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to Triangle in the Common Lisp Track

Published at Sep 03 2019 · 0 comments
Test suite

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.


Check out Installing Common Lisp for instructions to get started or take a look at the guides available in the track's side bar.


While Common Lisp doesn't care about indentation and layout of code, nor whether you use spaces or tabs, this is an important consideration for submissions to exercism.io. Excercism.io's code widget cannot handle mixing of tab and space characters well so using only spaces is recommended to make the code more readable to the human reviewers. Please review your editors settings on how to accomplish this. Below are instructions for popular editors for Common Lisp.


Use the following commands to ensure VIM uses only spaces for indentation:

:set tabstop=2
:set shiftwidth=2
:set expandtab

(or as a oneliner :set tabstop=2 shiftwidth=2 expandtab). This can be added to your ~/.vimrc file to use it all the time.


Emacs is very well suited for editing Common Lisp and has many powerful add-on packages available. The only thing that one needs to do with a stock emacs to make it work well with exercism.io is to evaluate the following code:

(setq-default indent-tabs-mode nil)

This can be placed in your ~/.emacs (or ~/.emacs.d/init.el) in order to have it set whenever Emacs is launched.

One suggested add-on for Emacs and Common Lisp is SLIME which offers tight integration with the REPL; making iterative coding and testing very easy.


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.


(ql:quickload "lisp-unit")
#-xlisp-test (load "triangle")

(defpackage #:triangle-test
  (:use #:cl #:lisp-unit))

(in-package #:triangle-test)

(define-test equilateral-1
  (assert-equal  :equilateral (triangle:triangle 2 2 2)))
(define-test equilateral-2
  (assert-equal  :equilateral (triangle:triangle 10 10 10)))
(define-test isoceles-1
  (assert-equal  :isosceles (triangle:triangle 3 4 4)))
(define-test isoceles-2
  (assert-equal  :isosceles (triangle:triangle 4 3 4)))
(define-test scalene
  (assert-equal  :scalene (triangle:triangle 3 4 5)))
(define-test invalid-1
  (assert-equal  :illogical (triangle:triangle 1 1 50)))
(define-test invalid-2
  (assert-equal  :illogical (triangle:triangle 1 2 1)))

(let ((*print-errors* t)
      (*print-failures* t))
  (run-tests :all :triangle-test))
(in-package #:cl-user)
(defpackage #:triangle
  (:use #:cl)
  (:export #:triangle))

(in-package #:triangle)

(defun count-unique (seq)
  (length (delete-duplicates seq)))

(defun greater-than-zero-p (n)
  (> n 0))

(defun valid-p (a b c)
  (and (every #'greater-than-zero-p (list a b c))
       (< a (+ b c))
       (< b (+ a c))
       (< c (+ a b))))

(defun triangle (a b c)
  (if (valid-p a b c)
    (case (count-unique (list a b c))
      (1 :equilateral)
      (2 :isosceles)
      (otherwise :scalene))

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