Clojure Conditionals

Clojure Conditionals
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#1

You’re not going to get anywhere in a language if all you can do is define functions that print things and do simple arithmetic. Conditionals and logic are a fundamental part of making code that does interesting, useful things. Try and imagine a world without logic in programs: you wouldn’t even be able to simple things, like checking if two numbers are equal!

Logical Operators

Clojure, like most languages, has 3 logical operators: and, or and not. These functions take booleans (true or false) as arguments, and return booleans based on what those booleans are. Like everything in a Lisp, these operators use prefix notation, which means they might look weird to you.

(and true false)
; => false
(and true true)
; => true
(or false false)
; => false
(or true false)
; => true
(not true)
; => false
(not false)
; => true

:rocket: IDEOne it!

If

if allows you to execute code based on whether a boolean is true or false. if in Clojure looks quite weird, not because it uses prefix notation, but because there is no else keyword. If the condition is true, it evaluates the first expression. If it’s false, it executes the second.

(if (= (+ 2 2) 4)
  (println "Maths works!") ; this gets evaluated if 2 + 2 = 4
  (println "UH OH"))       ; this gets evaluated if 2 + 2 != 4
; => Maths works!
;    nil

This presents a problem: what if we want to do multiple things?

(if (= (+ 2 2) 4)
  (println "Maths works!")
  (println "Maths still works!")
  (println "UH OH"))
; => CompilerException java.lang.RuntimeException: Too many arguments to if

Thankfully, we have the do function to solve this problem. do evaluates multiple expressions, one after the other.

(if (= (+ 2 2) 4)
  (do                               ; all of this gets evaluated if 2 + 2 = 4
    (println "Maths works!")
    (println "Maths still works!"))
  (println "UH OH"))
; => Maths works!
;    Maths still works!
;    nil

Note: since if is, itself, an expression, there’s no need for a ternary operator like in many C-like languages.

var doesMathsWork = 2 + 2 === 4 ? "Maths works!" : "UH OH";
console.log(doesMathsWork);
// => Maths works!

Now that you look at it, doesn’t that look so weird? This is much easier to read and understand (ignoring the lack of the word else):

(def does-maths-work (if (= (+ 2 2) 4) "Maths works!" "UH OH"))
(println does-maths-work)
; => Maths works!
;    nil

:rocket: IDEOne it!

Alternatives to If

Clojure also has some macros that behave similarly to if, and can sometimes be more concise.

if-not is perhaps the most simple example - it’s if inverted. These two pieces of code are exactly the same:

(def does-maths-work (if (not (= (+ 2 2) 4)) "UH OH" "Maths works!"))
(def does-maths-work (if-not (= (+ 2 2) 4) "UH OH" "Maths works!"))

The first expression gets evaluated if it’s false, and the second gets evaluated if it’s true. Notice that using if-not avoids nesting our condition inside not, which can help make our code easier to understand.

when is another useful macro. These two pieces of code are also the same:

(if (= (+ 2 2) 4) (do (println "Maths works!") (println "Hooray!")))
(when (= (+ 2 2) 4) (println "Maths works!") (println "Hooray!"))

:rocket: IDEOne it!

Note: There is no when/else. when only executes if the condition is true.

Clojure has plenty of useful macros like these for all kinds of tasks. Try having a look at the Clojure documentation and see if you can find any more!

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