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stevejb71's solution

to Change in the OCaml Track

Published at Jul 13 2018 · 1 comment
Test suite


This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Correctly determine the fewest number of coins to be given to a customer such that the sum of the coins' value would equal the correct amount of change.

For example

  • An input of 15 with [1, 5, 10, 25, 100] should return one nickel (5) and one dime (10) or [0, 1, 1, 0, 0]
  • An input of 40 with [1, 5, 10, 25, 100] should return one nickel (5) and one dime (10) and one quarter (25) or [0, 1, 1, 1, 0]

Edge cases

  • Does your algorithm work for any given set of coins?
  • Can you ask for negative change?
  • Can you ask for a change value smaller than the smallest coin value?

Getting Started

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


To work on the exercises, you will need Opam and Core. 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 core:

opam install core

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.

Feedback, Issues, Pull Requests

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!


Software Craftsmanship - Coin Change Kata https://web.archive.org/web/20130115115225/http://craftsmanship.sv.cmu.edu:80/exercises/coin-change-kata

Submitting Incomplete Solutions

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


(* Test/exercise version: "1.1.0" *)

open Base
open OUnit2
open Change

let printer = Option.value_map ~default:"None" ~f:(fun xs -> String.concat ~sep:";" (List.map ~f:Int.to_string xs))
let ae exp got _test_ctxt = assert_equal ~printer exp got

let tests = [
   "single coin change" >::
     ae (Some [25]) 
       (make_change ~target:25 ~coins:[1; 5; 10; 25; 100]);
   "multiple coin change" >::
     ae (Some [5; 10]) 
       (make_change ~target:15 ~coins:[1; 5; 10; 25; 100]);
   "change with Lilliputian Coins" >::
     ae (Some [4; 4; 15]) 
       (make_change ~target:23 ~coins:[1; 4; 15; 20; 50]);
   "change with Lower Elbonia Coins" >::
     ae (Some [21; 21; 21]) 
       (make_change ~target:63 ~coins:[1; 5; 10; 21; 25]);
   "large target values" >::
     ae (Some [2; 2; 5; 20; 20; 50; 100; 100; 100; 100; 100; 100; 100; 100; 100]) 
       (make_change ~target:999 ~coins:[1; 2; 5; 10; 20; 50; 100]);
   "possible change without unit coins available" >::
     ae (Some [2; 2; 2; 5; 10]) 
       (make_change ~target:21 ~coins:[2; 5; 10; 20; 50]);
   "another possible change without unit coins available" >::
     ae (Some [4; 4; 4; 5; 5; 5]) 
       (make_change ~target:27 ~coins:[4; 5]);
   "no coins make 0 change" >::
     ae (Some []) 
       (make_change ~target:0 ~coins:[1; 5; 10; 21; 25]);
   "error testing for change smaller than the smallest of coins" >::
     ae None 
       (make_change ~target:3 ~coins:[5; 10]);
   "error if no combination can add up to target" >::
     ae None 
       (make_change ~target:94 ~coins:[5; 10]);
   "cannot find negative change values" >::
     ae None 
       (make_change ~target:(-5) ~coins:[1; 2; 5]);

let () =
  run_test_tt_main ("change tests" >::: tests)
open Core.Std

let on ~f ~cmp x y = cmp (f x) (f y)

let find_shortest_coins_list_summing_to_target (shortest_coins_lists: int list option array) (target: int) (coins: int list): int list option =
  |> List.filter_map ~f:(fun coin ->
    if coin > target then None
    else shortest_coins_lists.(target - coin) |> Option.map ~f:(List.cons coin) 
  |> List.min_elt ~cmp:(on ~cmp:Int.compare ~f:List.length)

let make_change ~target ~coins = 
  if target < 0 then None
    let shortest_coins_lists = Array.init (target+1) ~f:(function | 0 -> Some [] | _ -> None) in
    for i = 1 to target do
      shortest_coins_lists.(i) <- find_shortest_coins_list_summing_to_target shortest_coins_lists i coins

Community comments

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Avatar of stevejb71

Made a few small changes:

Naming changes Use List.min_elt rather than List.sort + List.hd in find_shortest_coins_list_summing_to_target. Replaced some two branch pattern matches with if-else.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?