Datastructures - Swift

Data Structures

Arrays

The syntax of array looks like this

    var someArray = [SomeType](count: nElements, repeatedValue: InitValue)

You can use the following statement to create an empty array of Int type having 3 elements and the initial value as zero 

    var someInts = [Int](count: 3, repeatedValue: 0)

Following is one more example to create an array of three elements and assign three values to that array 

    var someInts:[Int] = [10, 20, 30]

The following example shows how to create, initialize, and access arrays − 

    var someInts = [Int](count: 3, repeatedValue: 10)

    var someVar = someInts[0]
    print( "Value of first element is \(someVar)" )
    print( "Value of second element is \(someInts[1])" )
    print( "Value of third element is \(someInts[2])" )

OUTPUT:

Value of first element is 10
Value of second element is 10
Value of third element is 10

Modifying Arrays

You can use append() method or addition assignment operator (+=) to add a new item at the end of an array. Take a look at the following example. Here, initially, we create an empty array and then add new elements into the same array − 

    var someInts = [Int]()

    someInts.append(20)
    someInts.append(30)
    someInts += [40]

    // Modify last element

    someInts[2] = 50

    var someVar = someInts[0]

    print( "Value of first element is \(someVar)" )
    print( "Value of second element is \(someInts[1])" )
    print( "Value of third element is \(someInts[2])" )

OUTPUT:

Value of first element is 20
Value of second element is 30
Value of third element is 50

Iterating over an array : 

    var someStrs = [String]()

    someStrs.append("Apple")
    someStrs.append("Amazon")
    someStrs += ["Google"]
    for item in someStrs {
        print(item)
    }

You can use enumerate() function which returns the index of an item along with its value as shown below 

    var someStrs = [String]()

    someStrs.append("Apple")
    someStrs.append("Amazon")
    someStrs += ["Google"]

    for (index, item) in someStrs.enumerated() {
        print("Value at index = \(index) is \(item)")
    }

Adding Two Arrays

You can use the addition operator (+) to add two arrays of the same type which will yield a new array with a combination of values from the two arrays as follows 

    var intsA = [Int](count:2, repeatedValue: 2)
    var intsB = [Int](count:3, repeatedValue: 1)

    var intsC = intsA + intsB
    for item in intsC {
        print(item)
    }

OUTPUT:

2
2
1
1
1

Sets

Swift 4 sets are used to store distinct values of same types but they don’t have definite ordering as arrays have.

A type must be hashable to be stored in a set. A hash value is a Int value that is equal for equal objects. For example, if x == y, then x.hashvalue == y.hashvalue.

All the basic swift values are of hashable type by default and may be used as set values.

Creating Sets

You can create an empty set of a certain type using the following initializer syntax 

    var someSet = Set<Character>()     
    //Character can be replaced by data type of set.


    //OR you can create it with below way too

    let letters: Set = [a,b,c,d]

Accessing and modifying Sets

    someSet.count  // prints the number of elements

    someSet.insert("c")  // adds the element to Set.

    someSet.isEmpty  // returns true or false depending on the set Elements.

    someSet.remove("c")  // removes a element 

    //removeAll() can be used to remove all elements


    someSet.contains("c")  // to check if set contains this value.

Iterating over a Set You can iterate over a set using for-in loop − 

    for items in someSet {
        print(someSet)
    }

    //Swift sets are not in an ordered way, 

    //to iterate over a set in ordered way use

    for items in someSet.sorted() {
        print(someSet)
    }

Performing Set Operations

Following are the methods for performing set operations −

  • Intersection

  • Union

  • subtracting

    let evens: Set = [10,12,14,16,18]
    let odds: Set = [5,7,9,11,13]
    let primes = [2,3,5,7]
    odds.union(evens).sorted()
    // [5,7,9,10,11,12,13,14,16,18]

    odds.intersection(evens).sorted()
    //[]

    odds.subtracting(primes).sorted()
    //[9, 11, 13]

Dictionaries

Swift 4 dictionaries are used to store unordered lists of values of the same type.

A dictionary key can be either an integer or a string without a restriction, but it should be unique within a dictionary.

If you assign a created dictionary to a variable, then it is always mutable which means you can change it by adding, removing, or changing its items. But if you assign a dictionary to a constant, then that dictionary is immutable, and its size and contents cannot be changed.

