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angelikatyborska's solution

to Triangle in the Erlang Track

Published at Oct 12 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

Determine if a triangle is equilateral, isosceles, or scalene.

An equilateral triangle has all three sides the same length.

An isosceles triangle has at least two sides the same length. (It is sometimes specified as having exactly two sides the same length, but for the purposes of this exercise we'll say at least two.)

A scalene triangle has all sides of different lengths.


For a shape to be a triangle at all, all sides have to be of length > 0, and the sum of the lengths of any two sides must be greater than or equal to the length of the third side. See Triangle Inequality.

Dig Deeper

The case where the sum of the lengths of two sides equals that of the third is known as a degenerate triangle - it has zero area and looks like a single line. Feel free to add your own code/tests to check for degenerate triangles.

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


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.


The Ruby Koans triangle project, parts 1 & 2 http://rubykoans.com

Submitting Incomplete Solutions

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


%% Based on canonical data version 1.2.0
%% https://github.com/exercism/problem-specifications/raw/master/exercises/triangle/canonical-data.json
%% This file is automatically generated from the exercises canonical data.



'1_equilateral_true_if_all_sides_are_equal_test'() ->
    ?assertMatch(equilateral, triangle:kind(2, 2, 2)).

'2_equilateral_all_zero_sides_are_illegal_so_the_triangle_is_not_equilateral_test'() ->
		  "all side lengths must be positive"},
		 triangle:kind(0, 0, 0)).

'3_equilateral_sides_may_be_floats_test'() ->
		 triangle:kind(5.0e-1, 5.0e-1, 5.0e-1)).

'4_isosceles_true_if_last_two_sides_are_equal_test'() ->
    ?assertMatch(isosceles, triangle:kind(3, 4, 4)).

'5_isosceles_true_if_first_two_sides_are_equal_test'() ->
    ?assertMatch(isosceles, triangle:kind(4, 4, 3)).

'6_isosceles_true_if_first_and_last_sides_are_equal_test'() ->
    ?assertMatch(isosceles, triangle:kind(4, 3, 4)).

'7_isosceles_sides_that_violate_triangle_inequality_are_not_isosceles_even_if_two_are_equal_1_test'() ->
		  "side lengths violate triangle inequality"},
		 triangle:kind(1, 1, 3)).

'8_isosceles_sides_that_violate_triangle_inequality_are_not_isosceles_even_if_two_are_equal_2_test'() ->
		  "side lengths violate triangle inequality"},
		 triangle:kind(1, 3, 1)).

'9_isosceles_sides_that_violate_triangle_inequality_are_not_isosceles_even_if_two_are_equal_3_test'() ->
		  "side lengths violate triangle inequality"},
		 triangle:kind(3, 1, 1)).

'10_isosceles_sides_may_be_floats_test'() ->
		 triangle:kind(5.0e-1, 4.0e-1, 5.0e-1)).

'11_scalene_true_if_no_sides_are_equal_test'() ->
    ?assertMatch(scalene, triangle:kind(5, 4, 6)).

'12_scalene_sides_that_violate_triangle_inequality_are_not_scalene_even_if_they_are_all_different_test'() ->
		  "side lengths violate triangle inequality"},
		 triangle:kind(7, 3, 2)).

'13_scalene_sides_may_be_floats_test'() ->
		 triangle:kind(5.0e-1, 4.0e-1,


kind(A, B, C) when (A =< 0) or (B =< 0) or (C =< 0) ->
  {error, "all side lengths must be positive"};
kind(A, B, C) when (A + B < C) or (A + C < B) or (B + C < A) ->
  {error, "side lengths violate triangle inequality"};
kind(A, A, A) -> equilateral;
kind(A, A, _) -> isosceles;
kind(_, A, A) -> isosceles;
kind(A, _, A) -> isosceles;
kind(_, _, _) -> scalene.

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