Lets Not Make Excuses

Making decisions is an common part of writing code, so in this section we will cover functions in Clojure that help us take decisions.

You will be introduced to the following functions for the first time:

• let - assign a name to a value (a value being a number, string, collection or function)
• cond, case - make a decision based on some condition

Local names with let

So far we have given names to values using the def function. This makes the name available throughout our code (and the page in this curriculum).

If we only need to use a name in the same few lines of code, we can use the let function instead.

In our previous making decisions examples using the if function, we could have used the let function instead

(let [sleepy true]
  (if sleepy
    "Drink some coffee or a nice cup of tea"
    "You are awake, lets code...")

A let function gives a name to a value, just like the def function. However, you can only use the name inside of the parenthesis around the let function.

Making decisions with cond

So far we have made a “true or false” decision using the if statement. With the cond function you can make a decision based on the value of a condition. Which ever condition matches, its associated expression will be evaluated and returned.

The cond function returns the answer based on the first matching condition it finds. Each condition will return either true or false. The first condition to return true will return its associated answer.

We need to have some value to check in the cond function, so either a name defined by def or let can be used.

(let [sleepy "maybe"]
  (cond
    (= sleepy "no")    "Lets code some more..."
    (= sleepy "maybe") "Take a few deep breaths"
    (= sleepy "yawn")  "Time for a brisk walk"
    (= sleepy "zzzz")  "Drink some coffee or a nice cup of tea"
    :else "You must be asleep"))

Homework Excuse Generator

Create a simple excuses generator that will return a suitable excuse based on the reason specified

Fill in the following code, especially where there are ,,, characters.

(let [reason ,,,]
  (cond
    (= reason ,,, )      "I see green elephant-shaped spots"
    ,,,
    ))

Suggested approach

• create a local name using the let function for the value of reason
• Use cond to match the reason with the different excuses you want to generate

Once you have tried to solve this exercise, take a look at the suggested example

Note: This example could have also used the def function to assign the name reason to the value of the excuse, eg. “sick”. The main difference is that the name assignment only works within the let function, meaning that once the closing bracket ) of the let function is reached then the name reason can no longer be used.

Bonjour Clojure with case

The case function is slightly simpler version of the cond funciton, simply returning an expression based on which value matches the case.

In our example, we have joined an international company and we want our code to speak a few different languages. Lets set the language we want to greet people in and return the appropriate message.

Add different greetings in other languages

(let [greeting :fr]
  (case greeting
    :fr "bonjour monde clojure"
    ,,,
     ))

In Italian you would say "ciao mondo clojure" or in Spanish you could say "hola mundo clojure".

Once you have tried to solve the challenge, see the suggested example.

Bonus challenge

Now lets also decide if we want to return the message in uppercase. We will change our code around a little first though. We will define a name called language to hold the keyword for the language we want to use. Then we will define the message we want to return, using the case function. Finally we will decide whether to convert the message to uppercase characters.

Try changing the value of upper-case? to false and changing the language value

(let [language :fr
      message (case language
                :fr "bonjour monde clojure"
                ,,, )
      upper-case? true]
  (if upper-case?
    (clojure.string/upper-case message)
    message))

suggested example