Scala for the Impatient

Arrays, Maps, Tuples

Copyright © Cay S. Horstmann 2015

Work with arrays and array buffers


Variable-Length Arrays

Transform arrays

Transforming Arrays

Use common array algorithms

Common Algorithms

Work with maps and tuples


Updating Maps

Iterating over Maps


Practice array transformations and working with maps and tuples


Scary looking lab

Part 1: Removing All But the First Negative Number

  1. You are given an array buffer with positive and negative integers. Your task is to remove all but the first negative number. (Don't ask why.) How do you do that in Java? Just write some pseudocode.
  2. You probably set a flag when you saw the first negative number, and then when you saw the second, you unset the flag. And you were careful not to increment the index after removal. Or maybe you found the first negative index, and then went backward. Either way, that's a lot of detail that can go wrong. In Scala, you can do better. First, get the indexes where a(i) < 0. Use until and for/yield.
  3. Now drop the first index. (Hint: drop in Seq)
  4. Now you want to call a.remove(i) for the remaining ones. Except, you need to do that in reverse order. Use for and reverse.

Part 2: Word Count

  1. A classic application for maps is word counting. Read a file, a word at a time, and keep a map that yields the frequency of each word.
    val in = new java.util.Scanner(new
    val count = scala.collection.mutable.Map[String, Int]()
    while (in.hasNext) {
      val word =;
      count(word) = ...
    What is count("Alice")? count("Rabbit")?
  2. Repeat with an immutable map. (Hint: var count)

Part 3: Grouping

  1. Read through the description of the groupBy method in the Array class. It may not sound like much, but it is really useful in applications. Try this:
    val words = Array("Mary", "had", "a", "little", "lamb", "its", "fleece",
      "was", "white", "as", "snow", "and", "everywhere", "that", "Mary", "went",
      "the", "lamb", "was", "sure", "to", "go")
    What do you get for
    words.groupBy(_.substring(0, 1))
  2. How do you put all words with the same length into one bucket?

Part 4: Partitions and Zips

  1. Tuples are useful for methods that yield more than one result. Try this out:
    "New York".partition(_.isUpper)
    What do you get? Why does the partition method yield a tuple?
  2. Look again at the exercise where you are supposed to print all positive values and only the first negative value. Suppose ordering doesn't matter. How can you trivially solve this with partition? (Hint: Pass the function _ < 0.)
  3. The zip method takes two collections of the same length and “zips” them together into a collection of tuples. Try this:
    val symbols = Array("<", "-", ">")
    val counts = Array(2, 10, 2)
    val pairs =
    What do you get?
  4. Now iterate over the result to produce a printout <<---------->>. That is, repeat the < twice, the - ten times, and the > twice. (Hint: s * n yields the string s repeated n times)