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Conditionals & Logic

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if Statement

An if statement executes a code block when its condition evaluates to true. If the condition is false, the code block does not execute. 

var halloween = true 

if halloween {
  print("Trick or treat!")
}

// Prints: Trick or treat!

else Statement

An else statement is a partner to an if statement. When the condition for the if statement evaluates to false, the code within the body of the else will execute.

var turbulence = false 

if turbulence {
  print("Please stay seated.")
} else {
  print("You may freely move around.")
}

// Prints: You may freely move around.

else if Statement

An else if statement provides additional conditions to check for within a standard if/else statement. else if statements can be chained and exist only after an if statement and before an else. 

var weather = "rainy" 

if weather == "sunny" {
  print("Grab some sunscreen")
} else if weather == "rainy" {
  print("Grab an umbrella")
} else if weather == "snowing" {
  print("Wear your snow boots")
} else {
  print("Invalid weather")
}

// Prints: Grab an umbrella

Comparison Operators

Comparison operators compare the values of two operands and return a Boolean result:

  • < less than
  • > greater than
  • <= less than or equal to
  • >= greater than or equal to
  • == equal to
  • != not equal to 
5 > 1       // true 
6 < 10      // true 
2 >= 3      // false
3 <= 5      // true
"A" == "a"  // false
"B" != "b"  // true

Ternary Conditional Operator

The ternary conditional operator, denoted by a ?, creates a shorter alternative to a standard if/else statement. It evaluates a single condition and if true, executes the code before the :. If the condition is false, the code following the : is executed.

var driverLicense = true

driverLicense ? print("Driver's Seat") : print("Passenger's Seat") 

// Prints: Driver's Seat

switch Statement

The switch statement is a type of conditional used to check the value of an expression against multiple cases. A case executes when it matches the value of the expression. When there are no matches between the case statements and the expression, the default statement executes.

var secondaryColor = "green"

switch secondaryColor {
  case "orange":
    print("Mix of red and yellow")
  case "green":
    print("Mix of blue and yellow")
  case "purple":
    print("Mix of red and blue") 
  default: 
    print("This might not be a secondary color.") 
}

// Prints: Mix of blue and yellow

switch Statement: Interval Matching

Intervals within a switch statement’s case provide a range of values that are checked against an expression. 

let year = 1905
var artPeriod: String

switch year {
  case 1860...1885:
    artPeriod = "Impressionism"
  case 1886...1910:
    artPeriod = "Post Impressionism"
  case 1912...1935: 
    artPeriod = "Expressionism"
  default:  
    artPeriod = "Unknown"
}

// Prints: Post Impressionism

switch Statement: Compound Cases

A compound case within a switch statement is a single case that contains multiple values. These values are all checked against the switch statement’s expression and are separated by commas. 

let service = "Seamless"

switch service {
  case "Uber", "Lyft":
    print("Travel")
  case "DoorDash", "Seamless", "GrubHub":
    print("Restaurant delivery")
  case "Instacart", "FreshDirect":
    print("Grocery delivery")
  default: 
    print("Unknown service")
}

// Prints: Restaurant delivery

switch Statement: where Clause

Within a switch statement, a where clause is used to test additional conditions against an expression. 

let num = 7

switch num {
  case let x where x % 2 == 0:
    print("\(num) is even")
  case let x where x % 2 == 1:
    print("\(num) is odd")
  default:
    print("\(num) is invalid")
}

// Prints: 7 is odd

Logical Operator !

The logical NOT operator, denoted by a !, is a prefix operator that negates the value on which it is prepended. It returns false when the original value is true and returns true when the original value is false.

!true          // false
!false         // true

Logical Operator &&

The logical AND operator, denoted by an &&, evaluates two operands and returns a Boolean result. It returns true when both operands are true and returns false when at least one operand is false.

true && true    // true
true && false   // false 
false && true   // false 
false && false  // false

Logical Operator ||

The logical OR operator, denoted by ||, evaluates two operands and returns a Boolean result. It returns false when both operands are false and returns true when at least one operand is true.

true || true    // true
true || false   // true
false || true   // true 
false || false  // false

Combining Logical Operators

Logical operators can be chained in order to create more complex logical expressions. When logical operators are chained, it’s important to note that the && operator has a higher precedence over the || operator and will get evaluated first. 

!false && true || false  // true

/* 
!false && true evaluates first and returns true. Then, the expression, true || false evaluates and returns the final result, true. 
*/ 

false || true && false   // false

/* 
true && false evaluates first which returns false. Then, the expression, false || false evaluates and returns the final result, false. 
*/

Controlling Order of Execution

Within a Swift logical expression, parentheses, (), can be used to organize and control the flow of operations. The usage of parentheses within a logical expression overrides operator precedence rules and improves code readability. 

// Without parentheses:

true || true && false || false      // true 

// With parentheses:

(true || true) && (false || false)  // false

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