What is flow control?

Decisions how to react Details

“Flow control” is the programming term for deciding how to react to a given circumstance. We make decisions like this all the time. If it’s a nice day out, then we should visit the park; otherwise we should stay inside and play board games. If your car’s tank is empty, then you should visit a gas station; otherwise you should continue to your destination.

Testing conditions to react Details

Software is also full of these decisions. If the user’s input is valid, then we should save their data; otherwise we show an error message. The common pattern here is that you test some condition and react differently based on whether the condition is true or false.

if

Details

In Clojure, the most basic tool we have for the flow control is the if operator. It allows you to choose between two options depending upon a condition.

Reference: Conditional if

(if (< age legal-drinking-age)
  ["water" "soda"]
  ["water" "soda" "beer" "wine"])

(if conditional-expression
  expression-to-evaluate-when-true
  expression-to-evaluate-when-false)

Truthiness Details

When testing the truth of an expression, Clojure considers the values nil and false to be false and everything else to be true. Here are some examples:

Reference: Truthiness

(if "anything other than nil or false is considered true"
  "A string is considered true"
  "A string is not considered true")
;=> "A string is considered true"

(if nil
  "nil is considered true"
  "nil is not considered true")
;=> "nil is not considered true"

(if (get {:a 1} :b)
  "expressions which evaluate to nil are considered true"
  "expressions which evaluate to nil are not considered true")
;=> "expressions which evaluate to nil are not considered true"

cond

Details 1 Details 2

The if operator takes only one predicate. When we want to use multiple predicates, if is not a good option. We have to write nested, nested, … and nested if conditions. To branch to multiple situations, cond operator works well.

Reference: Conditional cond

(if (= n 1)
  "1st"
  (if (= n 2)
    "2nd"
    (if (= n 3)
      "3rd"
      (str n "th"))))

In this case cond comes in handy.

(cond
  (= n 1) "1st"
  (= n 2) "2nd"
  (= n 3) "3rd"
  :else   (str n "th"))

General form of cond operator

(cond
  predicate1 expression-to-evaluate-when-predicate1-is-true
  predicate2 expression-to-evaluate-when-predicate2-is-true
  ...
  :else      expression-to-evaluate-when-all-above-are-false)

cond example

(cond
  (< x 10)    "x is smaller than 10"
  (< 10 x 20) "x is between 10 and 20"
  (< 20 x 30) "x is between 20 and 30"
  (< 30 x 40) "x is between 30 and 40"
  :else       "x is bigger than 40")

Boolean logic with and, or, and not

Intro

if statements are not limited to testing only one thing. You can test multiple conditions using boolean logic. Boolean logic refers to combining and changing the results of predicates using and, or, and not.

If you’ve never seen this concept in programming before, remember that it follows the common sense way you look at things normally. Is this and that true? Only if both are true. Is this or that true? Yes, if either – or both! – are. Is this not true? Yes, if it’s false.

Truthy and falsey table Details

and, or, and not work like other functions (they aren’t exactly functions, but work like them), so they are in prefix notation, like we’ve seen with arithmetic.

and, or, and not combination Details

and, or, and not can be combined. This can be hard to read. Here’s an example:

(defn leap-year?
  "Every four years, except years divisible by 100, but yes for years divisible by 400."
  [year]
  (and (zero? (mod year 4))
       (or (zero? (mod year 400))
           (not (zero? (mod year 100))))))