What does the ? operator do in Elixir?

Percy Grunwald's Profile Picture

Written by Percy Grunwald

— Last Updated February 25, 2019

Thanks to my friend @visarz for pointing out that I should have used this operator in my Exercism solutions.

I’ve been doing a lot of Exercism problems recently where I’ve been dealing with charcodes/code points of characters.

By charcode/code point, I mean the integer character code for a certain UTF-8 character: e.g. capital A has a UTF-8 character code of 65.

We saw this in the RNA transcription and Hamming problems:

iex> IO.inspect('GCTAU', charlists: false)
[71, 67, 84, 65, 85]

@dna_nucleotide_to_rna_nucleotide_map %{
  # `G` -> `C`
  71 => 67,

  # `C` -> `G`
  67 => 71,

  # `T` -> `A`
  84 => 65,

  # `A` -> `U`
  65 => 85

It turns out I’ve forgotten about a key Elixir operator that would have massively simplified the code above!

How to get the integer charcode or code point for a UTF-8 character

The ? operator allows you to get the integer code point/charcode for a UTF-8 character in Elixir:

iex> ?A

iex> ?å

iex> ?Д

iex> ?😃

The map I showed in the section above can be better represented like this:

@dna_nucleotide_to_rna_nucleotide_map %{
  ?G => ?C,
  ?C => ?G,
  ?T => ?A,
  ?A => ?U

Much more readable! We can also do something like this:

iex> 'ABCABC' |> Enum.filter(fn char -> char == ?A end)

Looks like I will need to go back and update my solutions! Thanks again, @visarz.

Further reading

Binaries, strings, and charlists on elixir-lang.org

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