Parser in Ruby: #slice! inside #each_with_index = missing element

Question

Let's say, I want to separate certain combinations of elements from an array. For example

data = %w{ start before rgb 255 255 255 between hex FFFFFF after end }
rgb, hex = [], []
data.each_with_index do |v,i|
  p [i,v]
  case v.downcase
    when 'rgb' then rgb  = data.slice! i,4
    when 'hex' then hex  = data.slice! i,2
  end
end
pp [rgb, hex, data]
# >> [0, "start"]
# >> [1, "before"]
# >> [2, "rgb"]
# >> [3, "hex"]
# >> [4, "end"]
# >> [["rgb", "255", "255", "255"],
# >>  ["hex", "FFFFFF"],
# >>  ["start", "before", "between", "after", "end"]]

The code have done the correct extraction, but it missed the elements just after the extracted sets. So if my data array is

data = %w{ start before rgb 255 255 255 hex FFFFFF after end }

then

pp [rgb, hex, data]
# >> [["rgb", "255", "255", "255"],
# >>  [],
# >>  ["start", "before", "hex", "FFFFFF", "after", "end"]]

Why does it happen? How to get those missed elements inside #each_with_index? Or may be there is a better solution for this problem assuming that there are much more sets to extract?

Answer 1

Because you are manipulating data in place.

When you hit rgb the next element in the loop would be 255, but you are deleting those elements so now between is in the place that rgb was, so the next element is hex

Something like this may work better for you:

when 'rgb' then rgb  = data.slice! i+1,3
when 'hex' then hex  = data.slice! i+1,1

Answer 2

Here is a bit nicer solution

data = %w{ start before rgb 255 255 255 hex FFFFFF hex EEEEEE after end }
rest, rgb, hex = [], [], []
until data.empty?
  case (key = data.shift).downcase
    when 'rgb' then rgb  += [key] + data.shift(3)
    when 'hex' then hex  += [key] + data.shift(1)
    else rest << key
  end
end
p rgb, hex, rest
# >> ["rgb", "255", "255", "255"]
# >> ["hex", "FFFFFF", "hex", "EEEEEE"]
# >> ["start", "before", "after", "end"]