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Conditional Expressions

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If Expressions

An if expression is a conditional that runs a block of code when its condition has a true value.

var morning = true

if (morning) {
  println("Rise and shine!")
}
// Prints: Rise and shine!

Else Expressions

An else expression is a conditional that runs a block of code only when the conditions contained in the previous expressions have false values.

var rained = false

if (rained) {
  println("No need to water the plants today.")
} else {
  println("Plants need to be watered!")
}
// Prints: Plants need to be watered!

Else-If Expressions

An else-if expression allows for more conditions to be evaluated within an if/else expression.

You can use multiple else-if expressions as long as they appear after the if expression and before the else expression.

var age = 65

if (age < 18 ) {
  println("You are considered a minor.")
} else if (age < 60) {
  println("You are considered an adult.")
} else {
  println("You are considered a senior.")
}

// Prints: You are considered a senior.

Comparison Operators

Comparison operators are symbols that are used to compare two values in order to return a result of true or false. Comparison operators include >, <, >=, <=.

var myAge = 19
var sisterAge = 11
var cousinAge = 11

myAge > sisterAge  // true
myAge < cousinAge  // false
myAge >= cousinAge // true
myAge <= sisterAge // false

Logical Operators

Logical operators are symbols used to evaluate the relationship between two or more Boolean expressions in order to return a true or false value.

Logical operators include !, &&, and ||.

var humid = true
var raining = true
var jacket = false

println(!humid)
// Prints: false

println(jacket && raining)
// Prints: true

println(humid || raining)
// Prints: true

The AND Operator: &&

The logical AND operator (&&) is used to compare the relationship between two Boolean expressions and will only return a true value if both expressions are true.

var humid = true
var raining = true
var shorts = false
var sunny = false

// true AND true
println(humid && raining) //  true

// true AND false
println(humid && shorts) //  false

// false AND true
println(sunny && raining) //  false

// false AND false
println(shorts && sunny) // false

The OR Operator : ||

The logical OR operator (||) is used to compare the relationship between two Boolean expressions and will return true when at least one of the expressions are true.

var late = true
var skipBreakfast = true
var underslept = false
var checkEmails = false


// true OR true
println(skipBreakfast || late) //  true

// true OR false
println(late || checkEmails) //  true

// false OR true
println(underslept || late) //  true

// false OR false
println(checkEmails || underslept) // false

The NOT Operator: !

The logical NOT operator (!) evaluates the value of a Boolean expression and then returns its negated value.

var hungry = true
var full = false

println(!hungry) //  false
println(!full) //  true

Order of Evaluation

The order of evaluation when using multiple logical operators in a single Boolean expression is:

  1. Expressions placed in parentheses.
  2. NOT(!) operator.
  3. AND(&&) operator.
  4. OR(||) operator.
!true && (false || true) // false
/*
(false || true) is evaluated first returning true. Then,
!true && true is evaluated returning the final result, false. 
*/

!false && true || false // true
/*
!false is evaluated first returning true. Then true && true are evaluated, returning true. Then, true || false is evaluated which ends up returning true.
*/

Nested Conditionals

A nested conditional is a conditional that exists within another conditional.

var studied = true
var wellRested = true

if (wellRested) {
	println("Best of luck today!")  
  if (studied) {
    println("You should be prepared for your exam!")
  } else {
    println("Take a few hours to study before your exam!")
  }
}

// Prints: Best of luck today!
// Prints: You should be prepared for your exam!

When Expressions

A when expression controls the flow of code by evaluating the value of a variable in order to determine what code gets executed.

var grade = "A"

when(grade) {
  "A" -> println("Excellent job!")
  "B" -> println("Very well done!")
  "C" -> println("You passed!")
  else -> println("Close! Make sure to perpare more next time!")
}
// Prints: Excellent job!

The Range Operator

The range operator (..) is used to create a succession of number or character values.

var height = 46 // inches

if (height in 1..53) {
  println("Sorry, you must be at least 54 inches to ride the rollercoaster.")
}
// Prints: Sorry, you must be at least 54 inches to ride the rollercoaster.

Equality Operators

Equality operators are symbols that are used to compare the equivalence of two values in order to return true or false. Equality operators include == and !=.

var myAge = 22
var sisterAge = 21

myAge == sisterAge // false
myAge !== sisterAge // true

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