convertToLowercase()

Step 1: Understanding the Task

First, we need to understand what the task requires. We're asked to convert a string into lowercase, meaning all uppercase letters in the string should be turned into lowercase letters. In Swift, there's a built-in method called lowercased() which can be directly used to transform all uppercase letters into lowercase ones.

let string = "Hello WORLD!"
let lowercaseString = string.lowercased()
print(lowercaseString) // prints: hello world!

Step 2: Building the Function

In Swift, we define a function using the func keyword. For our task, we will be needing a function that takes one parameter, the string to be converted to lowercase. The function will return the lowercase version of the string.

func convertToLowerCase(input: String) -> String {
    return input.lowercased()
}

Step 3: Testing the Function

We can now test out our function by calling it and passing a string with uppercase letters.

let result = convertToLowerCase(input: "Hello WORLD!")
print(result) // prints: hello world!

Step 4: Consider Edge Cases

Our function should already be working for most inputs. But what about edge cases? What if an empty string is passed as input? Since the lowercased() method does not have any side effects or does not throw any errors when called on an empty string, our function should perfectly handle this case as well.

let result = convertToLowerCase(input: "")
print(result) // prints: 

Step 5: Conclusion

As we can see, Swift provides us with efficient methods which makes our task of converting a string into lowercase very straightforward. This function should work for any given string.

Here is the complete function:

func convertToLowerCase(input: String) -> String {
    return input.lowercased()
}
let result = convertToLowerCase(input: "Hello WORLD!")
print(result) // prints: hello world!