# Stack2_class.rb - a class to implement a stack-like 
# structure in an array
class Stack2_class

# Constructor - parameter is the size of the stack - default is 100
 def initialize(len = 100)
 @stack_ref = Array.new(len)
 @max_len = len
 @top_index = -1
 end

# push method
 def push(number)
 if @top_index == @max_len
 puts "Error in push - stack is full"
 else
 @top_index += 1
 @stack_ref[@top_index] = number
 end
 end

# pop method
 def pop()
 if @top_index == -1
 puts "Error in pop - stack is empty"
 else
 @top_index -= 1
 end
 end

# top method
 def top()
 if @top_index > -1
 return @stack_ref[@top_index]
 else
 puts "Error in top - no elements"
 end
 end

# top2 method
 def top2
 if @top_index > 0
 return @stack_ref[@top_index - 1]
 else
 puts "Error in top2 - there are not 2 elements"
 end
 end

# empty method
 def empty()
 @topIndex == -1
 end

end