Add nil object as an element of an array in Ruby

Use:

a = [nil]

Example:

> a = nil
=> nil
> a.each{|x|puts x}
NoMethodError: undefined method `each' for nil:NilClass
        from (irb):3
        from :0

> a= [nil]
=> [nil]
> a.each{|x|puts x}
nil
=> [nil]

I believe your problem lies in when you "assign nil" to the array

arr = []
arr = nil

Is this something like what you tried doing? In this case you do not assign nil to the array you assign nil to the variable arr, hence arr is now nil giving you errors concerning a "nil class"

Sure you can. You are probably trying to do something with the nil object without checking to see if it's nil? first.

C:\work>irb
irb(main):001:0> a = [1,2,nil,3]
=> [1, 2, nil, 3]
irb(main):003:0> a.each{|i|
irb(main):004:1* if i.nil? then
irb(main):005:2* puts ">NADA>"
irb(main):006:2> else
irb(main):007:2* puts i
irb(main):008:2> end
irb(main):009:1> }
1
2
>NADA>
3
=> [1, 2, nil, 3]
irb(main):010:0>

I think you're confusing adding an item to an array with assigning a value of nil to a variable.

Add an item to (the end of) an array (two ways):

array.push(item)
# or if you prefer
array << item
# works great with nil, too
array << nil

I'm assuming that the array already exists. If it doesn't, you can create it with array = [] or array = Array.new.

On the other hand, array = nil assigns nil to a variable that happens to be (misleadingly) named 'array'. If that variable previously pointed to an array, that connection is now broken.

You may be thinking of assignment with an index position, but array[4] = nil is very different from array = nil.

There is a problem with how you're storing the nil value in the array. Here is an example to show how arrays work:

a = nil
b = [nil]
c = []
c.push(nil)

# a is nil
# b is [nil]
# c is now [nil]

So, 'b' and 'c' both created an array with a nil attribute while 'a' just set a variable to nil (NOT an array).

With either 'b' or 'c', you should now be able to run .each without any problem

use Enumerable#map

ruby-1.8.7-p249 > [1,2,nil,4].map{|item| puts item}
1
2
nil
4
 => [nil, nil, nil, nil] 

note that even though the return is nil for each item in the array, the original array is as it was. if you do something to operate on each item in the array, it will return the value of each operation. you can get rid of nils buy compacting it.

ruby-1.8.7-p249 > [1,2,nil,4].map{|item| item + 1 unless item.nil? }
 => [2, 3, nil, 5] 
ruby-1.8.7-p249 > [1,2,nil,4].map{|item| item + 1 unless item.nil? }.compact
 => [2, 3, 5]