W1301 Arrays

From Coder Merlin
Within these castle walls be forged Mavens of Computer Science ...
— Merlin, The Coder


Background[edit]

Key ConceptsKeyConceptsIcon.png
  • Abstract Data Types, or ADTs, represent models for data structures. The model specifies the required behavior (semantics) of the type, such as supported operations and the effect of those operations on the type.
  • The model specifies the required semantics (behavior) of the type, such as supported operations and the effect of those operations on the type.
  • ADTs are theoretical and are used in designing and analyzing algorithms and data structures. Compilers, however, might implement specific data structures fulfilling most of, if not all, the requirements of an ADT.

Arrays[edit]

The array data structure is a collection of elements each of which can be identified by an index. An array is stored so that the position of the element in memory (or other storage) can be easily calculated using only the index. For example, consider an array of integers:

Array Data Structure


In this array, each integer requires 32 bits (or 4 bytes). Thus, four bytes exist from the beginning of each element to the beginning of the next element. This allows us to calculate the position of an integer in this array by adding the product of the element index (starting from zero) and the bytes required per element to the foundation address. In this case, the formula would be:

0x0800 + (4 * index)

Arrays are very common data structures and are used in almost every program. CPUs generally enable assembly language programs to easily work with arrays. As an example, a base plus index mode enables a program to access an element by storing the foundation address in the base register and specifying the index (scaled by the size of each element) in the index register.

ADT Properties[edit]

As an abstract data type, arrays are:

  • A collection type, meaning that one variable conceptually “contains” many elements
  • Each item in the data structure is called an element
  • The collection is ordered, that is, the order of the elements is maintained
  • Elements can be accessed randomly, that is, there is no need to access elements in a certain order
  • The array is homogeneous, that is, all the elements are of the same type


One-Dimensional Array Declaration[edit]

The first step in using a one-dimensional array is to declare the array by using the outlined syntax below.

The following is a syntax template you can use for a one-dimensional array declaration:

Data_Type Array_Name [Const_Int_Expression];

Reference information for the array declaration:

  • Data_Type defines what type of data is stored in the array. Commonly used array types in a one-dimensional array are Int, String, Char, Bool, Var.
  • Array_Name defines the name of the array in the array declaration.
  • Const_Int_Expression defines the array size. This value must be greater than zero. When an array is defined as size 'n', the array indices will range from 0 to n-1.

You can create an array with a determined size and initialize it with a value using the below convention:

 var initailizedArray = [ArrayType](count: NumbeOfElements, repeatedValue: InitialValue) 

For example, the array can be set to hold four elements, with each element of the array holding an initial value of zero with this array definition:

var holderArray = [Int](count: 4, repeatedValue: 0)

Fun Use Case[edit]

Arrays in Swift can store elements of any type; these are known as heterogeneous collections. To define an array in Swift to store elements of any type, simply specify the variable as [Any].

Example

var test = [1, 2.4, "abc", "def", 4] as [Any]

To print the contents of the array, use the following statement:

print("The Array: \(test)")

This results in the following:

The Array: [1, 2.4, "abc", "def", 4] 

Performing Operations on a One-Dimensional Array[edit]

An array can be instantiated using a series of literals. For example:

   let oddNumbers = [1, 3, 5, 7, 9, 11]
   var states = ["California", "Florida", "Texas"]

An empty array can be instantiated by explicitly specifying its type:

    var emptyDoubleArray = [Double]()

The first element of a non-empty array is accessed using an index or subscript of 0. For example:

    let firstState = states[0]

You can determine if an array is empty using the isEmpty property. And you can determine the number of items in the array using the count property:

    if !states.isEmpty {
        print("The array contains \(states.count) states.")
    }

A shortcut to access the first and last elements uses the first and last property that return a type of Element? (optional Element).

    print("The first state in the array is \(states.first!)")
    print("The last state in the array is \(states.last!)")

Adding one element to the end of an array can be done with append:

    states.append("New York")

You can also insert an element at a specific index in the array:

    states.insert("New Jersey", at:1)

This shifts the other elements in the array to "make room" for the new element.

Finally, elements can be removed with the remove method:

    states.remove(at: 2)

Traversing One-Dimensional Arrays[edit]

Traversing is another key operation used when working with one-dimensional arrays. Traversing—also known as looping—is a great way to print specific items in an array, or iterate through a one-dimensional array in a method.

Let's use a simple example of iterating through a small array of state capitals to show the benefits of traversing a one-dimensional array.

