Most developers default to a for loop when they must loop through a collection or perform an operation several times. I find that the value of the while loop is often overlooked. It has some advantages over Swift's for loop in some scenarios. In this episode of Swift Fundamentals, you learn how to use a while loop in Swift and, more important, when it is appropriate to use a while loop over a for loop. Let's get started.
What Is a While Loop?
Like a for loop, a while loop is a control flow statement for repeatedly executing a block of code. The difference with a for loop is that the block of code is executed based on a boolean condition. You can also think of a while loop as an if statement that is executed multiple times until the statement of the if clause evaluates to false.
As I said, the block of code you pass to the while loop is only executed if a condition is met. Only if the condition evaluates to true, the code block is executed. What sets a while loop apart from a for loop is that the number of repetitions is undefined. In other words, the condition defines when the while loop stops executing the statements in its body. Let's look at an example to better understand what I just explained.
Fire up a playground in Xcode if you want to follow along. Choose the Blank template from the iOS > Playground section.
In this example, the code block we pass to the while loop is executed ten times. We declare a variable i of type Int and assign it a value of 10. The statements of the while loop are only executed if the condition evaluates to true, that is, if the value stored in i is less than or equal to 10.
import Foundation
var i = 1
while i <= 10 {
print(i)
i += 1
}
Because we increment the value of i with 1 every time the code block is executed, the while loop stops executing the statements in its body after 10 iterations.
When to Use a While Loop?
While we could replace the while loop in the example with a for loop, that isn't always possible or recommended. A while loop is a good choice if you don't know how often the code block is executed. In the example we increment the value of i with 1 every time the code block of the while loop is executed, but the logic is typically more complex.
You need to remember that a while loop is typically more appropriate than a for loop, and even necessary when you don't know in advance how many times you need to loop through a block of code. Remember what I said earlier. A while loop is nothing more than an if statement repeated several times until the condition of the if statement evaluates to false.
do {
let reader = try FileReader(url: fileURL)
defer {
reader?.close()
}
while let line = reader?.nextLine() {
print(line)
}
} catch {
print("Unable to Read File at \(fileURL), \(error)")
}
In this example, we use the FileReader class to read the lines of a file one by one. Let's focus on the while loop. In the condition of while loop, we ask the file reader for the next line of the file. The return type of the nextLine is String? so we use optional binding to unwrap the next line. This is another excellent example of when a while loop is a great fit. The while loop stops executing the statements in its body when the nextLine() method returns nil.
Infinite Loops
Let's revisit the first example. What happens if you forget to increment i in the code block of the while loop? You end up with an infinite loop, the nightmare of every developer. The application enters an infinite loop if the condition of the while loop never evaluates to false.
From the user's perspective, the application becomes unresponsive if this occurs on the main thread. After a while, pun intended, the system may terminate your application. Entering an infinite loop is the most important risk of a while loop. Be careful.
Don't Use a While Loop If ...
The while loop isn't as common as the for loop and that is a good thing. A for loop is often easier to implement and understand, and the risk of entering an infinite loop is much. smaller. If you know how many times a block of code needs to be executed, then a for loop is the better choice. Optimize for readability and ease of use.
