Question

To learn Ruby, I’m implementing different data structures starting with nodes and a simple stack. If I matching each def with a corresponding end, there are lots of error about expecting $end (EOF) but getting end. So I could fix it by stacking some ends at the end of the class, but obviously I don’t know why that works.

require "Node"
class Stack
    attr_accessor :top

    def size
            @size
    end

    def push(node)
        if node && node.next
            node.next = top
            top = node
        end
        size++
    def pop()
        if top != nil
            top = top.next
        end
        size--


    def to_s
        if top != nil
            temp = top
            while temp != nil
                puts temp.value
                temp = temp.next
            end
        else
            puts "The stack is empty"
        end
    end
end
end
end

The node class is very simple and shouldn’t cause any problems:

class Node
    attr_accessor :next
    def initialize(value)
        @value = value
    end
end

Everything works fine on that Frankenstein Stack, except pushing a node results in NoMethodError: undefined method +@' for nil:NilClass. Not sure if that is related, but I’m mostly concerned with the syntax of method/class declaration and using end

Answer 1

You get an error because ruby does not have ++ and -- operators.

Ruby understand the following constructs

size++
def pop()
# and
size--
def to_s()

like

size + +def pop()
# and 
size - -def to_s()

Ruby syntax is expression-oriented and method definition is expression in Ruby. Method definition expressions (def pop() and def to_s()) are evaluated to nil (in your code you actually define method pop inside push method body and to_s inside pop method body). And this is why you get NoMethodError: undefined method +@' for nil:NilClass error – it evaluates expression size + +nil and nil does not define unary plus operator. In this expression first + is an Fixnum addition operator (size is Fixnum), and second + is unary plus operator of nil (result of def pop() expression).

Use += 1 and -= 1 instead of ++ and --. Your code should look like this:

class Stack
    attr_accessor :top

    def size
            @size
    end

    def push(node)
        if node && node.next
            node.next = top
            top = node
        end
        @size += 1 # @size, not `size` because you have `size` getter and you cannot modify size with getter method
    end

    def pop()
        if top != nil
            top = top.next
        end
        @size -= 1
    end

    def to_s
        if top != nil
            temp = top
            while temp != nil
                puts temp.value
                temp = temp.next
            end
        else
            puts "The stack is empty"
        end
    end
end