SYNTAX: 

    var someDict = [Int: String]()

EXAMPLE: 

    var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

Sequence Based Initialization

Swift 4 allows you to create Dictionary from arrays (Key-Value Pairs.) 

    var cities = ["Delhi","Bangalore","Hyderabad"]

You can use the following simple syntax to create an empty dictionary whose key will be of Int type and the associated values will be strings − 

    var Distance = [2000,10, 620]

Here is an example to create a dictionary from a set of given values − 

let cityDistanceDict = Dictionary(uniqueKeysWithValues: zip(cities, Distance))

The above lines of code will create a dictionary with Cities as key and Distance as Value

Filtering

Swift 4 allows you to filter values from a dictionary. 

    var closeCities = cityDistanceDict.filter { $0.value < 1000 }

If we run the above code our closeCities Dictionary will be.

OUTPUT:

["Bangalore" : 10 , "Hyderabad" : 620]

Dictionary Grouping

Swift 4 allows you to create grouping of Dictionary values. 

    var cities = ["Delhi","Bangalore","Hyderabad","Dehradun","Bihar"]

You can use the following simple syntax to group the values of dictionary according to first alphabet. 

    var GroupedCities = Dictionary(grouping: cities ) { $0.first! }

The result of above code will be

OUTPUT: 

["D" :["Delhi","Dehradun"], "B" : ["Bengaluru","Bihar"], "H" : ["Hyderabad"]]

Accessing & Modifying Dictionaries

You can use updateValue(forKey:) method to add an existing value to a given key of the dictionary. This method returns an optional value of the dictionary’s value type. Here is a simple example − 

    var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
    var oldVal = someDict.updateValue("New value of one", forKey: 1)
    var someVar = someDict[1]

    print( "Old value of key = 1 is \(oldVal)" )
    print( "Value of key = 1 is \(someVar)" )
    print( "Value of key = 2 is \(someDict[2])" )
    print( "Value of key = 3 is \(someDict[3])" )

Or you may do like this as well.. both produces the same output 

    var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
    var oldVal = someDict[1]
    someDict[1] = "New value of one"
    var someVar = someDict[1]

    print( "Old value of key = 1 is \(oldVal)" )
    print( "Value of key = 1 is \(someVar)" )
    print( "Value of key = 2 is \(someDict[2])" )
    print( "Value of key = 3 is \(someDict[3])" )

OUTPUT:

    Old value of key = 1 is Optional("One")
    Value of key = 1 is Optional("New value of one")
    Value of key = 2 is Optional("Two")
    Value of key = 3 is Optional("Three")

Remove Key-Value Pairs

You can use removeValueForKey() method to remove a key-value pair from a dictionary. This method removes the key-value pair if it exists and returns the removed value, or returns nil if no value existed. Here is a simple example − 

    var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
    
    var removedValue = someDict.removeValue(forKey: 2)
    // or you may just set the value to nil.. both works..  

    // someDict[2] = nil


    print( "Value of key = 1 is \(someDict[1])" )
    print( "Value of key = 2 is \(someDict[2])" )
    print( "Value of key = 3 is \(someDict[3])" )

OUTPUT:

    Value of key = 1 is Optional("One")
    Value of key = 2 is nil
    Value of key = 3 is Optional("Three")

Iterating Over a Dictionary

You can use enumerate() function which returns the index of the item along with its (key, value) pair as shown below in the example − 

    var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]
    for (key, value) in someDict.enumerated() {
        print("Dictionary key \(key) - Dictionary value \(value)")
    }

OUTPUT:

    Dictionary key 0 - Dictionary value (key: 2, value: "Two")
    Dictionary key 1 - Dictionary value (key: 3, value: "Three")
    Dictionary key 2 - Dictionary value (key: 1, value: "One")

Convert to Arrays

You can extract a list of key-value pairs from a given dictionary to build separate arrays for both keys and values. Here is an example 

    var someDict:[Int:String] = [1:"One", 2:"Two", 3:"Three"]

    let dictKeys = [Int](someDict.keys)
    let dictValues = [String](someDict.values)

    print("Print Dictionary Keys")

    for (key) in dictKeys {
        print("\(key)")
    }
    print("Print Dictionary Values")

    for (value) in dictValues {
        print("\(value)")
    }

OUTPUT:

Print Dictionary Keys
2
3
1
Print Dictionary Values
Two
Three
One

count property of a dictionary to find out the number of items in a dictionary
empty property of a dictionary to find out whether a dictionary is empty or not