Associative Arrays - D Programming Language
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# Associative Arrays

Contents

1. Literals
2. Removing Keys
3. Testing Membership
4. Using Classes as the KeyType
5. Using Structs or Unions as the KeyType
6. Construction or Assignment on Setting AA Entries
7. Inserting if not present
8. Advanced updating
9. Static Initialization of AAs
10. Runtime Initialization of Immutable AAs
11. Construction and Reference Semantics
12. Properties
13. Examples
    1. Associative Array Example: word count
    2. Associative Array Example: counting pairs

Associative arrays have an index that is not necessarily an integer, and can be sparsely populated. The index for an associative array is called the key, and its type is called the KeyType.

Associative arrays are declared by placing the KeyType within the [ ] of an array declaration:

    int[string] aa;   // Associative array of ints that are
                      // indexed by string keys.
                      // The KeyType is string.
    aa["hello"] = 3;  // set value associated with key "hello" to 3
    int value = aa["hello"];  // lookup value from a key
    assert(value == 3);

Neither the KeyTypes nor the element types of an associative array can be function types or void.

Implementation Defined: The built-in associative arrays do not preserve the order of the keys inserted into the array. In particular, in a foreach loop the order in which the elements are iterated is typically unspecified.

## Literals

    auto aa = [21u: "he", 38: "ho", 2: "hi"];
    static assert(is(typeof(aa) == string[uint]));
    assert(aa[2] == "hi");

See Associative Array Literals.

## Removing Keys

Particular keys in an associative array can be removed with the remove function:

    aa.remove("hello");

remove(key) does nothing if the given key does not exist and returns false. If the given key does exist, it removes it from the AA and returns true.

All keys can be removed by using the method clear.

## Testing Membership

The InExpression yields a pointer to the value if the key is in the associative array, or null if not:

    int* p;

    p = "hello" in aa;
    if (p !is null)
    {
        *p = 4;  // update value associated with key
        assert(aa["hello"] == 4);
    }

Undefined Behavior: Adjusting the pointer to point before or after the element whose address is returned, and then dereferencing it.

## Using Classes as the KeyType

Classes can be used as the KeyType. For this to work, the class definition must override the following member functions of class Object:

- size_t toHash() @trusted nothrow
- bool opEquals(Object)

Note that the parameter to opEquals is of type Object, not the type of the class in which it is defined.

For example:

    class Foo
    {
        int a, b;

        override size_t toHash() { return a + b; }

        override bool opEquals(Object o)
        {
            Foo foo = cast(Foo) o;
            return foo && a == foo.a && b == foo.b;
        }
    }

Implementation Defined: opCmp is not used to check for equality by the associative array. However, since the actual opEquals or opCmp called is not decided until runtime, the compiler cannot always detect mismatched functions. Because of legacy issues, the compiler may reject an associative array key type that overrides opCmp but not opEquals. This restriction may be removed in future versions.
Undefined Behavior:

1. If toHash must consistently be the same value when opEquals returns true. In other words, two objects that are considered equal should always have the same hash value. Otherwise, undefined behavior will result.

Best Practices:

1. Use the attributes @safe, @nogc, pure, const, and scope as much as possible on the toHash and opEquals overrides.

## Using Structs or Unions as the KeyType

If the KeyType is a struct or union type, a default mechanism is used to compute the hash and comparisons of it based on the fields of the struct value. A custom mechanism can be used by providing the following functions as struct members:

    size_t toHash() const @safe pure nothrow;
    bool opEquals(ref const typeof(this) s) const @safe pure nothrow;

For example:

    import std.string;

    struct MyString
    {
        string str;

        size_t toHash() const @safe pure nothrow
        {
            size_t hash;
            foreach (char c; str)
                hash = (hash * 9) + c;
            return hash;
        }

        bool opEquals(ref const MyString s) const @safe pure nothrow
        {
            return std.string.cmp(this.str, s.str) == 0;
        }
    }

The functions can use @trusted instead of @safe.

Implementation Defined: opCmp is not used to check for equality by the associative array. For this reason, and for legacy reasons, an associative array key is not allowed to define a specialized opCmp, but omit a specialized opEquals. This restriction may be removed in future versions of D.
Undefined Behavior:

1. If toHash must consistently be the same value when opEquals returns true. In other words, two structs that are considered equal should always have the same hash value. Otherwise, undefined behavior will result.

Best Practices:

1. Use the attributes @nogc as much as possible on the toHash and opEquals overrides.

## Construction or Assignment on Setting AA Entries

When an AA indexing access appears on the left side of an assignment operator, it is specially handled for setting an AA entry associated with the key.

    string[int] aa;
    string s;

    //s = aa[1];        // throws RangeError in runtime
    aa[1] = "hello";    // handled for setting AA entry
    s = aa[1];          // succeeds to lookup
    assert(s == "hello");

If the assigned value type is equivalent with the AA element type:

1. If the indexing key does not yet exist in AA, a new AA entry will be allocated, and it will be initialized with the assigned value.
2. If the indexing key already exists in the AA, the setting runs normal assignment.

