1. Introduction

In Kotlin, as in many other programming languages, loops are a fundamental control structure that allows for the repeated execution of a block of code. The while loop is particularly useful for executing code as long as a certain condition remains true. However, Kotlin differs from some languages in how it treats variable assignment within the condition of a while loop. Unlike languages such as Java, where assignment within a while loop’s condition is common practice, Kotlin restricts this behavior.

This tutorial explores the rationale behind this design choice, discusses the implications for Kotlin developers, and presents alternative approaches to achieving similar functionality in a Kotlin-friendly way.

2. Understanding the Restriction

The restriction against assignment in while loop expressions in Kotlin is grounded in the language’s emphasis on clarity and safety. Kotlin aims to reduce the likelihood of common programming errors, such as accidentally using an assignment (=) when an equality check (==) was intended. This error can lead to elusive bugs that are hard to trace and fix.

Let’s consider the following Java snippet, which is prone to error due to the assignment within the while condition:

int value;
while ((value = getValue()) != sentinelValue) {
    // Process value

If we intended to compare value to sentinelValue but accidentally used a single = instead of ==, the code would compile and run, possibly leading to unintended behavior. Kotlin addresses this by disallowing assignments within expressions expected to return a value, including while loop conditions, guiding developers toward safer coding practices.

3. Alternative Patterns in Kotlin

To achieve functionality similar to assignment within a while loop condition in Kotlin, we have several alternatives. These alternatives adhere to Kotlin’s language design principles and enhance code readability and maintainability.

3.1. Simple Variable Assignment Outside the Loop

First, we can initialize a variable outside the loop and re-assign it within the loop based on a condition:

var value: Int = 0
while (value != sentinelValue) {
    // do something...

This pattern makes the assignment explicit and separates it from the loop’s continuation condition, aligning with Kotlin’s emphasis on explicitness.

3.2. Using a while Loop With a break Statement

Sometimes, we need to continue until a certain condition is met. Kotlin encourages an explicit assignment inside the loop and to exit the loop using a break statement when necessary:

while (true) {
    val value = getValue()
    if (value == sentinelValue) break
    // Process value

This approach emphasizes explicitness in separating the assignment from the loop’s continuation condition.

3.3. Using do-while for Initialization and Condition

The do-while loop offers another alternative, ensuring that the body executes at least once before evaluating the exit condition:

do {
    val value = getValue()
    if (value == sentinelValue) break
    // Process value
} while (true)

This pattern helps when initialization needs to be within the loop’s execution, providing a clear structure for handling such cases.

4. Conclusion

While Kotlin’s restriction against assignment in while loop expressions might initially seem limiting, it reflects the language’s overall design philosophy of prioritizing safety and clarity. Developers can write more robust and readable code by understanding the rationale behind this restriction and embracing Kotlin’s idiomatic patterns. The alternatives presented in this article, such as using a while with a break statement or opting for a do-while loop, demonstrate Kotlin’s flexibility and the powerful features for effective loop control.

As always, the code used in this article is available over on GitHub.

Inline Feedbacks
View all comments
Comments are open for 30 days after publishing a post. For any issues past this date, use the Contact form on the site.