Module 7

Flow control

Example: validating data

If the user's input is valid, then save their data; otherwise, show an error message.

(if (valid? data)
  (save! data)
  (error "Your data was invalid"))

if

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

if examples

(if (> 3 1)
  "3 is greater than 1"
  "3 is not greater than 1")
;;=> "3 is greater than 1"

(if (> 1 3)
  "1 is greater than 3"
  "1 is not greater than 3")
;;=> "1 is not greater than 3"

What is truthy?

Everything except false or nil.

Truthy examples

(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"

Exercise: more name formatting

Write a function format-name that takes a map representing a user, with keys :firstname, :lastname, and possibly :middlename. It should return their name as a string, like so:

(format-name {:firstname "Margaret" :lastname "Atwood"})
;;=> "Margaret Atwood"

(format-name {:firstname "Ursula" :lastname "Le Guin" :middlename "K."})
;;=> "Ursula K. Le Guin"

BONUS: Flexible name formatting

Change format-name to take a second argument, order. If order equals :lastname, then the format should be "Last, First Middle"; otherwise, it should be "First Middle Last."

Boolean logic

and, or, and not

Let

let

(defn spread
  "Given a collection of numbers, return the difference
  between the largest and smallest number."
  [numbers]
  (let [largest (reduce max numbers)
        smallest (reduce min numbers)]
    (- largest smallest)))

(spread [10 7 3 -3 8]) ;=> 13

Exercise: Rewrite average

Go back to the average function you created before and use let to make it easier to read.

Bonus: Ordinal numbers

Given a number (positive integer), return the string representing the ordinal number. For many numbers, this is done by adding "th" to the end.

Part one: return the ordinal number string, but for numbers ending in 1, 2, or 3, add "st", "2nd", and "rd" respectively. Ex:

(ordinal 1)  ;=> "1st"
(ordinal 2)  ;=> "2nd"
(ordinal 3)  ;=> "3rd"
(ordinal 4)  ;=> "4th"
(ordinal 5)  ;=> "5th"
(ordinal 21) ;=> "21st"
(ordinal 22) ;=> "22nd"

You will need the rem function, which takes 2 integers and returns the remainder from dividing the first by the second. You will also find that nesting if forms (putting one inside another) can be useful.

Bonus: Ordinal numbers

Part two: our exceptions for numbers ending in 1, 2, or 3 are superceded when the last 2 digits are 11, 12, or 13, in which case just add "th". Ex:

(ordinal 10) ;=> "10th"
(ordinal 11) ;=> "11th"
(ordinal 12) ;=> "12th"
(ordinal 13) ;=> "13th"
(ordinal 14) ;=> "14th"

Part three: rewrite the nested if statements using the cond function.