Swift Programming - String

Strings in Swift are an ordered collection of characters, such as "Hello, World!" and they are represented by the Swift data type String, which in turn represents a collection of values of Character type.

Create a String

You can create a String either by using a string literal or creating an instance of a String class as follows −

import Cocoa

 // String creation using String literal

var stringA = "Hello, Swift!"

println( stringA )

 // String creation using String instance

var stringB = String("Hello, Swift!")

println( stringB )

Result:

Hello, Swift!

Hello, Swift!

Empty String

You can create an empty String either by using an empty string literal or creating an instance of String class as shown below. You can also check whether a string is empty or not using the Boolean property isEmpty.

import Cocoa

// Empty string creation using String literal

var stringA = ""

if stringA.isEmpty {

   println( "stringA is empty" )

}else {

   println( "stringA is not empty" )

}

// Empty string creation using String instance

let stringB = String() 

if stringB.isEmpty {

   println( "stringB is empty" )

}else {

   println( "stringB is not empty" )

}

Result:

stringA is empty

stringB is empty

String Constants

You can specify whether your String can be modified (or mutated) by assigning it to a variable, or it will be constant by assigning it to a constant using let keyword as shown below −

import Cocoa

// stringA can be modified

var stringA = "Hello, Swift!"

stringA + = "--Readers--"

println( stringA )

// stringB can not be modified

let stringB = String("Hello, Swift!")

stringB + = "--Readers--"

println( stringB )

Result:

Playground execution failed: error: <EXPR>:10:1: error: 'String' is not convertible to '@lvalue UInt8'

stringB + = "--Readers--"

String Interpolation

String interpolation is a way to construct a new String value from a mix of constants, variables, literals, and expressions by including their values inside a string literal.

Each item (variable or constant) that you insert into the string literal is wrapped in a pair of parentheses, prefixed by a backslash. Here is a simple example −

import Cocoa

var varA   = 20

let constA = 100

var varC:Float = 20.0

var stringA = "\(varA) times \(constA) is equal to \(varC * 100)"

println( stringA )

Result:

20 times 100 is equal to 2000.0

String Concatenation

You can use the + operator to concatenate two strings or a string and a character, or two characters. Here is a simple example,

import Cocoa

let constA = "Hello,"

let constB = "World!"

var stringA = constA + constB

println( stringA )

Result:

Hello,World!

String Length

Swift strings do not have a length property, but you can use the global count() function to count the number of characters in a string. Here is a simple example

import Cocoa

var varA   = "Hello, Swift!"

println( "\(varA), length is \(count(varA))" )

When the above code is compiled and executed, it produces the following result −

Hello, Swift!, length is 13

String Comparison

You can use the == operator to compare two strings variables or constants. Here is a simple example

import Cocoa

var varA   = "Hello, Swift!"

var varB   = "Hello, World!"

if varA == varB {

   println( "\(varA) and \(varB) are equal" )

}else {

   println( "\(varA) and \(varB) are not equal" )

}

Result: 

Hello, Swift! and Hello, World! are not equal

Unicode Strings

You can access a UTF-8 and UTF-16 representation of a String by iterating over its utf8 and utf16 properties as demonstrated in the following example −

import Cocoa

var unicodeString   = "Dog‼🐶"

println("UTF-8 Codes: ")

for code in unicodeString.utf8 {

   print("\(code) ")

}

print("\n")

println("UTF-16 Codes: ")

for code in unicodeString.utf16 {

   print("\(code) ")

}

Result:

UTF-8 Codes:

68 111 103 226 128 188 240 159 144 182

UTF-16 Codes:

68 111 103 8252 55357 56374

String Functions & Operators

Swift supports a wide range of methods and operators related to Strings −

S.No	Functions/Operators & Purpose
1	isEmpty A Boolean value that determines whether a string is empty or not.
2	hasPrefix(prefix: String) Function to check whether a given parameter string exists as a prefix of the string or not.
3	hasSuffix(suffix: String) Function to check whether a given parameter string exists as a prefix of the string or not.
4	toInt() Function to convert numeric String value into Integer.
5	count() Global function to count the number of Characters in a string.
6	utf8 Property to return a UTF-8 representation of a string.
7	utf16 Property to return a UTF-16 representation of a string.
8	unicodeScalars Property to return a Unicode Scalar representation of a string.
9	+ Operator to concatenate two strings, or a string and a character, or two characters.
10	+= Operator to append a string or character to an existing string.
11	== Operator to determine the equality of two strings.
12	<  Operator to perform a lexicographical comparison to determine whether one string evaluates as less than another.
13	== Operator to determine the equality of two strings.