Why does map enumerable return value not return expected value

Question

I'm pretty familiar with the Ruby Enumerable module, or at least so I thought. Take the following code snippet:

names = [ "Donald", "Daisy", "Daffy" ]

new_empty_array = []

new_names_array = names.map do |name|
  new_empty_array << name
end

puts new_names_array.inspect
# prints [["Donald", "Daisy", "Daffy"], ["Donald", "Daisy", "Daffy"], ["Donald", "Daisy", "Daffy"]]

I know I'm not using map correctly, but I was teaching a lesson on Ruby enumerables and came across this example when a student was testing map out. The return value of the shovel (<<) operator is the array after an element has been added. Shouldn't the result instead be:

[["Donald"], ["Donald", "Daisy"], ["Donald", "Daisy", "Daffy"]]

It seems that the entire loop processes and the final return value of the shovel operator is processed? What gives?

Is first_names supposed to be names?

squiguy
– squiguy

2016-07-19 20:26:20 +00:00
Commented Jul 19, 2016 at 20:26 — squiguy
– squiguy, Commented Jul 19, 2016 at 20:26

the Tin Man · Accepted Answer · 2016-07-19 22:11:02Z

5

The result of map, in your case, is an array which consists of references to same array new_empty_array multiple times. You are not creating three different arrays, but modifying the same array in the map block.

To get the output you are expecting, you need to do:

new_names_array = first_names.map do |name|
  (new_empty_array << name).dup
end

As a side note, you could use this code, which is more obvious than the code above, for the output you desire:

(1..first_names.size).map do |num|
  first_names.take(num)
end
#=> [["Donald"], ["Donald", "Daisy"], ["Donald", "Daisy", "Daffy"]]

edited Jul 19, 2016 at 22:11

the Tin Man

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answered Jul 19, 2016 at 20:23

Wand Maker

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2 Comments

Andrew Kim Over a year ago

Let me get this straight, the return value for each iteration is new_empty_array and because the last iteration of the .map returns an array with 3 elements, what gets returned from the map is new_empty_array in it's last state 3 times

Wand Maker Over a year ago

@AndrewKim new_empty_array is reference to an array, and you return reference to it in map and hence, end up with 3 references to it as array - all 3 references point to same physical array which at the end of iteration has 3 elements

the Tin Man · Accepted Answer · 2016-07-19 22:12:35Z

0

new_names_array = first_names.map do |name|
  name
end

would give ["Donald", "Daisy", "Daffy"].

Each element of first_names returns a value to the new array created by map but instead you return for each element a new array, new_empty_array.

This object is << three times and at the end has the three names, so 3 x 3 elements:

[new_empty_array, new_empty_array, new_empty_array] = [["Donald", "Daisy", "Daffy"], ["Donald", "Daisy", "Daffy"], ["Donald", "Daisy", "Daffy"]]

edited Jul 19, 2016 at 22:12

the Tin Man

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answered Jul 19, 2016 at 20:39

peter

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2 Comments

peter Over a year ago

@the Tin Man, thnx again for the edit, You must be investing a lot of time in correcting non english speaking people like me

Wand Maker Over a year ago

May be you meant non english writing people ;-)

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