TypeError: can't convert String into Integer

Question

I have code:

class Scene
  def initialize(number)
    @number = number
  end
  attr_reader :number
end

scenes = [Scene.new("one"), Scene.new("one"), Scene.new("two"), Scene.new("one")]

groups = scenes.inject({}) do |new_hash, scene|
   new_hash[scene.number] = [] if new_hash[scene.number].nil?
   new_hash[scene.number] << scene
end

When I'm lauching it I get error:

freq.rb:11:in `[]': can't convert String into Integer (TypeError)
       from freq.rb:11:in `block in <main>'
       from freq.rb:10:in `each'
       from freq.rb:10:in `inject'
       from freq.rb:10:in `<main>'

If I change scenes to:

scenes = [Scene.new(1), Scene.new(1), Scene.new(2), Scene.new(1)]

the problem dissapear.

Why I get error message in the first case? Why Ruby decide to convert scene.number from String to Integer?

And one additional question about the 'inject' method. When Ruby initialize the 'new_hash' variable and how can Ruby know the type of this variable?

Btw: you can do what you want more easily by using groups = scenes.group_by(&:number) instead of inject. — sepp2k
– sepp2k, Commented Mar 21, 2010 at 11:54
The new_hash variable is initialized with the injected value (keep in mind: it's a folding operation which has a start value in the accumulator) — hurikhan77
– hurikhan77, Commented Mar 21, 2010 at 12:27

the Tin Man · Accepted Answer · 2010-03-21 10:14:06Z

10

try:

groups = scenes.inject({}) do |new_hash, scene|
   new_hash[scene.number] = [] if new_hash[scene.number].nil?
   new_hash[scene.number] << scene
   new_hash
end

Ruby takes the empty hash passed into inject() and sets new_hash to that. When the block ends the return value gets used to initialize new_hash the next time through, i.e., new_hash keeps accumulating the result of the block.

In your original code you were not returning the hash but an array (new_hash[scene.number] is an array) and the next loop through Ruby complained because new_hash[scene.number] was trying to do a lookup into the array with a string value, hence the error you got.

edited Mar 21, 2010 at 10:14

answered Mar 21, 2010 at 10:07

the Tin Man

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2 Comments

sepp2k Over a year ago

@shingara: No, it doesn't fail. His code runs without error and returns the expected result.

the Tin Man Over a year ago

Thank you sepp2k. I agree, "why was it downvoted?" It ran right, fixed the bug in his code and explained the problem.

benm · Accepted Answer · 2010-03-21 11:22:20Z

6

Z.E.D.'s right. See Jay Fields' Thoughts: Ruby: inject for a good explanation of inject by example.

As presented, your block returns an array. So the new_hash in |new_hash, scene| ends up being that array. When Ruby tries to find the array index 'one', it throws the error because 'one' is a String, not an Integer.

All you need to do is return new_hash as Z.E.D. showed, and you'll get something like this:

{
  "two" => [
    #<Scene:0x101836470 @number="two">
  ],
  "one" => [
    #<Scene:0x101836510 @number="one">,
    #<Scene:0x1018364c0 @number="one">,
    #<Scene:0x101836420 @number="one">
  ]
}

answered Mar 21, 2010 at 11:22

benm

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Comments

hurikhan77 · Accepted Answer · 2010-03-21 12:05:45Z

2

Why not use group_by which is probably exactly what you try to accomblish?

groups = scenes.group_by(&:number)
# => {"two"=>[#<Scene:0xb728ade0 @number="two">],
#     "one"=>
#       [#<Scene:0xb728ae30 @number="one">,
#        #<Scene:0xb728ae08 @number="one">,
#        #<Scene:0xb728ada4 @number="one">]}

inject is a folding operation and not exactly what you want. At least it's cumbersome to use in this way. merge with a block would probably be appropriate if you want to apply some algorithm during merging or grouping.

answered Mar 21, 2010 at 12:05

hurikhan77

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Comments

Dave Everitt · Accepted Answer · 2010-03-21 10:37:08Z

0

Also, to explain 'how can Ruby know the type of this variable' and why it tries to 'convert String into Integer' you might want to revise: Ruby variables and dynamic typing.

answered Mar 21, 2010 at 10:37

Dave Everitt

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Comments

Harish Shetty · Accepted Answer · 2010-03-25 08:03:07Z

0

I know an answer is accepted for this question, but I can't help but post my answer.

groups = scenes.inject({}) { |nh, s| nh.tap {|h| (h[s.number] ||= []) << s } }

answered Mar 25, 2010 at 8:03

Harish Shetty

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