How do I generate the first n prime numbers?

Question

I am learning Ruby and doing some math stuff. One of the things I want to do is generate prime numbers.

I want to generate the first ten prime numbers and the first ten

Answer 1

Here is a way to generate the prime numbers up to a "max" argument from scratch, without using Prime or Math. Let me know what you think.

def prime_test max
    primes = []
    (1..max).each {|num| 
        if
            (2..num-1).all? {|denom| num%denom >0}
        then
            primes.push(num)
        end
    }
    puts primes
end

prime_test #enter max

Answer 2

Ruby: Print N prime Numbers http://mishra-vishal.blogspot.in/2013/07/include-math-def-printnprimenumbernoofp.html

include Math

def print_n_prime_number(no_of_primes=nil)

  no_of_primes = 100 if no_of_primes.nil?

  puts "1 \n2"

  count = 1

  number = 3

  while count < no_of_primes

sq_rt_of_num = Math.sqrt(number)

number_divisible_by = 2

while number_divisible_by <= sq_rt_of_num

  break if(number % number_divisible_by == 0)

  number_divisible_by = number_divisible_by + 1

end

if number_divisible_by > sq_rt_of_num

  puts number

  count = count+1

end

number = number + 2

  end

end

print_n_prime_number

Answer 3

I did this for a coding kata and used the Sieve of Eratosthenes.

puts "Up to which number should I look for prime numbers?"
number = $stdin.gets.chomp
n = number.to_i
array = (1..n).to_a

  i = 0

while array[i]**2 < n

i = i + 1
array = array.select do |element|
  element % array[i] != 0 || element / array[i] == 1


end
end

 puts array.drop(1)

Answer 4

Here's a super compact method that generates an array of primes with a single line of code.

  def get_prime(up_to)
    (2..up_to).select { |num| (2...num).all? { |div| (num % div).positive? } }
  end

Answer 5

def get_prime(number)
  (2..number).each do |no|
      if (2..no-1).all? {|num| no % num  > 0}
        puts no
      end
  end
end

get_prime(100)

Answer 6

I think this may be an expensive solution for very large max numbers but seems to work well otherwise:

def multiples array
  target = array.shift 
  array.map{|item| item if target % item == 0}.compact
end

def prime? number
  reversed_range_array = *(2..number).reverse_each
  multiples_of_number = multiples(reversed_range_array)
  multiples_of_number.size == 0 ? true : false
end

def primes_in_range max_number
  range_array = *(2..max_number)
  range_array.map{|number| number if prime?(number)}.compact
end