Sum the prime item.

Pretty simple. Translating into Ruby code:

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def sum_prime_number(array)

  sum = 0

  for item in array

    if is_prime_number(item)

      sum += item

    end

  end

  return sum

end

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

puts sum_prime_number(array)

Cool. But what about the lovable Ruby way to solve problems?

#3 - Ruby Way

One important Ruby style is that we can manipulate arrays without worrying about temporary variables. Nice!

Going in parts, we can at least two favors to Ruby:

• Hey Ruby, given an array, select only items that match a specific condition

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array.select { |item| is_prime(item) }

Beautiful. Almost plain English.

If you execute the code above, you’ll end up with a smaller array, with only prime numbers:

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

puts array.select { |item| is_prime(item) }

# result: [2, 3, 5, 7]

The next step is asking Ruby another favor:

• Hey Ruby, by the way, now that you have an array with only prime numbers, reduce this array into a sum of all of the items

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array.select{ |item| is_prime(item) }.reduce(:+)

Beautifully, Ruby has the method reduce(), which receives the + method that will be used to sum all of the items

The complete code:

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def sum_prime_number(array)

    array.select{ |item| is_prime(item) }.reduce(:+)

end

def is_prime(item)

  return false if item == 1

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

    if item % number == 0

      return false

    end

  end

  return true

end

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

puts sum_prime_number(array)

Notice that we ended up having a more functional style since this code is more declarative. I’m preparing a series to explore Functional Programming in JavaScript, Python, Java, and Ruby with some comparisons with Haskell. I hope to get you into this journey :)

I hope that you’ve enjoyed 1 thing or maybe 2 and see you in the next coding challenge :)

Don’t forget to keep in touch and say hi Alex :)

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