Programmation appliquée en Scala

Copyright © Cay S. Horstmann 2015 Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License

Higher-Order Functions

Parameter Type Inference


What is the Value Of ... + _)
  1. The string "5050"
  2. The string "1 2 3 ... 98 99 100"
  3. A string of length 192
  4. Something else (syntax error, other value, etc.)


  • Given integer sequence s, easy to form all pairs (x, 0): => (x, 0))
        // Vector((1, 0), (2, 0), (3, 0), (4, 0))
  • What if we want to have all pairs (x, y) where x, y are elements of s?
  • Try it by calling map twice: => => (x, y)))
      // Vector(Vector((1,1), (2,1), (3,1), (4,1)),
      //        Vector((1,2), (2,2), (3,2), (4,2)),
      //        Vector((1,3), (2,3), (3,3), (4,3)),
      //        Vector((1,4), (2,4), (3,4), (4,4)))
  • Close, but not quite. To “flatten out” the result, use flatMap instead:
    s.flatMap(y => => (x, y)))
      // Vector((1, 1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2), ..., (4, 4)) 
  • Capturing the Enclosing Environment

    Where Did We Already Use Closures?

    1. In the reduceLeft slide
    2. In the flatMap slide
    3. In both
    4. In neither


    Scary looking lab

    1. You work with a buddy
    2. One of you (the coder) writes the code, the other (the scribe) types up answers
    3. When you get stuck, ask your buddy first!
    4. Switch coder/scribe roles each lab
    5. The coder submits the worksheet. Include the scribe's name in the worksheet!
    6. The scribe submits answers. Include the coder's name in the report!

    Part 1: Life without Loops

    1. Make a worksheet Day3. Type in
      val zones = java.util.TimeZone.getAvailableIDs
      . What do you get?
    2. We want to get rid of the continents. Try this: => s.split("/"))
    3. Ok, halfway there. Add a map that takes an array and yields a(1). What did you do? What happens?
    4. Hmmm, that's weird. There seem to be arrays of length < 2. How can you find them?
    5. Ok, now get rid of them and try again. What did you do?
    6. That's a lot of values. Can we get every tenth of them? (Hint: grouped(10))

    Part 2: Reductions

    1. Evaluate * _)
      What do you get?
    2. Write a function that computes n! in this way.
    3. Surely you have written a factorial function before. How did you used to do it? Which approach do you like better?
    4. Now we'd like to compute 2n with the same trick. How can you get a sequence of n copies of the number 2? Hint: map.
    5. What is your function that computes 2n?
    6. Given a Seq[String], how can you use reduceLeft to concatenate them with spaces in between? Write a function catSpace that does this. For example, catSpace(Vector("Mary", "had", "a", "lamb")) should give a string "Mary had a lamb"
    7. What happens when you try < _)? Why?

    Part 3: Fun with flatMap and reduceLeft

    Martin Odersky, the inventor of Scala (as well as the charming husband of one of your instructors) likes to show off the power of Scala with a solution of the “phone mnemonics” problem. Given a phone number, what word or word sequence spells it on a touch-tone pad? For example, given the phone number 435 569 6753, one should come up with the sequence HELLO WORLD (and maybe others).

    We can do better than what he did in his presentations. Over the next three labs, we'll solve this problem with our very restricted subset of Scala.

    1. What interesting word can you make from the number 72252?
    2. Declare a map that maps '2' to "ABC", '3' to "DEF", and so on. How do you do that? Pay attention that the keys should be characters and the values should be strings.
    3. Actually, we want '2' to map to a Vector with three elements, "A", "B", "C". But we don't want to type all that by hand. Transform your map by adding
      .map(e => (e._1, => c.toString)))
      Call the result letters—we'll need it in the next step.
    4. Now it's trivial to find out the words that one can make with one digit. It's just letters(d). Let's move on to two digits. Write a function that given two digits produces the set of all words (whether or not they make sense) that you can make with both.
      val wordsFor = (a: Char, b: Char) => ...
      For example, wordsFor('5', '9') contains "JW", "JX", "JY", "KW", "KX", "KY", "LW", "LX", "LY". Hint: flatMap