sieve

sieve.rb

def sieve(number)
  if number < 2
    raise TypeError.new("Must be a number greater than 2")
  end
  primes =* (2..number)
  primes.each do |prime|
    primes.reject! do |number|
      prime != number && number % prime == 0
    end
  end
end

sieve_spec.rb

require 'rspec'
require 'sieve'

describe 'prime_sift' do 
  it 'returns the prime number 2 for a range of 2' do
    sieve(2).should eq [2]
  end

  it 'returns the prime numbers 2 and 3 for a range of 3' do
    sieve(3).should eq [2, 3]
  end

  it 'does not return non-primes' do
    sieve(4).should eq [2, 3]
    sieve(100).should eq [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]
  end

  it 'raises an exception if given a number less than 2' do
    expect { sieve(1) }.to raise_exception
  end

  it 'raises an exception if given something other than a number' do
    expect { sieve("two") }.to raise_exception
  end
end