1. Introduction
Scala has a very powerful standard collections library. It provides many powerful functions for manipulating data within collections.
In this tutorial, we’ll examine two specific methods in collections; takeWhile() and dropWhile(). Both these functions are very useful for working with sequences based on predicate conditions.
2. Set-Up
Before we begin the implementation, let’s define the sample data that will be used in this article:
private val numbersAscending = List(1, 2, 3, 4, 5, 6, 7, 8)
private val numbersDescending = List(8, 7, 6, 5, 4, 3, 2, 1)
private val numbersMixed = List(1, 2, 3, 4, 3, 2, 1)
We’ll use the collection defined above to demonstrate the takeWhile() and dropWhile() functions.
3. takeWhile()
The takeWhile() function in Scala allows us to extract elements from the beginning of a collection as long as the provided predicate remains true. It stops taking elements as soon as the predicate is not satisfied.
Let’s look at a sample code that uses takeWhile() function:
val lessThanFive = numbersAscending.takeWhile(_ < 5)
lessThanFive shouldBe List(1, 2, 3, 4)
In the above example, the takeWhile() function returned the elements that are less than 5.
Unlike the filter() method which iterates through the entire collection, takeWhile() stops as soon as the predicate fails for the first time. Let’s verify this behavior with an example:
val lessThanThree = numbersMixed.takeWhile(_ < 3)
lessThanThree shouldBe List(1, 2)
In this case, although there are more elements less than 3, the takeWhile() function stopped as soon as it encountered element 3, which caused the predicate to fail.
Let’s see what happens when the predicate is satisfied for all elements:
val positive = numbersAscending.takeWhile(_ > 0)
positive shouldBe numbersAscending
Since all the numbers are positive, the takeWhile() predicate is satisfied for all elements, resulting in the entire collection being iterated.
As a result, the order of the collection impacts the output of the takeWhile operation.
The takeWhile function is useful in scenarios where we need to process elements until a threshold is reached, such as analyzing temperature readings to identify a continuous streak of increasingly warm days.
4. dropWhile()
The dropWhile() function in Scala is designed to skip elements from the beginning of a collection as long as a specified predicate holds true. Once the predicate fails for the first time, dropWhile() returns the remaining elements in the collection. As a result, the dropWhile operates in the opposite manner compared to the takeWhile.
Let’s look at an example using the dropWhile() function:
val dropLessThan5 = numbersAscending.dropWhile(_ < 5)
dropLessThan5 shouldBe List(5, 6, 7, 8)
Here, the dropWhile() function skipped elements until it encountered the occurrence of five, causing the predicate to fail. From that point forward, it returned the entire remaining collection.
Now, let’s look at what happens when the order of elements changes:
val dropLessThan5 = numbersDescending.dropWhile(_ < 5)
dropLessThan5 shouldBe numbers
In this case, since the predicate fails for the first element itself, dropWhile() doesn’t drop any elements and returns the entire collection unchanged.
Similar to the takeWhile() function, the order of elements is crucial when using dropWhile().
A practical use case for dropWhile() is in processing sorted data where we want to skip initial elements until a specific condition is met. For example, in financial data analysis, we might use dropWhile() to skip past historical data until reaching a certain financial year or quarter, focusing only on recent data for further analysis or reporting.
5. Conclusion
In this article, we looked at dropWhile() and takeWhile() functions in Scala. The takeWhile() allows for the extraction of elements until a predicate fails, while dropWhile() skips elements until the predicate fails and returns the remaining ones. We also explored how they are different from the filter() function on collections. Depending on the requirement, we can use the appropriate function.
As always, the sample code used in this article is available over on GitHub.