String Prefix methods in Swift explanation with example

String Prefix methods in Swift

Swift string provides a couple of methods to get and check prefix of a string. In this tutorial, I will show you these methods with examples :

1. hasPrefix : Check if a string starts with a prefix :

This method takes one string and checks if that string is prefix or not. It is defined as below :

func hasPrefix(String) -> Bool

It returns true or false.

Let’s take a look at the below example :

var givenString = "Hello String"

print(givenString.hasPrefix("Hello"))
print(givenString.hasPrefix("World"))

It prints the below output :

true
false

2. prefix(int) : Get subsequence up to a maximum length :

This method returns one subsequence up to a maximum length, i.e. the length we are passing as the parameter. It returns the starting characters :

var givenString = "Hello String"

print(givenString.prefix(5))
print(givenString.prefix(1))

It will print :

Hello
H

3. prefix(through: Index) : This method returns a substring from the start index to the index defined :

Similar to the above method, this is defined as below :

func prefix(through: Index) -> Substring

For example :

var givenText = "Hello World !!"

print(givenText.prefix(through: givenText.firstIndex(of: "o")!))

This example returns the substring from the start character to the first occurrence of ‘o’ .

Hello

4. prefix(upTo: Index): returns a substring from the start index to the provided index, excluding the provided index :

This method is defined as below :

func prefix(upTo: Index) -> Substring

For example :

var givenText = "Hello World !!"

print(givenText.prefix(upTo: givenText.firstIndex(of: "o")!))

This will return one text from the start character up to ‘o’, excluding it.

Hell

5. prefix(while: ch → Bool): Get initial subsequence using a predicate :

This method takes one predicate and returns all characters until the predicate returns false.

**

var givenText = "Hello World !!"

print(givenText.prefix(while: { (ch) -> Bool in
    return ch != "l"
}))

This example gets the initial subsequence of the string givenText using the provided predicate. It prints all characters up to the character before l.

It prints the below output :

He

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