Appending dynamic value to ruby instance variable

Question

I know this code doesn't look good at all , but i just want to explain my requirement. I want to know is there any good or alternative approach to it.

Actually, i want to create a new stack and whenever one stack has reached its capacity. I want to keep track of number of stacks created like @stack_1, @stack_2 ...by incrementing @number += 1 like @stack_@number. And for every stack, i want to maintain a @current_position pointer which is specific to every stack like @stack_2 has @current_position_2. So i want to create dynamic instance variables.

Example:

def initialize
  @number = 1
  @stack+"#{@number}" = Array.new(10)
  @current_position_"#{@number}" = 0
end

Output should be something like @stack1 = Array.new(10).
Lets say if i increment value of @number += 1, it should look like @stack2 = Array.new(10)

hi @Stefan , added more info to the question on what i want to achieve. — Rahul Dess
– Rahul Dess, Commented Oct 8, 2014 at 16:57

Raghuveer · Accepted Answer · 2014-10-08 06:12:48Z

2

Instead of array I suggest you to use Hash Map

@stack = Hash.new

@stack[@number] = <Your Array>

Be Careful if the @number is same your array will be replaced..

For more information about hash maps http://www.ruby-doc.org/core-1.9.3/Hash.html

answered Oct 8, 2014 at 6:12

Raghuveer

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5 Comments

Rahul Dess Over a year ago

can you explain why to use hash instead of array ? ..my requirement is to create many stacks. Does hash act like place holder for all stacks ?

Raghuveer Over a year ago

@RahulSambari yes.. it will

Rahul Dess Over a year ago

ill try to fit this suggestion into my scenario and let you know

Rahul Dess Over a year ago

hey, i updated my question.. lets say i want to maintain a position pointer for every stack..how to go about it

Rahul Dess Over a year ago

i liked this approach ..but how to maintain @current_position_@number in this scenario ?

Cary Swoveland · Accepted Answer · 2014-10-08 06:42:37Z

1

You can do it like this:

instance_variable_set("@stack#{@number}", Array.new(10, :a))
@stack1
  #=> [:a, :a, :a, :a, :a, :a, :a, :a, :a, :a]

instance_variable_set("@stack#{@number+1}", Array.new(10, :b))
@stack2
  #=> [:b, :b, :b, :b, :b, :b, :b, :b, :b, :b]

instance_variable_set("@current_position_#{@number}", 0)
@current_position_1
  #=> 0

edited Oct 8, 2014 at 6:42

answered Oct 8, 2014 at 6:31

Cary Swoveland

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8 Comments

Rahul Dess Over a year ago

how to achieve for @current_position_"#{@number}" = 0 ? which is not an instance_variable

Cary Swoveland Over a year ago

Does the last addition answer your question?

Rahul Dess Over a year ago

yep..so can i use both @stack1 and @current_position_1 regularly in my programs ?

Cary Swoveland Over a year ago

Yes, just as though you defined them in the normal way.

Rahul Dess Over a year ago

how to go in situation like if @current_position_@number < @[email protected]

|

Stefan · Accepted Answer · 2014-10-08 19:13:45Z

1

Instead of creating instance variables to track a stack's state from the outside, you could create a Stack class that tracks its state internally. Here's a very simple one:

class StackOverflow < StandardError; end

class Stack
  def initialize
    @stack = []
  end

  def position
    @stack.size
  end

  def full?
    position == 2 # small size for demonstration purposes
  end

  def push(obj)
    raise StackOverflow if full?
    @stack << obj
  end
end

stack = Stack.new
stack.push "foo"
stack.full?       #=> false

stack.push "bar"
stack.full?       #=> true

stack.push "baz"  #=> StackOverflow

Having a working stack, you can build something like a StackGroup to handle multiple stacks:

class StackGroup
  attr_reader :stacks

  def initialize
    @stacks = [Stack.new]
  end

  def push(obj)
    @stacks << Stack.new if @stacks.last.full?
    stacks.last.push(obj)
  end
end

stack_group = StackGroup.new
stack_group.push "foo"
stack_group.push "bar"
stack_group.stacks.size #=> 1

stack_group.push "baz"  # no error here
stack_group.stacks.size #=> 2

stack_group.stacks
#=> [#<Stack:0x007f9d8b886b18 @stack=["foo", "bar"]>,
#    #<Stack:0x007f9d8b886a50 @stack=["baz"]>]

answered Oct 8, 2014 at 19:13

Stefan

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2 Comments

Rahul Dess Over a year ago

i am sorry, but somehow .. i am little confused with your approach. Can you add some comments or is there any easy way to dynamically add numbers to instance variables ? it would be really helpful.

Stefan Over a year ago

@RahulSambari there's no need for dynamically created instance variables here. stack_groups tracks and crates the stacks and each stack tracks its position. stack_groups.stacks.size returns the total number of stacks, stack_groups.stacks[0].position returns the first stack's position, stack_groups.stacks[1].position returns the second stack's position and so on.

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