Collection types & Control Flow

Slides

Agenda

Learning Objectives
Collection Types
Activity on Arrays & Dictionaries
Break
Control Flow
Activity on Control Flow
Wrap Up

Learning Objectives

By the end of this lesson, students should be able to:

Create Arrays to store collections of data
Create Dictionaries to store collections of data
Differentiate & use the various types of loops in Swift
Apply Optionals in creating and using different collection types

Arrays

The most common collection type in Swift.

Collections are containers that let us store multiple values together.

Arrays store multiple values of the same type in a list.

An ordered collection of values of the same type

Arrays are zero-indexed.

Zero-indexed means the index of the first element is always 0, the second is 1, third is 2 and so on.

The last element has an index equal to the number of values in the array minus one.

In the example above, there are 4 elements of type String. Their indices go from 0 to 3.

Creating an array

Using an array literal

let concentrations = ["MOB", "BEW", "FEW", "DS"]

The type is [String]

An array literal is a list of values separated by commas, inside square brackets.

The type inside the square brackets tell us the type of values the array can store.

Creating an empty array

var concentrations = []

Empty collection literal requires an explicit type

If we need an empty array, we need to specify it's type since Xcode can't infer it.

Here's two ways to do it.

var concentrations : [String] = []

var concentrations = [String]()

Appending elements

We can add elements to an array using different methods:

concentrations.append("ROB")
concentrations += ["ROB"]
concentrations.insert("ROB", at: 2)

The append method will add the new element at the end of the array.

+= operator will also add it at the end.

The insert method lets us define the position in the array where we want the new element.

Removing elements

There are several alternatives to remove elements.

concentrations.removeLast()
concentrations.remove(at:4)

Both methods will do two things, they will remove the element while also returning it in case you need to store it and use it.

Updating elements

There are several alternatives to remove elements.

concentrations[0] = "MOBILE"

Here we use the subscript syntax to update the content.

Important: Be sure to NOT use an index that goes beyond the bounds of the array. Or else the program will crash.

Swapping

concentrations.swap(1,2)

The swap method lets us exchange the position of two elements.

Sorting

concentrations.sort()
concentrations.sorted()

The sort method will order the elements in the array.

The sorted method will return a sorted copy of the array.

Practice Problems

Complete these practice problems in a playground.

Download the playground here

Dictionaries

An unordered collection of pairs.

Each pair has a key and a value.

Keys are unique, they can't appear more than once in a dictionary.

All keys have to be of the same type and all values have to be of the same type.

Dictionaries are useful when when want to look up values given an identifier. Just like looking up words in a dictionary.

Creating a dictionary

Using a dictionary literal.

var coursesAndStudents = ["MOB":30, "BEW":40, "FEW":30, "DS":40]

The type is [String:Int]

A list of key-value pairs separated by commas inside square brackets.

Creating an empty dictionary

var coursesAndStudents : [String:Int] = [:]

Accessing values in a dictionary

We can use subscripting just like in arrays. But since elements are not ordered, instead of looking for an index, we look for a key.

print(coursesAndStudents["FEW"]!)

Why are we using force unwrapping?

The return type is an optional. Meaning the dictionary will first check if there is a value with the key provided. If there is it will return the value, nil otherwise.

Adding to and Updating a dictionary

coursesAndStudents.updateValue(15, forKey: "ROB")
coursesAndStudents["ROB"] = 15

These both serve as a way to add a new pair and also to update an existing pair. The code will update the value for the key given or create a new pair if it can't find it.

Removing elements

coursesAndStudents.removeValue(forKey:"ROB")
coursesAndStudents["ROB"] = nil

These will remove the key and the corresponding value from the dictionary.

There is a difference between the two methods. Assigning a key to nil will remove the value and the key entirely. If we wanted to keep the key and set the value to nil (in case we are dealing with optionals) we should use the removeValue method.

Sets

An unordered collection of unique values of the same type.

let plantCollection: Set<String> = ["Pothos", "Monstera", "Calathea"]
let plantCollection = Set(["Pothos", "Monstera", "Calathea"])

The first option uses a type annotation while the second one let's the compiler infer the type.

let plantCollection: Set<String> = ["Pothos", "Monstera", "Calathea", "Pothos"]

print(plantCollection)

What will be the result in the console?

