Getting the indexes of duplicate elements in arrays (Ruby)

Question

I have an array in Ruby that has some duplicate elements. E.g.:

fruits = ["apples", "bananas", "apples", "grapes", "apples"]

When I do the following:

fruits.index("apples")
# returns 0

I only get the first occurrence of "apples" which is, in this case, fruits[0]. Is there a way that I can run something similar to the above code and get the indexes of the other occurrences of "apples"? If I can't run something similar to the above code, how else can I get the indexes of the duplicate elements?

@CarySwoveland Sorry, I had meant to; simply forgot. Your answer was useful; thanks. — GDP2
– GDP2, Commented Sep 26, 2016 at 15:35

Cary Swoveland · Accepted Answer · 2015-04-14 21:18:56Z

11

Taking a page from procedural languages, we could write:

fruits.each_index.select { |i| fruits[i]=="apples" }
  #=> [0, 2, 4]

answered Apr 14, 2015 at 21:18

Cary Swoveland

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sawa · Accepted Answer · 2015-04-14 20:13:03Z

3

You can do this:

fruits.to_enum.with_index.select{|e, _| e == "apples"}.map(&:last)
# => [0, 2, 4]

answered Apr 14, 2015 at 20:13

sawa

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2 Comments

Tucker Over a year ago

The "_" just means the variable is not going to be used.

Cary Swoveland Over a year ago

Note that _ is indeed a variable: _=3; puts _#=> 3. Some prefer writing something like |e,_ndx|.

steenslag · Accepted Answer · 2015-04-14 20:24:58Z

0

fruits = ["apples", "bananas", "apples", "grapes", "apples"]

p fruits.each_with_index.group_by{|f,i| f}.each{|k,v| v.map!(&:last)}
# => {"apples"=>[0, 2, 4], "bananas"=>[1], "grapes"=>[3]}

answered Apr 14, 2015 at 20:24

steenslag

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