Swift 2.2 Brief Introduction for Experienced Programmer - Part 2 - Control Flow

Control Flow 

• For conditional branch we use if and switch.   
• For loops we use for-in, while and repeat-while.   
• Parentheses around the condition or loop variable are optional. 
• Braces around the body are required. 

if  Syntax: 

     if (Boolean expression) 

          {...} 

          else 

          {...} 

          elseif 

          {...}

See example below: 

     if someVar3 >= 50 { 

          //Do something for pass 

      } else { 

           //Do someThing for fail 

      } 

Below is an example using if:

var student1 = 40
var student2 = 80

if student1 > 50
{
    print("Pass")
}
else
{
    print("Fail")
}

if student2 > 50
{
    print("Pass")
}
else
{
    print("Fail")
}

• Please note that previously in C, any variable contains a 1 is considered true. However, in Swift, conditional expression must be evaluated to true or false.

let checksum1 = 1
//if checksum1 { print("0k") } else { print("Not OK") }  //This expression failed because checksum1 is 1 not True

let checksum2 = true
if checksum2 { print("0k") }  else { print("Not OK") }

Optional Variable 

Now, we would like to introduce optionals. Optionals are variable that could be nil (no value at all). The syntax of these variables are added with ? after the variable names and data type declaration.

•  var someData = 12 is not optional 
•  var someData: Int? is optional

To test optional, we use two methods. One of them is to use ?? as conditional test, if test failed, a substitute will be use

See example below:

let someVar3: Int? = nil
let someVarCheck1 = someVar3 ?? 0  //In this statement if someVar3 is nil, we will assign 0 to someVarCheck1

• Another way to test optional is to use if and let.

let someVar4: Int? = nil
if let someVarCheck2 = someVar4
{
    print(someVarCheck2)
}
else
{
    print("No Values in someVar4")
}

• Another example

var optionalNumber: Int? = 3
if let checkNum = optionalNumber
{
    print("Got value")
}
else
{
    print("No Value")
}

optionalNumber = nil
if let checkNum = optionalNumber
{
    print("Got value")
}
else
{
    print("No Value")
}

• If the optional is nil, the let statement is false, thus it will skipped to else statement 
• Note: Both methods of testing optional are very common in Swift programming. 
• If an optional must not be nil then we should use `??` to substitute a default value when the optional happen to be nil. 
• If we just want to trap an error using optional than we should use if and let method. 

Switches 

We use switch and case to support multiple conditional testing. Please note that the testing must be exhaustive, therefore, we need to add a `default` as a catch all condition at the last conditional check. 

switch keyVariable 
{ 
     case (number or string): code 
     case (number or string): code 
     default: doSomeOtherThing 
} 

• Please note that default is required, no break is necessary, and the testing of each condition must be exhaustive. 
• We usually use switch where there are few option that we want to test.

let option = 5

switch option {
    
case 1 : print("You have selected option 1")
case 2 : print("You have selected option 2")
case 3 : print("You have selected option 3")
case 4 : print("You have selected option 4")
case 5 : print("You have selected option 5")
default: print("Your selection is invalid!")
    
}

• Another example

var someNum1 = 3000
switch someNum1
{
case 1000 : let result1 = "your case is 1000"
case 2000 : let result1 = "your case is 2000"
case 3000, 4000 : let result1 = "your case is 3000 or 4000"
case 5000: let result1 = "your case is 5000"
default : let result1 = "you have no case"
}

• We can use string for switch, we can use let x where x.hasPrefix("somePrefix"): as a conditional text for string.

let food = "curry rice"
switch food {
    
case "chicken salad" : print("You have chicken rice")
case "duck rice" : print("You have duck rice")
case let x where x.hasPrefix("curry"): print("You like curry")
case "fried chicken": print("You have fried chicken")
default: print("Your food choice is something we don't know")
    
}

• let can be used with where in a pattern to assign the value that matched that part of a pattern to a constant Another example

let fruits = "waterchestnut"
switch fruits {
case "orange":
    let fruitComm = "This is orange"
case "apple":
    let fruitComm = "An Apple a day"
case let x where x.hasPrefix("water"):
    let fruitComm = "Is this fruit \(x) taste watery?"
default:
    let fruitComm = "Fruit is good for you."
}

• We can also allow multiple condition to flow through as the example below:

let partNumber = "7500A"
switch partNumber {
    
case "6100A", "7500A", "7000C" : print("The part \(partNumber) is in our store")
default : print("We do not sell this part")
    
}

for-in

• We use for and in to iterate through every items in a collection. Iterating through an array:

let someArray1 = [12, 34, 65, 87, 23, 87]
var sum: Int = 0

for eachItem in someArray1 {
    sum += eachItem
}

print(sum)

• We can also use dictionary to iterate each set of item

let someDictionary1 = [1:"A1", 2:"B2", 3:"C3"]
var total = 0
for (key, description) in someDictionary1 {
    
    print(key)
    print(description)
    total += 1
    
}

print(total)

• We can also have nested loop using for and in
• We can use range indicator such as ..< or .... 
• For example, we use 0..<100 to indicate a range of number 0 to 99. This statement include 0 but exclude 100. 
• We use 0...100 to indicate the range of number 0 to 100. 
• To exclude the lower range we can just start the lower value from where we want such as: 1..<99 to exclude 0.

Please check the playground for more examples.

while, repeat-while

• Usually we have while and do-while. In Swift, we use while and repeat-while. We list the basic example where every programmer knows

var sum1 = 0
var n1 = 1

while n1 <= 100 {
    
    sum1 += n1
    n1 += 1
}

print(sum1)

• Our next example use repeat and while. This is similar with do-while, just replace do with repeat.

var sum2 = 0
var n2 = 1

repeat {
    
    sum2 += n2
    n2 += 1
    
} while n2 = 100

print(sum2)

• Usually we use repeat-while to run the loop at least once

var sum3 = 0
var n3 = 20
repeat {
    
    sum3 += n3
    n3 += 1
    
} while n3 = 20

print(sum3)

• We also use repeat-while to run the loop and check for condition as shown in the example below.

var sum4 = 0
var additive = 20
repeat {
    
    sum4 += additive
    
} while sum4 < 100

print(sum4)

A collection of playgrounds file for this tutorial can be found at https://github.com/SwiftiCode/Swift-Brief-Note-Playground

*** End ***