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4d47's solution

to Accumulate in the Erlang Track

Instructions
Test suite
Solution

Implement the accumulate operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.

Given the collection of numbers:

  • 1, 2, 3, 4, 5

And the operation:

  • square a number (x => x * x)

Your code should be able to produce the collection of squares:

  • 1, 4, 9, 16, 25

Check out the test suite to see the expected function signature.

Restrictions

Keep your hands off that collect/map/fmap/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

Running tests

In order to run the tests, issue the following command from the exercise directory:

For running the tests provided, rebar3 is used as it is the official build and dependency management tool for erlang now. Please refer to the tracks installation instructions on how to do that.

In order to run the tests, you can issue the following command from the exercise directory.

$ rebar3 eunit

Test versioning

Each problem defines a macro TEST_VERSION in the test file and verifies that the solution defines and exports a function test_version returning that same value.

To make tests pass, add the following to your solution:

-export([test_version/0]).

test_version() ->
  1.

The benefit of this is that reviewers can see against which test version an iteration was written if, for example, a previously posted solution does not solve the current problem or passes current tests.

Questions?

For detailed information about the Erlang track, please refer to the help page on the Exercism site. This covers the basic information on setting up the development environment expected by the exercises.

Source

Conversation with James Edward Gray II https://twitter.com/jeg2

Submitting Incomplete Solutions

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

accumulate_tests.erl

-module(accumulate_tests).

-include_lib("erl_exercism/include/exercism.hrl").
-include_lib("eunit/include/eunit.hrl").

accumulate_squares_test() ->
  Fn = fun(Number) -> Number * Number end,
  Ls = [1, 2, 3],
  ?assertEqual([1, 4, 9], accumulate:accumulate(Fn, Ls)).

accumulate_upcases_test() ->
  Fn = fun(Word) -> string:to_upper(Word) end,
  Ls = string:tokens("hello world", " "),
  ?assertEqual(["HELLO", "WORLD"], accumulate:accumulate(Fn, Ls)).

accumulate_reversed_strings_test() ->
  Fn = fun(Word) -> lists:reverse(Word) end,
  Ls = string:tokens("the quick brown fox etc", " "),
  ?assertEqual(["eht", "kciuq", "nworb", "xof", "cte"], accumulate:accumulate(Fn, Ls)).

accumulate_recursively_test() ->
  Chars = string:tokens("a b c", " "),
  Nums = string:tokens("1 2 3", " "),
  Fn = fun(Char) -> [Char ++ Num || Num <- Nums] end,
  ?assertEqual([["a1", "a2", "a3"], ["b1", "b2", "b3"], ["c1", "c2", "c3"]], accumulate:accumulate(Fn, Chars)).

version_test() ->
  ?assertMatch(1, accumulate:test_version()).
-module(accumulate).

-export([accumulate/2, test_version/0]).

test_version() -> 1.

accumulate(Fn, Ls) -> [Fn(X) || X <- Ls].

What can you learn from this solution?

A huge amount can be learnt 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 I could read more about to develop my understanding?