Reverse string

A string contains characters. In Swift 5.8, no reverse() func is available to reverse the ordering of these characters. But an extension can be built.

To reverse a string, we must create a new buffer (an array of characters). Then, we loop in reverse over the existing string, adding characters to the new character buffer.

Func

Here we use the extension keyword to add a reverse() method to the String type. The logic has some tricks to it. We use the endIndex and startIndex properties on the "self" String.

extension String {
    func reverse() -> String {
        let data = self
        var array = [Character]()
        // Begin at endIndex.
        var i = data.endIndex
        // Loop over all string characters.
        while (i != data.startIndex) {
            // Get previous index if not on first.
            if i > data.startIndex {
                i = data.index(i, offsetBy: -1)
            }
            // Add character to array.
            array.append(data[i])
        }
        // Return reversed string.
        return String(array)
    }
}

// Test string reversal extension method.
var animal = "bird"
let result = animal.reverse()
print(result)

var plant = "carrot"
let result2 = plant.reverse()
print(result2)
drib
torrac

Some notes

The reverse() method above is not marked as "mutating," so it cannot change the self String instance. Instead it returns a new String.

With extension method syntax, we can add useful (even if not common) funcs to types like String. To reverse the characters in a String, we use a simple looping algorithm.