Ruby - Prime Number calculator

Question

I need some feedback to figure out why I cant puts or print anything from my methods on the screen. This is a simple script I wrote to solve the problem of finding the 1001st prime number. Thanks

def primes
  # iterates through numbers until it has the 1001th prime number and returns it.
  # I chose to create the num_primes variable instead of counting the number of 
  # elements in in_prime_array every iteration
  # based upon a guess that it would be faster to check.

is_prime_array = []
  num_primes = 0
  i = 2
  loop do
    is_prime_array << i && num_primes += 1 if is_prime?(i) == true 
    i += 1
    break if num_primes == 1001
    end
  is_prime_array[1001]
end


def is_prime? (num)
# Checks to see if the individual number given is a prime number or not.
i = 2
  loop do
    if i == num
      return true
    elsif num % i == 0
      return false
    else
      i += 1
    end
  end
end

Thanks for any help!

EDIT

I took your advice and tried this pice of code:

def is_prime? (num)
  # Checks to see if the individual number given is a prime number or not.
  i = 2
  loop do
    if i == num
      return true
    elsif num % i == 0
      return false
    else
      i += 1
    end
  end
end

i = 0
count = 0
loop do
  count += 1 if is_prime?(x)
  puts "#{i}" if count == 1001
  break
end

It still returns nothing. Hummm

for better efficiency, in your `is_prime?` inside the `loop`, change it to `if i*i > num` `return true` `elsif ...`. — Will Ness, Apr 29 '13 at 11:36
Being rigorous with indentation will make your life better. `i = 2` and `is_prime_array = []` should be 2 spaces in. — Nathan Lilienthal, Apr 29 '13 at 15:59

score 2 · Accepted Answer · answered Apr 29 '13 at 10:05

2

i = 0
count = 0
loop do
  if is_prime(i)
    count += 1
  end

  if count == 10001
    puts "#{i}"
    break
  end
end

Simple method :)

answered Apr 29 '13 at 10:05

Kiattisak Anoochitarom

2,157
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Good suggestion. I implemented it but it still wont return anything. – JDillon522 Apr 29 '13 at 23:32

score 1 · Answer 2 · answered Apr 29 '13 at 09:45

1

It's an off-by-one error. If you have 1001 elements in an array, the last element will be at index 1000.

Where you have

is_prime_array[1001]

Change it to

is_prime_array[1000]

And you can do this:

puts primes
 => 7927

You could also have

is_prime_array.last

instead of a specific index number.

answered Apr 29 '13 at 09:45

Neil Slater

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Even after 30+ years of coding, I still make the same mistake on new projects. Which is why I saw this one. – Neil Slater May 02 '13 at 08:11

score 1 · Answer 3 · answered Apr 29 '13 at 09:46

1

What are you trying to "puts"? The first thing I notice is that there is no call to primes in the file, so nothing will happen if you try to run this code by itself. Maybe that's why you don't see anything printed.

Here's an example of how to print a few variables inside your loop:

loop do
  ...
  puts "At iteration #{i}, we have prime=#{is_prime?(i)}"

If you don't know, enclosing a statement with #{<statement goes here>} inside a string is the same as appending the return value of <statement goes here> to the string at that position. This is the same as "Str " + blah + " rest of str" in a language like Java.

answered Apr 29 '13 at 09:46

Benjewman

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Sorry, I only wrote down the methods in the script and not where I tried to call them. Even when I write something like `puts primes` it still returns nothing. I dont even get an error. – JDillon522 Apr 29 '13 at 23:30
I ran your code with my added puts statement and call to `primes()` on my local machine with Ruby 1.9.3 -- the puts does work. Make sure you're calling the function from somewhere (and if it's inside another function, make sure THAT function is being called). I added `primes()` at the bottom of the file to test this. – Benjewman May 02 '13 at 13:09

Ruby - Prime Number calculator

3 Answers3