Question

I have a code to get list of prime numbers:

def primes_numbers num
    primes = [2]

    3.step(Math.sqrt(num) + 1, 2) do |i|
        is_prime = true     

        primes.each do |p|   # (here)
            if (p > Math.sqrt(i) + 1)
                break
            end

            if (i % p == 0)
                is_prime = false
                break
            end
        end

        if is_prime
            primes << i
        end     
    end

    primes
end

Is it possible rewrite code using Array methods (select, collect and so on…)?
Something like:

s = (3..n)
s.select { |x| x % 2 == 1}.select{ |x| ..... }

The problem is that I need to iterate throught result array (comment ‘here’) in the select method.

Answer 1

Ruby 1.9 has a very nice Prime class:

http://www.ruby-doc.org/core-1.9/classes/Prime.html

But I’m assuming you don’t care about any standard classes, but want to see some code, so here we go:

>> n = 100 #=> 100
>> s = (2..n) #=> 2..100
>> s.select { |num| (2..Math.sqrt(num)).none? { |d| (num % d).zero? }} 
#=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

Note: I wrote it this way because you wanted Enumerable methods, for efficiency’s sake you probably want to read up on prime finding methods.