dna.erl

-module(dna).
-export([count/2, nucleotide_counts/1]).

nucleotides()-> ["A", "T", "C", "G"].

count(Strand, TargetNucleotide)->
  case lists:member(TargetNucleotide, nucleotides()) of
    true ->
      erlang:length(
        lists:filter(
          fun(CandidateNucleotide)-> [CandidateNucleotide] == TargetNucleotide end,
          Strand
         )
       );
    false -> erlang:error("Invalid nucleotide")
  end.

nucleotide_counts(Strand)->
  lists:map(
   fun(Nucleotide)-> {Nucleotide, count(Strand, Nucleotide)} end,
   nucleotides()
   ).