Day 3 of 30  Ruby Coding Challenge  Better Prime Algorithm :)
It's time to use some Ruby power to solve the previous coding challenge, which was writing an algorithm to count how many prime items exist in an array
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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!
Published at DZone with permission of Alexandre Gama. See the original article here.
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