Ruby Prime number program - ruby

Question

I know similar questions have been asked before but they all seem so over complicated and I don't think i'm that far off, so was wondering if anyone knows how to tweak my code to get it to work. My goal is to save all prime numbers of a range into an array and then print that array. Below is my code so far
prime_array = []
(1...100).each do |num|
if Math.sqrt(num) % 2 == 0
prime_array.push(num)
end
end
prime_array
#=> [4, 16, 36, 64]
I've tried a few different options but have hit a wall and can't see what i need to do. I'm trying to keep it as simple as possible as I feel a lot of the answers out there a little over complicated. Thanks in advance.

Answer 1

Let's calculate the square root of 7 (a prime number) and 8 (a composite number):
Math.sqrt(7) #=> 2.6457513110645907
Math.sqrt(8) #=> 2.8284271247461903
This doesn't really help, does it? Apparently, you can't determine if a number is a prime number by calculating its square root.
Instead, you have to check the number's divisors. From Wikipedia:
A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.
Let's determine the divisors of 7: (using the modulo operator %)
7 % 1 #=> 0 <- 7 is divisible by 1
7 % 2 #=> 1
7 % 3 #=> 1
7 % 4 #=> 3
7 % 5 #=> 2
7 % 6 #=> 1
7 % 7 #=> 0 <- 7 is divisible by 7
This satisfies the above definition - 7 is a prime number.
Now, let's determine the divisors of 8:
8 % 1 #=> 0 <- 8 is divisible by 1
8 % 2 #=> 0 <- 8 is divisible by 2
8 % 3 #=> 2
8 % 4 #=> 0 <- 8 is divisible by 4
8 % 5 #=> 3
8 % 6 #=> 2
8 % 7 #=> 1
8 % 8 #=> 0 <- 8 is divisible by 8
8 has two additional divisors 2 and 4. Therefore, 8 is not a prime number.
In Ruby, you could use select to find the divisors:
(1..7).select { |d| 7 % d == 0 } #=> [1, 7]
(1..8).select { |d| 8 % d == 0 } #=> [1, 2, 4, 8]
Finally, here's a variant of your Ruby code that checks if a given number num has exactly two divisors, 1 and num itself:
prime_array = []
(1...100).each do |num|
if (1..num).select { |d| num % d == 0 } == [1, num]
prime_array.push(num)
end
end
prime_array
#=> [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]
The above code can be optimized. I leave that to you.

Answer 2

Method that returns prime numbers in an array in ruby
def is_prime?(number)
return false if number <2
(2..Integer.sqrt(number)).each do |divisor|
return false if number % divisor == 0
end
return number
end
def read_array(array)
primes = []
array.each do |num|
primes << num if is_prime?(num)
end
return primes
end