Start by defining the array:

let statecapitals = [
    "Arizona": "Phoenix",
    "California": "Sacramento",
    "Florida": "Tallahassee"
    "Texas": "Austin"
]

Now that the array is defined, let's use a for loop to iterate through it and print each state and its capital.

for statecapital in statecapitals {
    print("\(statecapital.value) is the state capital of\(statecapital.key)")
}

When traversing a one-dimensional array, it might be important to access or print array items starting at the end of the list. To achieve this, use the Swift method reversed().

for statecapital in statecapitals.reversed() {
    print("\(statecapitals.value) is the state capital \(statecapitals.key)")
}

Quiz[edit]

1 True or False: The best way to traverse over an array is by using an "if" statement.

True
False

2 How do you define an array that supports multiple types?

var holder: Any[] = ["a", false, "hello", 1]
char holder = ["a", false, "hello", 1]
char holder: Any[] = ["a", false, "hello", 1]
var holder = ["a", true, "hello", 1]

3 True or False: The sort() method creates a newly sorted array.

False
True

4 What does the enumerated() function return?

All three
The size of the array
Both the item in the array and its position
The position of the item in the array


Searching a One-Dimensional Array[edit]

Often, you will need to find a specific item in an array or determine if an object exists within an array. To find elements in an array, the contains() method can be very useful. Let's take a look at how contains() can be applied.

if statecapitals.contains(where: {$0.name == "Phoenix"}) {
print("\(statecapitals.key) exists")
   // Here, we're looking to see if the element name exists within the array.

} else {
print("\(statecapitals.key) does not exist in the array")
// If there is no "Phoenix" element in the array.
}

Once you've determined the object is in the array, you can get the element with the following function:

if let capital = statecapitals.first(where: {$0.name == "Phoenix"}) {
   // Now you can use capital as you wish
} else {
   // the capital could not be found.
}

Quiz[edit]

1 Select the proper declaration for the contains() method.

func contains(_ element: Element) -> Bool
bool contains(_ element: Element) -> Bool
var contains(_ element: Element) -> Bool
char contains(_ element: Element) -> Bool

2 What instance method is used to return the first element that satisfies a given predictive?

func contains()
func contains(where)
func return
func first()

3 How do you determine if a value in a var type array is even?

Use the even method
Iterate using a for loop and divide by the modulo operator %.
Use the isEven method
Return all values and sort

4 True or False: There's no way to find the last element in an array

False
True


Filtering One-Dimensional Arrays[edit]

When working with a one-dimensional array, it can be useful to remove specific elements that aren't needed from the array. Fortunately, this is straightforward. To filter out specific elements in an array, use the filter() method. Let's take a look at how it works.

let capitals = ["Phoenix", "Sacramento", "Austin", "Tallahassee", "Santa Fe"]
let filteredcapitals = capitals.filter { word in
  return word.count >= 9
} 
print(filteredcapitals)
// filtered is ["Sacramento", "Tallahassee"]

You should consider some key points when filtering an array. First and foremost, the filter(isIncluded:) method in Swift takes a closure for each element in the source array. Further, when you apply a filter on an array, the original array is not modified. If this method returns an element, this element is included in a new filtered array. In this case, filter(isIncluded:) creates a new array with only the elements you want. However, if the method returns false, the element is effectively filtered out and not included in this new array.

Quiz[edit]

1 True or False: The filter array lets you find matching elements in an array.

False
True

2 What is the proper declaration for the filter method?

var filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]
bool filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]
char filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]
func filter(_ isIncluded: (Self.Element) throws -> Bool) rethrows -> [Self.Element]

3 True or False: When you apply the filter method, the original array is not modified. Instead, filter(isIncluded) creates a new array with only the element you're searching for.

False
True

4 In an array called words, what is the proper way to find words that are 4 letters or longer?

let filtered = words.filter { word in return word.count >= 4
let filtered = words.filter { word in return word.count >= 3
let filtered = filter.words { word in return word.count >= 4
let filtered = filter.words { word in return word.count >= 3


Sorting One-Dimensional Arrays[edit]

When working with arrays in Swift, you can sort elements using two methods: sort() and sorted(). Both are effective for sorting arrays; however, they are slightly different in what they return. With sort(), the array is sorted in place, meaning the original array is sorted. With sorted(), the original array elements are copied to a new array with the new sorting. Let's take a look at how you can use these methods.

When working with a simple array of a single type, the easiest way to sort the array is with the sort() method. In this example, the array is sorted in place.

var capitals = ["Phoenix", "Sacramento", "Austin", "Tallahassee", "Santa Fe"]
capitals.sort()

To work with a copy of the original array that is sorted, use the sorted() method. Let's take a look at the subtle difference in using this method.

struct Capitals {
    var capitalnames: String
// creating a custom struct for holding capitals
}

var afewcapitals = [
    afewcapitals(capital: "Phoenix"),
    afewcapitals(capital: "Sacramento"),
    afewcapitals(capital: "Austin"),
    afewcapitals(capital: "Santa Fe"),
 
]

capitalnames.sort {
    $0.capital < $1.capital
}

//Once the custom struct is created, we can sort the struct, copying the elements of capitalnames to sortedCapitals and sorted.

let sortedCapitals = capitalnames.sorted {
    $0.firstName < $1.firstName
}

Quiz[edit]

