How to Get a String's Index in Swift: 3 Helpful Examples

Swift strings are very powerful, but they can be a little complicated when it comes to indexing into those strings. When you've got a string, how do you get a specific character or substring out of it?

Well, you have a few options. Here are three of the most helpful methods. 

What Is an Index in a String?

An index is a number that identifies a character in a string. It's also known as the position of the char in the string. 

The index of the first character in a string is zero. A vital thing to note is that the index of the last character in a string will be equal to the length of the string minus one. 

Understanding String in Swift

A string is a sequence of characters, and Swift provides a number of ways to work with them. You can create a string by enclosing a sequence of characters in quotation marks: 

let str = "Hello, world!"

Once you've created a string, you can perform a number of operations on it, including retrieving individual characters, finding the length of the string, and concatenating strings together. 

Swift also provides special support for working with Unicode strings, which can represent a wide variety of languages and symbols. You can use Unicode strings to perform operations such as comparing strings for equality, finding a string's length, and concatenating strings together. 

Also Read: Combine for SwiftUi Developers: What It Is and Why You Should Use It 

String Index in Swift

Indexing in Swift is a powerful and efficient way to access data stored in an array or dictionary. It enables you to quickly and easily find the data you need without having to search through the entire dataset. 

Indexing can be used to access data stored in any order, and it can be used to access data stored in a specific order. 

StartIndex and EndIndex in Swift (With Strings)

Swift provides two special methods on its string type that can be used to access substrings: startIndex and endIndex. These methods allow you to specify a range of characters to be extracted from a string. 

The startIndex is the index of the first character in a string. It's used to specify where a string starts. 

var str = "How are you doing?"// character
str[str.startIndex] // H
str[str.endIndex] // This will result in an error

Before getting into a more complex example, let's understand a few more concepts. 

index(after:) in Swift

The index(after:) method returns the index after the given index. If the given index is the last valid index in the string, this method returns nil. 

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This method is equivalent to the following code: 

str.index(str.startIndex, offsetBy: index)

Let's understand this with an example: 

let str = "How you doing?"
let index = str.index(after: str.startIndex) 
print(str[index]) // "o"

index(before:) in Swift

The index(before:) method is used to access the index of the element that's located before the given index. 

This method is used in conjunction with the index(after:) method to provide access to the elements of a collection in both directions. 

let str = "Character"
let index = str.index(before: str.endIndex)

print(str[index]) // "r"

For example, the following code extracts the substring "Hello," from a string: 

let str = "Hello, Swift!"
let substr = str[str.startIndex...str.index(str.startIndex, offsetBy: 5)]
print(substr) // Prints "Hello,"

As you can see, using startIndex and endIndex can make it easy to work with substrings in Swift. 

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Now, let's understand how you can find the index of a substring in a string using those concepts. 

let str = "Hello, playground"
let range = str.range(of: "playground")

if let range = range {
   let index = str.distance(from: str.startIndex, to: range.lowerBound)
   print(index)
}

// Output - 7

Writing a Function to Find an Index of the Substring (First Occurrence)

When you need to find the index of a substring within a string in Swift, you can use the index(of:) method. 

This function returns the index of the first occurrence(first index) of the specified substring or nil if the substring is not found. But in some cases, you might want to create a custom function to return the exact index of a substring in a string. 

import Foundation

let inputStr = "Hi! This is a test string"
func findIndex(of string: String, in str: String) -> Int? {
   for (index, char) in str.enumerated() {
       var found = true
       for (offset, char2) in string.enumerated() {
           if str[str.index(str.startIndex, offsetBy: index + offset)] != char2 {
               found = false
               break
           }
       }
       if found {
           return index
       }
   }
   return nil
}

if let index = findIndex(of: "test", in: inputStr) {
   print(index)
}

// Output - 14

Finding an Index of the Substring in a String (All Occurrences)

Now, we've written a function to find an index of a substring in a string when that function only returns the first occurrence, but there might be a scenario where we want all occurrences. Let's write a function to accomplish that too. 

func findSubstringOccurrences(in string: String, substring: String) -> [Int] 
{
    var indices = [Int]()
    var startIndex = string.startIndex
    while startIndex < string.endIndex,
        let range = string.range(of: substring, range: startIndex..

In the code above, we have a function named findSubstringOccurrences, which takes string and substring as arguments and returns back an array of indexes of the substring in the given string. 

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Also Read: Swift Struct vs. Class: Here’s How to Decide Which to Use 

To Sum Up Swift String Index

In this post, we covered how you can find the index of a substring in a given string, and we've written a function to find indexes of multiple occurrences of a substring in a string. With this knowledge, you'll be able to find a string's index—something that's useful when working with arrays, dictionaries, and other helpful data types in Swift. 

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