Unexpected $end after while loop and nested if

Question

I have this program that I am working on that is supposed to find the sum of the first 1000 prime numbers. Currently all I am concerned with is making sure that the program is finding the first 1000 prime numbers, I will add the functionality for adding them later. Here is what I have:

#!/usr/bin/ruby

def prime(num)

    is_prime = true

    for i in 2..Math.sqrt(num)

        if (num % i) == 0           
            is_prime = false
        else
            is_prime = true
        end
    end

    return is_prime
end


i = 2
number_of_primes = 0

while number_of_primes < 1000

    prime = prime(i)

    if prime == true
        number_of_primes++

    end
    i++
end

When i try to run the program I get the following feedback:

sumOfPrimes.rb:32: syntax error, unexpected keyword_end
sumOfPrimes.rb:34: syntax error, unexpected keyword_end

what gives? Any direction is appreciated.

ruby doesn't understand '++'.. use number_of_primes += 1

ilan berci
– ilan berci

2013-05-02 18:50:46 +00:00
Commented May 2, 2013 at 18:50 — ilan berci
– ilan berci, Commented May 2, 2013 at 18:50

Dogbert · Accepted Answer · 2013-05-02 18:48:19Z

13

Ruby doesn't have ++ operator, you need to do += 1

number_of_primes += 1

answered May 2, 2013 at 18:48

Dogbert

224k43 gold badges419 silver badges416 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Jason Thompson Over a year ago

Any reason why it doesn't have such a common operator? I must have spend 40 minutes counting my 'if's 'do's and 'end's

Russell · Accepted Answer · 2013-05-02 20:58:14Z

Unasked for, but a few pieces of advice if you're interested:

One of the cool things about Ruby is that question marks are legal in method names. As such you'll often find that 'predicate' methods (methods that test something and return true or false) end with a question mark, like this: odd?. Your prime method is a perfect candidate for this, so we can rename it prime?.

You use a local variable, is_prime, to hold whether you have found a factor of the number you're testing yet - this is the kind of thing you'd expect to do in an imperative language such as java or C - but Ruby has all sorts of cool features from functional programming that you will gain great power and expressiveness by learning. If you haven't come across them before, you may need to google what a block is and how the syntax works, but for this purpose you can just think of it as a way to get some code run on every item of a collection. It can be used with a variety of cool methods, and one of them is perfectly suited to your purpose: none?, which returns true if no items in the collection it is called on, when passed to the code block you give, return true. So your prime? method can be rewritten like this:

def prime? num
  (2..Math.sqrt(num)).none? { |x| num % x == 0 }
end

Apart from being shorter, the advantage of not needing to use local variables like is_prime is that you give yourself fewer opportunities to introduce bugs - if for example you think the contents of is_prime is one thing but it's actually another. It's also, if you look carefully, a lot closer to the actual mathematical definition of a prime number. So by cutting out the unnecessary code you can get closer to exposing the 'meaning' of what you're writing.

As far as getting the first 1000 primes goes, infinite streams are a really cool way to do this but are probably a bit complex to explain here - definitely google if you're interested as they really are amazing! But just out of interest, here's a simple way you could do it using just recursion and no local variables (remember local variables are the devil!):

def first_n_primes(i = 2, primes = [], n)
  if primes.count == n then primes
  elsif prime? i then first_n_primes(i + 1, primes + [i], n)
  else first_n_primes(i + 1, primes, n)
  end
end

And as far as summing them up goes all I'll say is have a search for a ruby method called inject - also called reduce. It might be a bit brain-bending at first if you haven't come across the concept before but it's well worth learning! Very cool and very powerful.

Have fun!

Awesome advice, I appreciate the response. Definitely something I will be looking into more. I'm brand new to the ruby language, and so far it has been a fun learning experience, so it's nice to see some of the things that you are able to do with Ruby that you wouldn't see with Java.
Great, glad to have helped! You've picked a good language in Ruby. A lot of fun awaits.

Collectives™ on Stack Overflow

Unexpected $end after while loop and nested if

2 Answers 2

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related