Conditionals

if

(def temperature 31)

(println (if (> temperature 30) "Hot" "Not hot"))

(if (> temperature 30)
  (do
    (println "Stay hydrated")
    (println "Find some shade"))
  (println "Enjoy the weather"))

Hot
Stay hydrated
Find some shade

• (if test then else) is an expression: it returns the value of the branch that ran.
• Each branch is one form. Use do to run several forms; do returns the last one's value.
• Without an else branch, if returns nil when the test is falsy.

when

(def items ["apple"])

(when (seq items)
  (println "You have" (count items) "item(s)")
  (println "First:" (first items)))

(when-not (seq [])
  (println "The list is empty"))

You have 1 item(s)
First: apple
The list is empty

when is if with no else and an implicit do — use it for side effects that should happen only sometimes.

cond

(defn describe [n]
  (cond
    (neg? n)  "negative"
    (zero? n) "zero"
    (< n 10)  "small"
    :else     "large"))

(println (map describe [-5 0 7 250]))

(negative zero small large)

cond checks test/result pairs in order and returns the first match. :else is just a keyword — always truthy — used as the catch-all.

case

(defn http-status [code]
  (case code
    200 "OK"
    404 "Not Found"
    (500 502 503) "Server Error"
    "Unknown"))

(println (map http-status [200 404 503 418]))

(OK Not Found Server Error Unknown)

case compares against constants (numbers, strings, keywords) in constant time. A list of constants matches any of them, and the final lone value is the default — without one, an unmatched value throws an exception.

Practice

Write fizzbuzz that returns "FizzBuzz", "Fizz", "Buzz", or the number, and print the results for 1 to 15.

(defn fizzbuzz [n]
  (cond
    (zero? (mod n 15)) "FizzBuzz"
    (zero? (mod n 3))  "Fizz"
    (zero? (mod n 5))  "Buzz"
    :else n))

(println (map fizzbuzz (range 1 16)))

(1 2 Fizz 4 Buzz Fizz 7 8 Fizz Buzz 11 Fizz 13 14 FizzBuzz)