Associative Domains and Arrays

View associative.chpl on GitHub

This is a tutorial on Chapel's associative domains and arrays.

It may be beneficial to read the arrays.chpl and domains.chpl primers before proceeding if you're not already familiar with Chapel's domains and arrays.

Associative Domains

Associative domains are commonly used to represent arbitrary sets, or to represent the set of keys for a dictionary/hash table.

The current implementation supports most Chapel types, and will eventually support any language-defined type.

The following block contains some examples of associative domains with various index types.

  var A : domain(int);     // a domain (set) whose indices are integers
  var B : domain(string);  // a domain (set) whose indices are strings
  var C : domain(real);    // a domain (set) whose indices are reals

  class X {
    var x : int;

  // A domain whose indices are classes of type 'X'. The indices are hashed by
  // identity.
  var D : domain(X);

  record Y {
    var y : real;

  // A domain whose indices are records of type 'Y'. The indices are hashed by
  // value.
  var E : domain(Y);

Below, Names is an associative domain of strings. Associative domains start out empty (no indices).

var Names: domain(string);
writeln("An empty associative domain: ", Names);

We can use the size method to confirm the emptiness of the Names domain.

writeln("The initial size of the 'Names' domain is ", Names.size);

We can also use the domain-literal syntax to create an associative domain.

Below, Days is an associative domain of strings.

var Days = {"Sunday", "Wednesday", "Saturday"};

This declaration of Days relies on Chapel's type inference, and is equivalent to

var Days : domain(string) = {"Sunday", "Wednesday", "Saturday"};

The += operator can be used to add indices to an associative domain.

Names += "Alice";
Names += "Bob";
Names += "Clark";
Names += "Dana";

The add method is equivalent to +=

The order in which indices are printed is not guaranteed for associative domains.

writeln("A populated associative domain of strings: ", Names);

Associative domains cannot contain duplicate indices.

const PreviousNamesSize = Names.size;
if Names.size != PreviousNamesSize then
  halt("Error: Added duplicate index, but size of domain changed");

Below we can add an array of strings with the += or add operators because of Chapel's standard function promotion semantics. For the ColorSet domain, those functions take string arguments. Because PrimaryColors is a collection of strings Chapel promotes the routine, calling it for each string represented in the array.

var PrimaryColors : [1..3] string = ["Red", "Green", "Blue"];
var ColorSet : domain(string);
ColorSet += PrimaryColors;

The -= operator is used to remove indices from an associative domain. The remove method performs the same function.

Names -= "Bob";
Names += "Robert";
writeln("Removed 'Bob' and 'Frank', added 'Robert': ", Names);

If we want to check that "Bob" and "Frank" were indeed removed we can use the member function, which returns a boolean.

if Names.member("Bob") then
  halt("Error: Bob should have been removed!");
if Names.member("Frank") then
  halt("Error: Frank should have been removed!");

Associative Arrays

Like all other domains, we can use associative domains to define an array. Scores is the Chapel-equivalent of a "map", "dictionary", or "hash table" found in other languages. Where Names provides the keys, Scores provides the values.

Scores: an associative array mapping strings to integers.

var Scores : [Names] int;

Like other arrays, we can initialize associative array elements to have an initial value.

var HoursInDay : [Days] int = 24;
for hours in HoursInDay do
  if hours != 24 then
    halt("HoursInDay's values should have all been initialized to '24'");

We could also use the array-literal syntax to create an associative array.

Below, DaysInMonth is an associative array mapping strings to integers.

var DaysInMonth = ["June" => 30, "January" => 31, "September" => 30];

All array elements are initialized to the default value for that type.

writeln("Our 'Scores' associative array: ", Scores);

Printing an array only prints its values. Let's write a function that prints our associative array in a more human-readable format.

