Code quizzes commonly have a prime number challenge, which got me to thinking, shouldn’t there already be an is_prime method? Well, yes, there should be. And in Ruby, there is.

Ruby has a Prime class and it does some useful things for you when you require 'prime' in your file. Two of my favorites are:

Return true if a given number is prime

require 'prime'

Prime.prime?(8753)
#=> true

Prime.prime?(4)
#=> false

Awwwwww yeah! That’s the stuff right there.

Generate a list of primes starting with 2 through a given number

require 'prime'

Prime.each(27) do |prime|
p prime
end

#=> 2, 3, 5, 7, 11, 13, 17, 19, 23

Those two methods alone have pretty much handled any prime number code challenge that’s comes my way. If the challenge is about researching a programming language to see what options are available, this approach wins – hooray!– because learning a language’s libraries and methods makes us stronger, faster, and more efficient. Ah yes, proficiency leads to efficiency… I like that.

Creating our own is_prime method

But let’s be honest, code challenges involving math are usually focused on manipulating the basics of a language to solve a math problem, not about accessing the libraries or gems you’d probably use in real life. Code challenges like to make you do logic calisthenics, so let’s do this thing the hard way.

Here’s some pseudocode for my approach:

  • Create a range of numbers from 2 to one less than the number in question (num - 1)
  • Loop through that range
    • Ask if the number in question (num) is cleanly divisible by each number in the range (n)
    • If any one of those range numbers (n) creates a 0 remainder when the number in question (num) is divided by that number (n), the number in question (num) is not prime
  • If at the end of the loop, none of the range numbers (n) created a 0 remainder, the number in question (num) is, indeed, prime

And now for the actual code

Using the Ruby each loop:

def is_prime(num)
(2..(num - 1)).each do |n|
return false if num % n == 0
end
true
end

is_prime(7)
#=> true

is_prime(4)
#=> false

Using a Ruby while loop:

def is_prime(num)
n = 2
while n < num
return false if num % n == 0
n += 1
end
true
end

is_prime(7)
#=> true

is_prime(4)
#=> false

The Ruby while loop approach might feel familiar to people coming to Ruby from JavaScript because its iteration is transparent like JavaScript’s for loop. After all, there’s none of that kludgy (2..(num - 1)) to define the iteration range like there is in the Ruby each loop. For good measure, here is a JavaScript version of isPrime.

Using the JavaScript for loop:

function isPrime(num){
for (n = 2; n < num; n++) {
if (num % n == 0) return false;
}
return true;
}

isPrime(7);
//=> true

isPrime(4);
//=> false