HackerRank Ruby – Methods – Introduction problem solution

In this HackerRank Ruby – Methods – Introduction problem solution we have been using methods (def method() .. end construct) to abstract compound operations, perform data manipulations and learn various concepts of the language, without talking in much detail about the concept of methods themselves, and how they are useful for a programmer. In this set of challenges, we will explore the guiding principles behind methods, learn more features and how to use them efficiently in our programs.

In simpler terms, a method is a group of several expressions (block, so to say) that can be referred with a name, can be passed some arguments (input) and are associated with one or more objects.

If you have programmed before, you might notice that the description above sounds almost same as functions in other languages (e.g, Python) except the last part which talks about association with one or more objects. It might be a bit non trivial to comprehend since Classes haven’t been introduced, but what it means is that these methods, even though they appear like global functions, are instead private methods of a root Object class on which they are implicitly defined and invoked automatically.

So, when you write –

def hello_world

    ‘Eha! Ruby’

end

> hello_world

‘Eha! Ruby’

You are essentially adding a private method to Object class –

class Object

    private

    def hello_world2

        ‘Eha! Ruby’

    end

end

> hello_world2

=> ‘Eha! Ruby’

This, however, is not the focus of this challenge. Instead, it was just to show you the true object nature of Ruby, and we’ll return to it again later during our challenges on classes.

In this challenge, you need to write a method prime? that takes an argument and returns true or false depending on if the number is prime or not.

> prime? 3

true

> prime? 17

true

> prime? 22

false

Further reading

These methods, unlike functions in other objects oriented programming languages (e.g., Python) are not first-class citizens, because they cannot be passed to other methods as arguments, returned by other methods, or assigned to variables.

Problem solution.

# Your code here
def prime?(number)
    Prime.prime? (number)
end 

Second solution.

def prime?(arg)
    return false if arg <= 1
    2.upto(Math.sqrt(arg).to_i) do |x|
        return false if arg%x == 0
    end 
    true
end

Third solution.

class Object
    def prime?(num)
        return false if num <= 1
        Math.sqrt(num).to_i.downto(2).each {|i| return false if num % i == 0}
        true
    end
end