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to Nucleotide Count in the OCaml Track

Published at Mar 08 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Given a single-stranded DNA string, compute how many times each nucleotide occurs in the string.

The genetic language of every living thing on the planet is DNA. DNA is a large molecule that is built from an extremely long sequence of individual elements called nucleotides. Four types exist in DNA and these differ only slightly and can be represented as the following symbols:

  • 'A' for Adenine,
  • 'C' for Cytosine,
  • 'G' for Guanine, and
  • 'T' for Thymine.

Since not all char are valid nucleotides and not all string are valid DNA strings, part of this exercise is to only give a result for meaningful input. See Real World Ocaml Chapter 7. Error Handling for more information.

The exercise consists of two parts:

  • Given an input DNA string and a nucleotide, count the number of times the nucleotide occurs in the string. If the nucleotide c is invalid, or the DNA string contains an invalid nucleotide c, the result should be Error c.
  • Given an input DNA string, count all the nucleotides in the string and gather the result in a Map. If the DNA string contains an invalid nucleotide c, the result should be Error c. Otherwise the result should be Ok map.

See test.ml for examples.

Getting Started

For installation and learning resources, refer to the exercism help page.


To work on the exercises, you will need Opam and Base. Consult opam website for instructions on how to install opam for your OS. Once opam is installed open a terminal window and run the following command to install base:

opam install base

To run the tests you will need OUnit. Install it using opam:

opam install ounit

Running Tests

A Makefile is provided with a default target to compile your solution and run the tests. At the command line, type:


Interactive Shell

utop is a command line program which allows you to run Ocaml code interactively. The easiest way to install it is via opam:

opam install utop

Consult utop for more detail.

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The exercism/ocaml repository on GitHub is the home for all of the Ocaml exercises.

If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue. We'll do our best to help you!


The Calculating DNA Nucleotides_problem at Rosalind http://rosalind.info/problems/dna/

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


open Base
open OUnit2

module NC = Nucleotide_count

(* Assert that two 'int option' values are equivalent. *)
let aire exp got _ctxt =
  let printer m =
    |> Sexp.to_string_hum ~indent:1
  in assert_equal exp got ~printer

(* Assert that two '(int Char.Map.t, char) Result.t' values are equivalent. *)
let amre exp got _ctxt =
  let sexp_of_map = Map.sexp_of_m__t (module Char) in
  let printer m =
    Result.sexp_of_t (sexp_of_map Int.sexp_of_t) Char.sexp_of_t m
    |> Sexp.to_string_hum ~indent:1
  let cmp exp got = match exp, got with
    | Ok exp_map, Ok got_map -> Map.equal Int.equal exp_map got_map
    | Error c1, Error c2     -> Char.equal c1 c2
    | _ -> false
  in assert_equal exp got ~cmp ~printer

let tests =
  [ "Empty DNA string has no invalid nucleotides" >:: aire (Error 'X') (NC.count_nucleotide "" 'X');
    "Non-empty DNA string has no invalid nucleotides" >:: aire (Error 'X') (NC.count_nucleotide "ACGT" 'X');
    "Invalid DNA string has no invalid nucleotides" >:: aire (Error 'X') (NC.count_nucleotide "ACGXT" 'A');

    "Empty DNA string has zero Adenine nucleotides" >:: aire (Ok 0) (NC.count_nucleotide "" 'A');
    "DNA string with one Adenine nucleotide" >:: aire (Ok 1) (NC.count_nucleotide "A" 'A');
    "DNA string with five Cytosine nucleotides" >:: aire (Ok 5) (NC.count_nucleotide "CCCCC" 'C');
    "DNA string with two Guanine nucleotides" >:: aire (Ok 2) (NC.count_nucleotide "ACGGT" 'G');
    "DNA string with three Thymine nucleotides" >:: aire (Ok 3) (NC.count_nucleotide "CACTAGCTGCT" 'T');

    "Invalid DNA string has no nucleotides" >::
      amre (Error 'X') (NC.count_nucleotides "ACGXT");

    "Empty DNA string has zero nucleotides" >::
      amre (Ok (Map.empty (module Char))) (NC.count_nucleotides "");

    "DNA string with two Adenine nucleotides" >::
      amre (Ok (Map.singleton (module Char) 'A' 2)) (NC.count_nucleotides "AA");

    "DNA string with one Adenine, two Cytosine nucleotides" >::
        let exp = Ok ((Map.of_alist_exn (module Char)) [('A', 1); ('C', 2)])
        in amre exp (NC.count_nucleotides "ACC")

    "DNA string with one Adenine, two Cytosine, three Guanine, four Thymine nucleotides" >::
        let exp = Ok ((Map.of_alist_exn (module Char)) [('A', 1); ('C', 2); ('G', 3); ('T', 4)])
        in amre exp (NC.count_nucleotides "CGTATGTCTG")

let () =
  run_test_tt_main ("nucleotide-counts tests" >::: tests)


open Base

let (=.) (a : 'a) (b : 'a) = Caml.(a = b)

let is_nucleotide : char -> bool = function
  | 'A' | 'C' | 'G' | 'T' -> true
  | _ -> false

let empty = Map.empty (module Char)

let count_nucleotide s c = if is_nucleotide c then
    let characters = String.to_list s in
    let rec loop chars needle acc = match chars with
      | [] -> Ok acc
      | ch :: _ when not (is_nucleotide ch) -> Error ch
      | ch :: tail ->
        loop tail needle (if ch =. needle then acc + 1 else acc) in
    loop characters c 0
    Error c

let count_nucleotides s =
  let add_result m c = (
    let r = count_nucleotide s c in
    Result.map r ~f:(fun it -> if it > 0 then Map.set m ~key:c ~data:it else m)
  ) in
  let rec gather_all nucleotides m = match nucleotides with
    | [] -> m
    | c :: tail -> (
      m |> Result.bind ~f:(fun it -> add_result it c)
        |> gather_all tail
    ) in
  let all_nucleotides = ['A'; 'C'; 'G'; 'T'] in
  gather_all all_nucleotides (Ok empty)


open Base

(* Count the number of times a nucleotide occurs in the string. *)
val count_nucleotide : string -> char -> (int, char) Result.t

(* Count the nucleotides in the string. *)
val count_nucleotides : string -> (int Map.M(Char).t, char) Result.t

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