The result will be an unordered list and it will also show unique values. So even if we added the same plant twice, the set will make sure all the elements are unique.

Finding elements in a Set

print(plantCollection.contains("Monstera"))

Adding and removing elements

plantCollection.insert("Ficus")

let removedPlant = plantCollection.remove("Calathea")

Practice Problems

Complete these practice problems in a playground.

Download the playground here

In Class Activity

Individually answer:

When would you use a Dictionary vs an Array? Are there advantages/disadvantages when using each?
When would you use a Set vs an Array? Are there advantages/disadvantages when using each?
Is type annotation better than type inference?

Discuss your answers with a neighbor.

Do you share the same answers?
Any disagreements?
A question you both have for the instructor? Raise your hand to get help.

[10m] BREAK

Loops

while loop

while <CONDITION> {
  // code that will loop
}

The loop checks the condition for every iteration. When the condition is false, it will stop.

repeat-while loops

repeat {
  // code that will loop
} while <CONDITION>

The condition is evaluated at the end of the loop.

var result = 0

while result < 5{
  result = result + (result + 1)
}

var result = 0

repeat{
  result = result + (result + 1)
} while result < 10

What will be the result in each loop? -->

var result = 0

while true{
  result = result + (result + 1)
  if result >= 10{
    break
  }
}

We can end the loop using the break statement. It will stop the execution of the loop and continue with the code after the loop.

Countable ranges

let closedRange = 0...8

Goes from 0 to 8 inclusive.

let halfOpenRange = 0..<8

Goes from 0 up to, but not including 8.

For loop

for <CONSTANT> in <RANGE> {
  // code that will loop
}

let count = 5
var result = 0
for i in 1...count{
  result += i
}

The for loop iterates from 1 tp 5, and each time it will update the value of result.

When working with loops that update the value of a variable it is useful to do a whiteboard test and check what is its value in each iteration.

Its important to notice that the constant i is only visible inside the scope of the for loop.

Sometimes we don't need the loop constant, we just want to run a block of code certain number of times.

for _ in 0..count{
  // code that will loop
}

var result = 0
for i in 1...10 where i % 2 == 1 {
  result += i
}

This for loop has a where clause. It will loop through all of the values in the range but will only execute the block then the where condition is true.

Looping items in an array

var coursesAndStudents = ["MOB", "BEW", "FEW", "DS"]
for course in coursesAndStudents {
  print(course)
}

Looping items in a dictionary

var coursesAndStudents = ["MOB":30, "BEW":40, "FEW":30, "DS":40]
for (course, countStudents) in coursesAndStudents {
  print("\(course): \(countStudents)")
}

Lab Practice Problems

Complete these practice problems in a playground.

Additional Resources

Challenge: Conway's Game of life found here: Array's & Loops Swift Playgrounds
Apple's documentation on Arrays & Dictionaries

For more practice, try this out: Optionals & Dictionaries Swift Playgrounds

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README.md

README.md

Collection types & Control Flow

Slides

Agenda

Learning Objectives

Arrays

Creating an array

Creating an empty array

Appending elements

Removing elements

Updating elements

Swapping

Sorting

Practice Problems

Dictionaries

Creating a dictionary

Creating an empty dictionary

Accessing values in a dictionary

Adding to and Updating a dictionary

Removing elements

Sets

Finding elements in a Set

Adding and removing elements

Practice Problems

In Class Activity

[10m] BREAK

Loops

while loop

repeat-while loops

Countable ranges

For loop

Lab Practice Problems

Additional Resources

Files

README.md

Latest commit

History

README.md

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Collection types & Control Flow

Slides

Agenda

Learning Objectives

Arrays

Creating an array

Creating an empty array

Appending elements

Removing elements

Updating elements

Swapping

Sorting

Practice Problems

Dictionaries

Creating a dictionary

Creating an empty dictionary

Accessing values in a dictionary

Adding to and Updating a dictionary

Removing elements

Sets

Finding elements in a Set

Adding and removing elements

Practice Problems

In Class Activity

[10m] BREAK

Loops

while loop

repeat-while loops

Countable ranges

For loop

Lab Practice Problems

Additional Resources