find the first index of a substring in a string in swift

To find the first index of a substring in a string in Swift, you can use the range(of:) method provided by the String class. Here's an example:

main.swift
let myString = "Hello, world!"
let mySubstring = "world"

if let range = myString.range(of: mySubstring) {
    let startIndex = myString.distance(from: myString.startIndex, to: range.lowerBound)
    print("Found substring at index \(startIndex)")
} else {
    print("Substring not found")
}
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In the above example, we first define the string that we want to search in (myString) and the substring that we want to find (mySubstring). We then use the range(of:) method to find the range of the substring within the string.

If the substring is found, range(of:) returns a Range<String.Index> object, which we can then use to calculate the index of the first character of the substring using distance(from:to:) method. If the substring is not found, range(of:) returns nil, and we print a message indicating that the substring was not found.