1 True or False: The sort() method creates a newly sorted array.

True
False

2 Create a new array named sortedUsers that is coped and sorted from the original array named users.

struct sortedUsers = users.sorted {$0.firstName < $1.firstName}
struct users = sortedUsers.sorted {$0.firstName < $1.firstName}
let users = sortedUsers.sorted {$0.firstName < $1.firstName}
let sortedUsers = users.sorted {$0.firstName < $1.firstName}

3 True or False: The sorted() method creates a newly sorted array.

False
True

4 What does the following return? mutating func sort(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows

Sorts the collection in a new array, using the given predicate as the comparison between array sizes
Sorts the collection in place, using the given predicate as the comparison between array sizes
Sorts the collection in a new array, using the given predicate as the comparison between elements
Sorts the collection in place, using the given predicate as the comparison between elements


Mutating Method for One-Dimensional Arrays[edit]

When creating structs that have a variable defined as a constant, some limitations apply in how you can use or alter the struct. Specifically, you may not change the properties of the struct once these properties have been defined as constant.

This can quickly become a challenge when you want to apply a different variable in that structure. This limitation might seem overly cautious; however, it protects you from unintentionally misapplying variable declarations to a struct. Fortunately, the mutating func() allows you to override the constant declaration in an array and change the property inside the method.

struct StateCapitals {
    var capitals: String

    mutating func newtype() {
        Capitals = "Hold"
    }
}
//This defines the structure, embedded with the property inside the method that can be altered.

//Now we can apply this mutating function as follows:

var capitals = StateCapitals(name: "Phoenix")
capitals.newtype()

Using the Reduce Method with One-Dimensional Arrays[edit]

The Reduce() method combines all the elements in an array and returns one new value. Often, Reduce() is used to sum all the integers or characters in an array.

The Swift declaration is as follows:

func reduce<Result>(_ initialResult: Result, _ nextPartialResult: 
(Result, Element) throws -> Result) rethrows -> Result

As seen above, the Reduce() function takes two arguments: the initial value and the closure.

  • The initial value stores the initial value. This argument can also store the value or result of returned by the closure.
  • The closure is used to capture and store references to any constants or variables, or both, from the context in which they're defined.

Keep in mind that Swift handles the memory management of capturing for you.


ComingSoonIcon.png
Coming Soon
  • Preconditions

Python

Python[edit]

Note: Python does not have arrays; lists are used instead

You can instantiate a list using a series of literals. For example:

oddNumbers = [1, 3, 5, 7, 9, 11]
states = ["California", "Florida", "Texas"]

To instantiate an empty list:

emptyArray = []

The first element of a non-empty list is accessed using an index or subscript of 0. For example:

firstState = states[0]

You can determine if a list is empty by checking its length:

if len( states ) > 0:
    print("The array contains " + str(len(states)) + " states.")

or by using if:

if states:
    print("The array contains " + str(len(states)) + " states.")

You can access the last element of a list by using -1 as the index. Access the first element by calling the negative length (e.g., states[-len(states)])

print("The first state in the array is " + states[-len(states)])
print("The last state in the array is " + states[-1])

To iterate over each element at a specific location in a list, use a for loop:

for i in range(0, len(states)):
    print(states[i])

A very easy way to iterate over all elements in a list, is to use a for loop:

for state in states:
    print("State: " + state)

To add one element to the end of a list, use append:

states.append("New York")

You can also insert an element at a specific index in the list:

states.insert(1,"New Jersey")

This shifts the other elements in the list to "make room" for the new element.

Finally, you can remove elements with the del keyword:

del states[2]

Exercises[edit]

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  •  M1301-15  Complete  Merlin Mission Manager  Mission M1301-15.

Quiz[edit]

1 For the array: int stats[4]; what is the range of the index?

1 to 6
0 to 3
0 to 5
0 to 4

2 True or False: Arrays can be passed as parameters to a method either by value or by reference.

True
False

3 What are the legal indexes for the array, if the array is defined as: int [] num = {3, 4, 5, 8,};

0,2,4,6
1,2,3,4
1,3,5,7
0,1,2,3

4 How do you create a one-dimensional string array called holder?

int holder = (String)()
int holder = [String]()
var holder = (String)()
var holder = [String]()

5 How do you set the first value of the array called holder to 50?

holder[one] = 50;
holder[zero] = 50;
holder[1] = 50;
holder[0] = 50;

6 For the array: int [] holder = {1, 2, 3, 4, 5}; How do you access the third element in the holder array?

holder(3)
holder(4)
holder[4]
holder[3]

7 True or False: A method can return a value of type array.

True
False

8 Which of the following statements outputs the fourth value in the holder array?

holder[3];
holder[3];
System.out.println(holder[4]);
System.out.println(holder[3]);

9 A one-dimensional array is an example of a structured data type.

True
False

10 What value is at index 1 in this array? String[] names = {"Michael", "Kobe", "Larry", "Magic"};

Larry
Magic
Kobe
Michael