Kotlin return@forEach from nested forEach

Kotlin return@forEach from nested forEach

Understanding the Problem

In Kotlin, using return inside nested forEach loops can lead to unexpected behavior. The return statement only exits the innermost loop, not the outer one. To break out of both loops, you need the return@forEach label.

The Issue with Simple `return`

Consider this example:

fun main() {
  val outerList = listOf(1, 2, 3)
  val innerList = listOf(4, 5, 6)

  outerList.forEach { outer ->
    innerList.forEach { inner ->
      if (inner == 5) {
        return // Exits only the inner loop
      }
      println("$outer, $inner")
    }
  }
}

1, 4
1, 5
2, 4
2, 5
3, 4
3, 5

The code above prints all combinations of elements from both lists even though we expected it to stop when inner is 5. This happens because return only exits the innerList.forEach loop.

Solution: Using `return@forEach`

To break out of both loops, use return@forEach with the label forEach:

fun main() {
  val outerList = listOf(1, 2, 3)
  val innerList = listOf(4, 5, 6)

  outerList.forEach { outer ->
    innerList.forEach { inner ->
      if (inner == 5) {
        return@forEach // Exits both loops
      }
      println("$outer, $inner")
    }
  }
}

1, 4
1, 5

Now, the code stops iterating after printing 1, 5 as intended.

Explanation

• return@forEach is a labeled return statement. It specifies which loop to exit.
• The @forEach label identifies the outer forEach loop.
• By using this label, we explicitly tell Kotlin to exit both the inner and outer loops.

Benefits of `return@forEach`

• Clearer intent: Explicitly states which loop to exit.
• Improved readability: Makes the code easier to understand.
• Less error-prone: Avoids confusion with simple return.

Alternative Solutions

• Use a boolean flag: Set a flag to indicate whether to break out of the loops.
• Use nested for loops: Explicitly control iteration using loop variables.
• Use a dedicated function: Encapsulate the nested iteration logic in a separate function.

Comparison

Conclusion

When dealing with nested forEach loops, using return@forEach with a label is the most efficient and reliable way to exit both loops. It provides clarity, avoids ambiguity, and promotes better code structure. Remember to choose the method that best suits your specific needs and context.