Scala for Backend Engineering & Functional Programming · Lesson

Grouping and Sorting

groupBy and sortBy.

Grouping and Sorting is a free Scala for Backend Engineering & Functional Programming lesson on CoddyKit. This is lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Scala for Backend Engineering & Functional Programming learning path, and your progress syncs across the web and the CoddyKit app. The Scala for Backend Engineering & Functional Programming course includes 4 lessons in total.

Organizing data

Real-world data often needs to be grouped by a property or sorted by a key. Scala collections offer groupBy, sortBy, sortWith, and friends to do this declaratively.

object Main {
  def main(args: Array[String]): Unit = {
    val nums = List(5, 3, 8, 1, 9, 2)
    println(nums.sorted)
  }
}

sorted: natural order

sorted sorts elements in their natural order (ascending for numbers, alphabetical for strings). It needs an ordering, which exists for common types.

object Main {
  def main(args: Array[String]): Unit = {
    println(List(3, 1, 2).sorted)
    println(List("pear", "apple", "fig").sorted)
  }
}

sortBy: sort by a key

sortBy sorts by a value derived from each element. Provide a function that extracts the sort key.

object Main {
  def main(args: Array[String]): Unit = {
    val words = List("banana", "fig", "apple")
    val byLength = words.sortBy(_.length)
    println(byLength)
  }
}

Sorting in descending order

To sort descending, negate a numeric key or use sorted(Ordering.Int.reverse). For derived keys, sortBy with a minus sign is concise.

object Main {
  def main(args: Array[String]): Unit = {
    val nums = List(5, 3, 8, 1)
    println(nums.sortBy(-_))
    println(nums.sorted(Ordering.Int.reverse))
  }
}

sortWith: custom comparator

sortWith takes a function returning true when the first argument should come before the second, giving you full control over ordering.

object Main {
  def main(args: Array[String]): Unit = {
    val nums = List(5, 3, 8, 1)
    val descending = nums.sortWith((a, b) => a > b)
    println(descending)
  }
}

Sorting by multiple keys

sortBy can return a tuple to sort by several keys at once: it compares the first element, then the second to break ties.

object Main {
  def main(args: Array[String]): Unit = {
    val people = List(("Ann", 30), ("Bob", 25), ("Ann", 22))
    val sorted = people.sortBy { case (name, age) => (name, age) }
    println(sorted)
  }
}

groupBy: partition into a Map

groupBy returns a Map where each key is the result of your function and each value is the list of elements sharing that key.

object Main {
  def main(args: Array[String]): Unit = {
    val nums = List(1, 2, 3, 4, 5, 6)
    val byParity = nums.groupBy(_ % 2 == 0)
    println(byParity)
  }
}

Grouping by a computed key

The grouping key can be anything: a first letter, a length, a category. Each distinct key becomes a bucket.

object Main {
  def main(args: Array[String]): Unit = {
    val words = List("apple", "avocado", "banana", "cherry", "cranberry")
    val byFirstLetter = words.groupBy(_.head)
    byFirstLetter.foreach { case (letter, ws) => println(s"$letter -> $ws") }
  }
}

Transforming groups

After grouping, you often want to summarize each bucket. Use view.mapValues (or map) to transform the values, for example counting elements per group.

object Main {
  def main(args: Array[String]): Unit = {
    val words = List("cat", "car", "dog", "deer", "cow")
    val counts = words.groupBy(_.head).view.mapValues(_.size).toMap
    println(counts)
  }
}

groupMapReduce in one step

groupMapReduce groups, maps each element, and reduces each group's values, all in a single pass. It is a concise way to build summaries like sums per category.

object Main {
  def main(args: Array[String]): Unit = {
    val sales = List(("books", 10), ("toys", 5), ("books", 7), ("toys", 3))
    val totals = sales.groupMapReduce(_._1)(_._2)(_ + _)
    println(totals)
  }
}

Combining group and sort

A frequent pattern: group data, summarize each group, then sort the summary. Here we count words per first letter and sort by count.

object Main {
  def main(args: Array[String]): Unit = {
    val words = List("apple", "avocado", "banana", "cherry", "cranberry", "apricot")
    val ranked = words
      .groupBy(_.head)
      .view.mapValues(_.size).toList
      .sortBy(-_._2)
    println(ranked)
  }
}

Quick Check

What does List(1,2,3,4).groupBy(_ % 2 == 0) return?

Recap

You learned grouping and sorting:

sorted — natural order
sortBy — sort by a derived key (tuple for multi-key)
sortWith — custom comparator
groupBy — partition into a Map of buckets
groupMapReduce — group, map, and reduce in one pass

Frequently Asked Questions

Is the “Grouping and Sorting” lesson free?

Yes — the full text of “Grouping and Sorting” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Scala for Backend Engineering & Functional Programming course, upgrade to CoddyKit PRO. The Scala for Backend Engineering & Functional Programming course includes 4 lessons in total.

What will I learn in “Grouping and Sorting”?

groupBy and sortBy. You practise Scala for Backend Engineering & Functional Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Scala for Backend Engineering & Functional Programming?

No prior experience is required. Scala for Backend Engineering & Functional Programming on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Grouping and Sorting” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Scala for Backend Engineering & Functional Programming lesson?

Yes. Every Scala for Backend Engineering & Functional Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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