Given an array, print in the following format: [ idx => val, ... ]

We'll use the sorted iterator to print in a consistent order. Otherwise, the order in which indices are yielded is nondeterministic.

proc prettyPrint(arr : [?dom]) {
  write("[ ");
  var first = true;
  for k in dom.sorted() {
    if !first {
      write(", ", k, " => ", arr[k]);
    } else {
      write(k, " => ", arr[k]);
      first = false;
  writeln(" ]");

Using our new routine, print our array.

write("Our 'Scores' array, pretty-printed: ");

Let's initialize our Scores array.

Traditional array operations like whole-array assignment and indexing are supported for associative arrays.

Scores = 100; // every value in 'Scores' is now '100'.

Scores["Robert"] = 33;
Scores["Alice"] = 42;
Scores["Dana"] = 91;
write("Our 'Scores' array initialized with some values: ");

Like other Chapel arrays, we can use reductions.

writeln("The average score is ", (+ reduce Scores) / Scores.size);

Because Chapel arrays are updated when their domain is changed, we can add indices after the array is declared. When we add an index to the defining domain, the array is extended to support an element for that index. The new element is initialized to the default value of its type.

Names += "John";
if Scores["John"] != 0 then
  halt("Scores[\"John\"] should have been initialized to 0.");

Once added to the Names domain, we can use it like any other index in the array.

Scores["John"] = 76;

We can also implicitly add indices to an associative array if it is the only array defined over its domain.

Here, the string "Gary" is added as an index, and the corresponding array element is initialized to 10.

Scores["Gary"] = 10;
write("Our updated 'Scores' array after adding some more names: ");

The first line of code in the previous block is equivalent to:

Names += "Gary";
Scores["Gary"] = 10;

We can also verify that the underlying Names domain was modified:

if !Names.member("Gary") then
  halt("Error: 'Gary' should be present in the 'Names' domain");

If we were to define another array with Names as the backing domain, then we could no longer use this convenient feature.

var Ages : [Names] int;

Here, since Ages and Scores are backed by the same associative domain, we can no longer write something like

Ages["Mark"] = 33;


Score["Sam"] = 42;

since "Mark" or "Sam" are not valid indices in the Names domain.

This restriction exists because Chapel binds arrays to their domains for their lifetime to support semantic reasoning by the compiler and users. Extending the array via indexing will modify its domain, which would in turn modify the keys and values of any other arrays sharing that domain.

We consider this behavior to be too surprising to support ("I modified Ages, why did Scores change as well?"). Thus, to extend an array which shares its domain, the user is required to modify the domain directly. By modifying such a domain, it should be less surprising that arrays bound to that domain are affected.

Ages["Mark"] = 33;
Scores["Mark"] = 81;

The program will still halt with an out-of-bounds error if one tries to access an index not in the domain:


Set Operations

Set operations are available on associative domains and arrays.

The supported set operations are:
  • Union (| or +)
  • Intersection (&)
  • Difference (-)
  • Symmetric Difference (^)
For both associative domains and arrays, the op= variants are supported:
|=, +=, &=, -=, and ^=
var primeDom = {2, 3, 5, 7, 11, 13, 17};  // some prime numbers
var fibDom   = {0, 1, 1, 2, 3, 5, 8, 13}; // part of the Fibonacci sequence

var primeAndFib = primeDom & fibDom;
writeln("Some primes in the Fibonacci sequence: ", primeAndFib);
writeln("Some primes not in the Fibonacci sequence: ", primeDom - primeAndFib);

var Women = {"Alice", "Dana", "Ellen"};
var Men = Names - Women;

writeln("Women = ", Women);
writeln("Men = ", Men);

if (Men | Women) != Names then
  halt("The union of the 'Men' and 'Women' sets should be equivalent to 'Names'");

Special rules exist for associative array set operations.

If performing a union on two associative arrays with overlapping indices, the value of the second array takes precedence.

For the op= variants on associative arrays, the array must not share its domain with another array.

Future Directions

Today, associative domains cannot be distributed across multiple locales. A prototype domain map for this exists, and the effort to make it a polished feature could be accelerated with sufficient user interest.