speed up array functions in Ruby

Question

I have a simple method that finds prime numbers, puts them in an array and then sums them up. What are some possible steps to speed it up?

def prime?(number, array)
  array.each do |x|
    if number % x == 0
      return false
    end
  end
  true
end

def sum_prime(number)
  i = 0
  prime = 1
  output = [2]
  while prime < number
    if prime?(prime, output)
      i += 1
      output << prime if prime != 1
      p prime
    end 
    prime += 2
  end
  output.inject(:+)
end

sum_prime(200000)

is array#each ok? Can I concatinate differently for faster results?

Assuming you must store the values in an array, array.inject(:+) seems to be about as fast as one can do it in ruby. The only other suggestion would be to use a compiled language like C but even there the improvement will probably be negligible. — Matt
– Matt, Commented Dec 4, 2013 at 2:01
Compare your results to using the built in Prime class (ruby-doc.org/stdlib-1.9.3/libdoc/prime/rdoc/Prime.html). Then you might know if you can get it faster :) — Rots
– Rots, Commented Dec 4, 2013 at 2:03
For really large sum_prime, you could send request to a faster language. You could also make a few short cuts, by priming a hash with existing known values and starting from there: shortCut={100 => sum_prime(100), 1000 => sum_prime(1000), ad-nausum. Then you can just count from the least value and save your self quite a bit of ram. — Daniel
– Daniel, Commented Dec 4, 2013 at 2:04
@mr.musicman If it doesn't need to be in an array, then add each value to a sum += prime as soon as each prime is made available. — Matt
– Matt, Commented Dec 4, 2013 at 2:04
If your goal is just to sum primes, you might be better off downloading a list. — phs
– phs, Commented Dec 4, 2013 at 2:10

Will · Accepted Answer · 2013-12-04 03:35:10Z

1

This should work. It uses others' suggestions:

require 'prime'
def sum_prime(limit)
  Prime.each(limit).inject(0) {|sum, num| sum + num}
end
puts sum_prime(200000)

answered Dec 4, 2013 at 3:35

Will

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