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String Interpolation in Swift

Ans : 

String interpolation means creating string from mix of constants, variables, literals, expressions.

let length:Float = 3.14
var breadth = 10
var myString = "Area of a rectangle is length*breadth"
myString = "\(myString) i.e. = \(length)*\(breadth)"

Output :

Area of a rectangle is length*breadth
Area of a rectangle is length*breadth i.e. = 3.14*10

Another way of string interpolcation in Swift :

Use the Swift String initializer:
String(format: <#String#>, arguments: <#[CVarArgType]#>)

For example: let stringFromNumber = String(format: "%.2f", number)

Swift 5.0 has super powerful string interpolation : 

1. Normal custom String interpolation

Code for Example :

Using the new string interpolation system in Swift 5.0 we can extend String.StringInterpolation to add our own custom interpolations

extension String.StringInterpolation {
    mutating func appendInterpolation(_ number: Int) {
        let formatter = NumberFormatter()
        formatter.numberStyle = .spellOut

        if let result = formatter.string(from: number as NSNumber) {

Use : print("My age is \(age).")
Output : My age is twenty nine.
Above custom interpolation tells to convert integer to word.


We can also write above function as following :

mutating func appendInterpolation(format number: Int) {

So, we can use like : print("Hi, I'm \(format: age)."). So there will be no confusion between default interpolation and custom interpolation.

2. String interpolation with parameters. : 

mutating func appendInterpolation(linkedin: String) {
    appendLiteral("<a href=\"\(linkedin)\">@\(linkedin)</a>")}

output :
"You should follow me on linkedin : <a href="">@twostraws</a>.\n"

We can also pass multiple parameters.

3. We can pass auto closure as parameter :

extension String.StringInterpolation {
    mutating func appendInterpolation(_ values: [String], empty defaultValue: String) {
        if values.count == 0 {
        } else {
            appendLiteral(values.joined(separator: ", "))

let names  = [String]()

print("Crew: \(names, empty: "None").")

Using @autoclosure means that we can use simple values or call complex functions for the default value. Here defaultValue() function called for set default value.

extension String.StringInterpolation {
    mutating func appendInterpolation(if condition: @autoclosure () -> Bool, _ literal: StringLiteralType) {
        guard condition() else { return }

let amIiOSDeveloper = true
print("Swift rocks: \(if: amIiOSDeveloper, "(*)")")
print("Swift rocks \(amIiOSDeveloper ? "(*)" : "")")

Normally we use 2nd print option at above. But due to custom string interpolation, we can directly use 1st print option.

4. String interpolation with custom type (class or struct)

struct Employee {
    var name: String
    var designation: String

extension String.StringInterpolation {
    mutating func appendInterpolation(_ emp: Employee) {
        appendLiteral("I am \( and my designation is \(amp.designation).")

let manan = Employee(name: "Manan", designation: "Senior Software Engineer")

Print("\(manan)") gives output as :

I am Manan and my designation is Senior Software Engineer.

if we use print(manan) then it gives class debug description because here no string interpolation used.

Thanks Swift 5.0...

If you have any comment, question, or recommendation, feel free to post them in the comment section below!  

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