Common Idioms 2

Associative arrays are one of the driving force behind the adoption of so-called scripting languages such as JavaScript or Ruby. Strangely enough, they're absent from the mainstream generalist languages of the C++/C#/Java family.

Associative arrays are like standard arrays but can be indexed with any given collection of values (whereas Java arrays can only be indexed with 0-based integers). Alternatively, associative arrays can be thought of as maps with built-in language support and a fixed an immutable set of keys (refering to a non-existent key raises an ArrayIndexOutOfBoundsException). Here is the OptimJ version of the canonical example from Wikipedia :

  String[String] phoneBook = {
    "Sally Smart" -> "555-9999",
    "John Doe" -> "555-1212",
    "J. Random Hacker" -> "553-1337"
  };

  // iterate over the values
  for(String number : phoneBook) {
    System.out.println(number);
  }

  // iterate over the keys
  for(String name : phoneBook.keys) {
    System.out.println(name + " -> " + phoneBook[name]);
  }

Notice the type String[String] that denotes an array of strings indexed by strings.

To see how associative arrays differ from Java arrays, consider the two definitions :

  String[] a1 = { "a", "b", "c" };
  String[int] a2 = { 12 -> "a", 3 -> "b", 7 -> "c" };

Here a1 is indexed by 0, 1, 2 and a2 is indexed by 3, 7, 12. Note how their types are different.

Java and associative array dimensions can be mixed freely:

  int[String][][double] a;

a is a 3-dimensional array. The first dimension is associative, indexed by doubles. The second dimension is a Java array dimension, indexed by 0-based integers. The third dimension is associative, indexed by strings.

As usual in Java, an array index expression is written with indices in the opposite order of the types:

  a[3.1416][10]["abc"]

Associative arrays are common in algebraic modeling languages such as AMPL, GAMS, OPL, etc., where they allow for a concise and mathematical-like expression of optimization problems.

Common Idioms 1

Most mainstream programming languages are pretty bad at initializating and populating structured data. Let's say you want a Java list containing "a" and "b".

First attempt :

List<String> a = new ArrayList<String>();
a.add("1");
a.add("2");

Pretty bad signal-to-noise ratio.

Second attempt :

List<String> a = Arrays.asList(new String[]{ "a", "b" });

Slightly more readable, but allocates a useless intermediate array.


Third attempt :

List<String> a = Arrays.asList("a", "b");

Much more readable, but doesn't work if you want a set rather than a list : there's no Arrays.asSet().

Here's the OptimJ version :

ArrayList<String> a = { "a", "b" };
HashSet<String> s = { "a", "b" };

But nausea quickly kicks in when you try to instanciate imbricated collection. A cooking recipe may be seen as a list (ordered) of sets of tasks (unordered because they can be done in parallel).

First attempt :

  List<Hashset<String>> irishStew = new ArrayList<Hashset <String>>();
  HashSet<String> set1 = new HashSet<String>();
  set1.add("Mince the meet");
  set1.add("Mash the potatoes");
  irishStew.add(set1);
  HashSet<String> set2 = new HashSet<String>();
  set2.add("Cook in oven");
  irishStew.add(set2);

Wow! This is called write-only code. Think about having to maintain pages of similar code written by an summer intern two years ago.

Second attempt :

... ?

The asList doesn't work because it requires expressions, and the code that builds our sets is a sequence of statements, not an expression.

Here's the OptimJ version :

  ArrayList<HashSet<String>> irishStew = {
    { "Mince the meet", "Mash the potatoes" },
    { "Cook in oven"}
  };

Do you feel more confident now taking this code maintenance assignment ?