Difference between revisions of "W1301 Arrays"

From Coder Merlin
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- func return
- func return


{
There's no way to fine the last element in an array
|type="()"}
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+ false
</quiz>
</quiz>



Revision as of 13:56, 7 January 2022

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 impact of those operations on the type.
  • The model specifies the required semantics (behavior) of the type, such as supported operations and the impact of those operations on the type.
  • ADTs are theoretical and are used in designing and analyzing algorithms and data structures. Compilers, however, may implement specific data structures fulfilling most, if not all, of 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 based only on the index. For example, consider an array of integers:

Array Data Structure


In this array, each integer requires thirty-two bits (or four bytes). Thus, there are four bytes 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 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 a single variable conceptually “contains” many elements
  • Each item in the data structure is termed 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 any particular order
  • The array is homogeneous, that is, all of 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 following the outlined syntax below.

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

Data_Type Array_Name [Const_Int_Expression];

Reference information regarding array declaration:

  • Data_Type defines what type of data is stored in the array. Commonly used array types within 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.

Arrays can also be created of a determined size and initialized with an initial value with the following convention:

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

For instance, the array can be set to hold four elements with each element of the array holding an initial value of zero with the following array definition.

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

Fun Use Case[edit]

Arrays in Swift can store elements of any type, 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]

Once you are ready to print the contents of the array simply use the following:

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

Resulting 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]

One can determine if an array is empty using the isEmpty property and 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 makes use of the first and last property which 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 a single element to the end of an array can be accomplished with append:

    states.append("New York")

We 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 within an array, or iterate through a one-dimensional array in a method.

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

We'll start by defining the array:

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

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

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

When traversing a one-dimensional array it may 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 The best way to traverse over an array is by using an "if" statement?

False
True

2 How to define an array that supports multiple types:

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

3 The sort() method creates a newly sorted array

False
True

4 What does the enumerated() function return?

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

5 Select the proper declaration for the for loop. The result should read "The number is ___" let count = 1...10

for count in number { print("Number is \(number)")}
for number in count { print("Number is (number)")}
for count in number { print("Number is (number)")}
for number in count { print("Number is \(number)")}


Searching a One-Dimensional Array[edit]

Oftentimes, you'll need to find a specific item in an array or determine if an object exists within an array. To find elements within 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 to be 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.

var contains(_ element: Element) -> Bool
func contains(_ element: Element) -> Bool
bool 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 return
func contains()
func contains(where)
func first()

3 There's no way to fine the last element in an array

false
true


Filtering One-Dimensional Arrays[edit]

When you're working with a one-dimensional array, it may be useful to remove specific elements that aren't needed from the array. Fortunately, removing elements from an array is quite straightforward. Simply by using the filter() method, one can filter out specific elements within an array. Let's take a look at how it works.

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

There are some key points to consider when filtering an array. First and foremost, the filter(isIncluded:) method in swift takes a closure for each element in the source Array that is enumerated over. Further, when you apply a filter on an array the original array is not modified. If this method returns an element, this element will be 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.

Sorting One-Dimensional Arrays[edit]

When working with arrays in Swift, users can sort elements using both the sort() and the sorted() methods. Both these methods 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, whereas, with sorted() the original array elements are copied to a new array in a sorted orientation. Let's take a look at how these methods can be used.

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

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

If you'd rather work with a copy of the original array that is sorted, using the sorted() method will be best. 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 in a sorted manner.

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

Mutating Method for One-Dimensional Arrays[edit]

When creating structs that have a variable defined as a constant, there are some limitations in how you can use or alter the struct. Specifically, users are unable to change the properties of the struct once these properties have been defined as constant.

This can quickly become a challenge when users want to apply a different variable within that structure. This limitation may seem overly cautious, however, the intention behind it is to protect users from unintentionally misapplying variable declarations to a struct. Fortunately, there is the mutating func() that allows users 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 in the following manner:

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

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

The Reduce() method combines all of the elements within an array and returns a single new value. Often, Reduce() is used to sum all of the integers or characters within 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 accepts two arguments, one is an initial value which is used to store the initial value or the value or and the next item in the iteration.


ComingSoonIcon.png
Coming Soon
  • Preconditions

Python

Python[edit]

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

A list can be instantiated using a series of literals. For example:

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

An empty list can be instantiated:

emptyArray = []

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

firstState = states[0]

One 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 simply using if:

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

The last element of a list can be accessed by using -1 as the index. The first element can be accessed 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])

We can use indices to iterate over each element at a specific location in a list with 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 makes use of a for loop:

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

Adding a single element to the end of a list can be accomplished with append:

states.append("New York")

We 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, elements can be removed with the del keyword:

del states[2]

Exercises[edit]

ExercisesExercisesIcon.png
  •  M1301-10  Complete  Merlin Mission Manager  Mission M1301-10.

Quiz[edit]

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

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

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

False
True

3 int [] nums = {2, 3, 5, 8, 9, 11};

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

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

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

5 The first value of the array called holder can set to 50 in the following way?

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

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

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

7 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?

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

Michael
Larry
Kobe
Magic