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xdl's solution

to Two Fer in the Common Lisp Track

Published at Apr 03 2020 · 0 comments
Instructions
Test suite
Solution

Two-fer or 2-fer is short for two for one. One for you and one for me.

Given a name, return a string with the message:

One for X, one for me.

Where X is the given name.

However, if the name is missing, return the string:

One for you, one for me.

Here are some examples:

Name String to return
Alice One for Alice, one for me.
Bob One for Bob, one for me.
One for you, one for me.
Zaphod One for Zaphod, one for me.

Setup

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

Formatting

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.

VIM

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

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.

Source

https://github.com/exercism/problem-specifications/issues/757

Submitting Incomplete Solutions

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

two-fer-test.lisp

;;;
;;; two-fer v1.2.0
;;;
(ql:quickload "lisp-unit")
#-xlisp-test (load "two-fer")

(defpackage #:two-fer-test
  (:use #:common-lisp #:lisp-unit))
(in-package #:two-fer-test)

(define-test
    no-name-given-nil
    (assert-equal
     "One for you, one for me."
     (two-fer:twofer nil)))


(define-test
  a-name-given
  (assert-equal
    "One for Alice, one for me."
    (two-fer:twofer "Alice")))


(define-test
    another-name-given
    (assert-equal
     "One for Bob, one for me."
     (two-fer:twofer "Bob")))

(define-test
    no-name-given
    (assert-equal
     "One for you, one for me."
     (two-fer:twofer)))

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

(defun twofer (&optional name)
  (format nil "One for ~A, one for me." (if name name "you")))

xdl's Reflection

Mentor pointed out more idiomatic way of writing the `if` form: `(or name "you")`