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LESSON 38 OF 415 · FUNCTIONS I

Using Parameters in Swift: Fixing a Greeting Function

Practice Read the full explanation

Inside the code editor we've tried to write a function that takes a person's name as argument and prints a tailored welcome message for that person.

Then, we welcomed Andy and John by calling the function for each of them.

So when we ran the code, we expected it to print:

Welcome, Andy!
Welcome, John!

but it seems like we made some mistakes because when we run our code, it prints:

Welcome, Andy!
Welcome, Andy!

Assignment:

Your task is to fix our code such that no errors will be produced and it will print the desired output.

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    SWIFT · FUNCTIONS I

    Buggy Code 5 in Swift, explained

    What the code is supposed to do

    The function welcomeUser takes a person's name and prints a welcome message for that person. Called for Andy and then for John, it should print:

    Welcome, Andy!
Welcome, John!

    This code compiles and runs, but it prints Welcome, Andy! twice:

    class Solution {
    func welcomeUser(_ name: String) {
        print("Welcome, Andy!")
    }

    func mainFunction() {
        welcomeUser("Andy")
        welcomeUser("John")
    }
}

    What goes wrong

    This bug produces no error message, which makes it harder to spot than a compiler error. Swift can't know that the text inside the quotes was meant to change. The string "Welcome, Andy!" is a fixed piece of text, so every call prints exactly that text.

    The function receives its argument in the parameter name, but the body never reads it. The value for John arrives and is dropped. Swift does not warn about an unused function parameter, so nothing flags the problem. The only clue is the output, which repeats the same line.

    Fixing it step by step

    1. Look at the output. Identical lines for different arguments mean the function ignores its input.
    2. Find the part that should vary. In "Welcome, Andy!", only Andy should change. The rest, "Welcome, " and "!", stays the same for everyone.
    3. Split the string at that point. Keep the fixed text in quotes and put the parameter name between the two pieces, outside any quotes.
    4. Join the pieces with +. For String values, + concatenates: it puts the pieces together with nothing in between, so the spaces and punctuation must be inside the quoted parts.

    The fixed code in Swift

    class Solution {
    func welcomeUser(_ name: String) {
        print("Welcome, " + name + "!")
    }

    func mainFunction() {
        welcomeUser("Andy")
        welcomeUser("John")
    }
}

/* Explanation:

Notice that in the original function we didn't actually use our parameter "name".

And so the function printed "Welcome, Andy!" no matter if we passed "Mike" or "Sally" etc. to it.

The fix is to use the parameter "name" with string concatenation or string formatting to dynamically construct the message:
print("Welcome, " + name + "!")

*/

    With the fix, the call for John produces Welcome, John!, and any other name works too.

    String interpolation: the other way to do it

    Swift also lets you put a value inside a string literal with \(...). This does the same job and is the more common Swift style:

    func welcomeUser(_ name: String) {
    print("Welcome, \(name)!")
}

welcomeUser("Mike")   // Welcome, Mike!
welcomeUser("Sally")  // Welcome, Sally!

    Inside the parentheses you can use any expression, not only a variable. With interpolation, you don't have to remember to close and reopen the quotes.

    Common mistakes

    • Putting the name inside the quotes. print("Welcome, name!") prints the letters name literally, because quoted text is never treated as a variable.
    • Forgetting the spaces. "Welcome," + name prints Welcome,John. The space after the comma must be inside the string.
    • Adding a number with +. "Welcome, " + 5 doesn't compile, because + joins two strings. Convert it with String(5) or use interpolation.
    • Changing the calls. The calls in mainFunction are correct. The mistake is in the function body.

    Checking the fix

    Try more than the two names in the lesson. A correct welcomeUser gives these results:

    • welcomeUser("Mike") prints Welcome, Mike!.
    • welcomeUser("Sally") prints Welcome, Sally!.
    • welcomeUser("") prints Welcome, !, because an empty string is still a valid String.

    If a name you pass shows up in the output, the function is reading its parameter.

    Key takeaways

    • A parameter only matters if the body uses it.
    • If different arguments give the same output, check whether the function reads its parameters.
    • Keep fixed text in quotes and put variables outside them, joined with +, or use \(name) inside the quotes.
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