Here’s a quick walk through of the prime?() lab, but I’m going to keep it brief—mainly because I am much more pleased with my solution to the OO Triangle lab. My solution to OO Triangle is novel (not the solution posted to GitHub) and I think it’s quite elegant and simple. So a quick look at prime?() then on to the fun stuff with OO Triangle in my next blog!
def prime?(number)
return false if number <= 1
root = Math.sqrt(number)
range = 2..root
range.none? {|i| number % i == 0}
end
The specs are really basic, just return true for prime and return false for numbers that are not prime. I followed the suggestions in the README file and did some quick online research about defining a prime number and the simplest algorithms for determining if a given number is a prime, and then I got to coding.
def prime?(number)
return false if number <= 1
First, I define the method to take a number as an argument. A prime number is defined as “A whole number greater than 1 that can not be made by multiplying other whole numbers.” So the first thing to check is that it is not 1, which is not considered a prime number but would return true for the algorithm I used, and also not zero or a negative number (none of which are prime numbers).
root = Math.sqrt(number)
range = 2..root
Then I find the square root of the number and set it equal to a local variable, “root.” Ruby has a built in module, Math, that lets you use some basic math functions like the .sqrt method I used here. You can read more about the Math module here.
I then set another local variable to be equal to the range of numbers from 2 to the square root. I need this range because if a number is not divisible by any number between 2 and its square root with no remainder, then it will not be divisible by any number greater than its square root with no remainder. By only checking up to the square root, I reduce the amount of computations my program needs to iterate through. It might not seem like a big deal with smaller numbers, but with super large numbers, this can make a difference.
range.none? {|i| number % i == 0}
end
Lastly, I iterate over the range and check if the original number given as an argument for the method is divisible by any number in the range with a remainder of zero, and end the method. The % operator is known as modulus; in x % y it returns the remainder of x divided by y. You can learn more about Ruby arithmetic operators here.
I used the .none? enumerable method because it has a true/false return value, and returns “true” if none of the enumerables has a true value in the iteration block. A number is a prime if none of the numbers in the range return a true value for the iteration block. At first I had .any? but quickly realized that was giving me the opposite of the true/false return value that I wanted.
And that’s it, a quick run down on prime?()! Happy coding :)