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LESSON 39 OF 415 · FUNCTIONS I

Why Parameters and Arguments in Swift

Lesson Read the full explanation

Now that we've learned about function parameters and arguments, let's see why they are so important in our programmers' life:

Consider this: Three students named Andy, Mike and Sally made a mistake. And so their teacher asked each of them to write an apology on the whiteboard.

We can put the apologies in a program like this:

print("This is Andy")
print("Andy made a mistake")
print("Andy suffers the consequences")

print("This is Mike")
print("Mike made a mistake")
print("Mike suffers the consequences")

print("This is Sally")
print("Sally made a mistake")
print("Sally suffers the consequences")

That's decent. But what if we want to change the students' names?

We would have to go through the whole program and change each individual name with the new one.

And if we dealt with 10 students instead of 3 and if the apology text consisted of more than 3 lines, this would quickly get out of hand.

The story with functions:

This is where functions with parameters shine. We can write a function that takes in a student's name as argument and prints the specific apology for that student.

Then, we'll just call the function for each individual student:

func printMessage(name: String) {
	print("This is \(name)")
	print("\(name) made a mistake")
	print("\(name) suffers the consequences")
}

printMessage(name: "Andy")
printMessage(name: "Mike")
printMessage(name: "Sally")

Changing the story:

Need to change the students' names? We only change the arguments in the function calls.

Need to change the apology text a little bit? We only change it inside the function instead of changing it in the whole program.

func printMessage(name: String) {
	print("This is the apology of \(name)")
	print("\(name) made a mistake and so \(name) suffers the consequences")
}

printMessage(name: "Andrei")
printMessage(name: "Paul")
printMessage(name: "Mary")

Assignment
Follow the Coding Tutorial and let's write some functions.

Hint
Look at the examples above if you get stuck.

STEP 1 OF 5

Guided

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SWIFT · FUNCTIONS I

Why Parameters And Arguments in Swift, explained

The problem: repeated code

Suppose three students each have to write the same apology. Without functions, the program repeats three lines for every student:

print("This is Andy")
print("Andy made a mistake")
print("Andy suffers the consequences")

print("This is Mike")
print("Mike made a mistake")
print("Mike suffers the consequences")

Only the name differs between the blocks. If the wording changes, you must edit every block, and with ten students and a longer apology the program becomes tedious and easy to break.

The fix: a function with a parameter

A parameter is a variable declared in a function's header. An argument is the value you pass for it in a call. Write the shared lines once, with the name as a parameter, and call the function once per student:

func printMessage(name: String) {
    print("This is \(name)")
    print("\(name) made a mistake")
    print("\(name) suffers the consequences")
}

printMessage(name: "Andy")
printMessage(name: "Mike")
printMessage(name: "Sally")

Now a change of wording happens in one place, and a new student is one more call. Inside the string, \(name) is string interpolation: Swift replaces it with the value of name.

Argument labels and the underscore

Swift calls usually name their arguments. In printMessage(name: "Andy") the word name: is an argument label, and leaving it out is an error. Writing an underscore before the parameter removes the label:

func greet(_ name: String) {
    print("Hi, " + name)
}

greet("Andy")  // Hi, Andy

The lesson's functions use the underscore, so calls pass plain values. The body still uses the name after the underscore.

The lesson's exercise, step by step

Define the function. Above mainFunction, write func introduceYourself(_ name: String, _ language: String). Each parameter has a type, and both are String.
Print the name. The text "My name is " ends with a space, because + adds nothing between the pieces. Joining it with name gives, for example, My name is Andy.
Print the language the same way, with "I code in ".
Call it for John. Inside mainFunction, introduceYourself("John", "Python"). Arguments match parameters by position: the first goes to name, the second to language.
Call it for Mary with "Mary" and "JavaScript". Only the arguments change, not the printing code.

The solution in Swift

func introduceYourself(_ name: String, _ language: String) {
    print("My name is " + name)
    print("I code in " + language)
}

func mainFunction() {
    introduceYourself("John", "Python")

    introduceYourself("Mary", "JavaScript")
}

The two calls print:

My name is John
I code in Python
My name is Mary
I code in JavaScript

Common mistakes

Swapping the arguments. introduceYourself("Python", "John") compiles, because both are strings, and prints My name is Python. Swift can't tell which string means what; the order is your responsibility.
Forgetting an argument. introduceYourself("John") fails with "missing argument for parameter #2 in call".
Passing the wrong type. A call such as introduceYourself("John", 5) fails because 5 is an Int and the parameter is a String.
Forgetting the trailing space. "My name is" + name prints My name isJohn.
Calling with labels after removing them. With the underscores in place, introduceYourself(name: "John", language: "Python") is an error. Without the underscores, leaving the labels out is the error.

Changing the story

The payoff comes when something changes. To change the students, edit only the arguments in the calls. To change the apology, edit only the function body, and every call follows:

func printMessage(name: String) {
    print("This is the apology of \(name)")
    print("\(name) made a mistake and so \(name) suffers the consequences")
}

printMessage(name: "Andrei")
printMessage(name: "Paul")

A parameter can appear as many times as you like in the body, as name does in the second line.

Key takeaways

A parameter lets one function handle many different values.
Write shared logic once, and vary only the arguments.
Swift matches arguments to parameters by position and checks their types.
An underscore before a parameter name removes its argument label.

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