Pal Picker

Pal Picker

Learning Exercise


Truthy And Falsy

In Common Lisp all values are "true" except for () which is "false". There are two special constant symbols t and nil whose values are true and false respectively.


Common lisp provides several different conditional expressions, the main difference being the number of branches they support.

  • when and unless allow for a single branch:
(when (= 2 2) "All is well")      ; => "All is well"
(unless (= 2 2) "Time to panic!") ; => NIL

The section after the test expression may be more than one expression.

  • if provides the classic if-then-else construct:
(if (= 2 2) 'how-honest 'you-liar) ; => HOW-HONEST

Note that both the then and else clauses can only be a single expression.

  • cond provides a way to have multiple branches without nesting if expressions:
(cond ((= 0 2) 'nope)
      ((= 1 2) 'try-again)
      ((= 2 2) 'quite-true)
      ((= 3 2) 'too-far)
      (t 'something-else))

Note that there is no limit to the number of expressions after the test expression.

  • case provides a classic 'switch' style construct: It checks a single value against a number of branches:
(case 'elder-beast
  (cat "Meow")
  (bird "Chirp")
  (dog "Bark")
  (otherwise "???"))
; => "???"

Note that like cond there is no limit to the number of expressions after the test expression.


When the Lisp aliens first came to Earth, they encountered countless weird and wonderful things, but one of their favorite discoveries was that of "pets". Let's be honest, what sort of alien wouldn't gush over a puppy or kitten?

Ludwig, after their recent holiday to Earth, has decided they want a pet of their own! The only issue? Far too many choices! Perhaps you could write a program to help Ludwig find and care for their next cuddly friend?

1. Picking a Pal

Though Ludwig is set on getting a pet, that's about as far as they've gotten in thinking things through. The first step, then, is deciding what sort of pet to get!

Ludwig wants to find a pal with a personality that complements their own. Could you write Ludwig a function called pal-picker that takes some personality trait (a keyword) and returns the type of pet (a string) with the given trait?

In this particular case:

  • :lazy -> "Cat"
  • :energetic -> "Dog"
  • :quiet -> "Fish"
  • :hungry -> "Rabbit"
  • :talkative -> "Bird"

For other, unknown personalities, your function should evaluate to: "I don't know... A dragon?"

(pal-picker :quiet) ; => "Fish"

2. In Their Natural Habitat

Now that Ludwig has a new friend, they'll need a place to stay! Can you help Ludwig select the right size bed / tank / cage for their pet?

Here, Ludwig needs a function called habitat-fitter for selecting the proper habitat size (a keyword) from their pet's weight in kilograms (an integer).

The habitats needed by each size pet are:

  • More than or equal to 40kg -> :massive
  • 20kg to 39kg inclusive -> :large
  • 10kg to 19kg inclusive -> :medium
  • 1kg to 9kg inclusive -> :small
  • Less than or equal to 0kg -> :just-your-imagination
(habitat-fitter 42) ; => :MASSIVE

3. And Now, We Feast

One thing all earthling pets have in common is their need for food! This concept is somewhat alien to Ludwig, however, as they are prone to forgetting to refill their pet's food-bowl.

Ludwig could use a simple function called feeding-time-p to alert them when the bowl needs refilling. The function would take a percent fullness (an integer) and return a message in the form of a string.

If the food level is:

  • Above 20% -> "All is well."
  • 20% or below -> "It's feeding time!"
(feeding-time-p 15) ; => "It's feeding time!"

4. A Code of Conduct

With all of the basics sorted, Ludwig is looking forward to trying out a number of exciting things like "petting" and playing "fetch". With that being said, not every pet is suitable for these activities.

Ludwig would like a pair of functions – pet and play-fetch – that take the type of pet (as a string) and return either nothing or a message like: "Maybe not with this pet..." if the action is unfitting.

Assume that only "Dog"s will play fetch and that all pets except "Fish" can be pet.

(pet "Dog")         ; => NIL
(play-fetch "Fish") ; => "Maybe not with this pet..."

This task requires string comparisons. This can be done using the string= function. We'll learn more about this later in the syllabus in the Strings lesson.

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