    struct S
    {
        int val;
        void opAssign(S rhs) { this.val = rhs.val * 2; }
    }
    S[int] aa;
    aa[1] = S(10);  // first setting initializes the entry aa[1]
    assert(aa[1].val == 10);
    aa[1] = S(10);  // second setting invokes normal assignment, and
                    // operator-overloading rewrites it to member opAssign function.
    assert(aa[1].val == 20);

If the assigned value type is not equivalent with the AA element type, the expression could invoke operator overloading with normal indexing access:

    struct S
    {
        int val;
        void opAssign(int v) { this.val = v * 2; }
    }
    S[int] aa;
    aa[1] = 10;     // is rewritten to: aa[1].opAssign(10), and
                    // throws RangeError before opAssign is called

However, if the AA element type is a struct which supports an implicit constructor call from the assigned value, implicit construction is used for setting the AA entry:

    struct S
    {
        int val;
        this(int v) { this.val = v; }
        void opAssign(int v) { this.val = v * 2; }
    }
    S s = 1;    // OK, rewritten to: S s = S(1);
    s = 1;      // OK, rewritten to: s.opAssign(1);

    S[int] aa;
    aa[1] = 10; // first setting is rewritten to: aa[1] = S(10);
    assert(aa[1].val == 10);
    aa[1] = 10; // second setting is rewritten to: aa[1].opAssign(10);
    assert(aa[1].val == 20);

This is designed for efficient memory reuse with some value-semantics structs, eg. std.bigint.BigInt.

    import std.bigint;
    BigInt[string] aa;
    aa["a"] = 10;   // construct BigInt(10) and move it in AA
    aa["a"] = 20;   // call aa["a"].opAssign(20)

## Inserting if not present

When AA access requires that there must be a value corresponding to the key, a value must be constructed and inserted if not present. The require function provides a means to construct a new value via a lazy argument. The lazy argument is evaluated when the key is not present. The require operation avoids the need to perform multiple key lookups.

    class C{}
    C[string] aa;

    auto a = aa.require("a", new C);   // lookup "a", construct if not present

Sometimes it is necessary to know whether the value was constructed or already exists. The require function doesn't provide a boolean parameter to indicate whether the value was constructed but instead allows the construction via a function or delegate. This allows the use of any mechanism as demonstrated below.

    class C{}
    C[string] aa;

    bool constructed;
    auto a = aa.require("a", { constructed=true; return new C;}());
    assert(constructed == true);

    C newc;
    auto b = aa.require("b", { newc = new C; return newc;}());
    assert(b is newc);

## Advanced updating

Typically updating a value in an associative array is simply done with an assign statement.

    int[string] aa;

    aa["a"] = 3;  // set value associated with key "a" to 3

Sometimes it is necessary to perform different operations depending on whether a value already exists or needs to be constructed. The update function provides a means to construct a new value via the create delegate or update an existing value via the update delegate. The update operation avoids the need to perform multiple key lookups.

    int[string] aa;

    // create
    aa.update("key",
        () => 1,
        (int) {} // not executed
        );
    assert(aa["key"] == 1);

    // update value by ref
    aa.update("key",
        () => 0, // not executed
        (ref int v) {
            v += 1;
        });
    assert(aa["key"] == 2);

For details, see update.

## Static Initialization of AAs

NOTE: Static initialization, e.g. assigning an AA value to a global or static variable, isn't implemented yet. AAs can be constructed and used at runtime and at compile time as usual. Pre-computed AA values embedded into the executable are however not yet possible.

    // in global scope
    immutable long[string] aa = [
      "foo": 5,
      "bar": 10,
      "baz": 2000
    ];

    unittest
    {
        assert(aa["foo"] == 5);
        assert(aa["bar"] == 10);
        assert(aa["baz"] == 2000);
    }

As a workaround you can use static constructors or CRT constructors to initialize global AAs immediately at program startup.

## Runtime Initialization of Immutable AAs

Immutable associative arrays are often desirable, but sometimes initialization must be done at runtime. This can be achieved with a constructor (static constructor depending on scope), a buffer associative array and assumeUnique:

    immutable long[string] aa;

    shared static this()
    {
        import std.exception : assumeUnique;
        import std.conv : to;

        long[string] temp; // mutable buffer
        foreach (i; 0 .. 10)
        {
            temp[to!string(i)] = i;
        }
        temp.rehash; // for faster lookups

        aa = assumeUnique(temp);
    }

    void main()
    {
        assert(aa["1"] == 1);
        assert(aa["5"] == 5);
        assert(aa["9"] == 9);
    }

## Construction and Reference Semantics

An Associative Array defaults to null, and is constructed upon assigning the first key/value pair. However, once constructed, an associative array has reference semantics, meaning that assigning one array to another does not copy the data. This is especially important when attempting to create multiple references to the same array.

    int[int] aa;             // defaults to null
    int[int] aa2 = aa;       // copies the null reference

    aa[1] = 1;
    assert(aa2.length == 0); // aa2 still is null
    aa2 = aa;
    aa2[2] = 2;
    assert(aa[2] == 2);      // now both refer to the same instance

## Properties

Properties for associative arrays are:

Associative Array Properties
Property | Description
--- | ---
.sizeof | Returns the size of the reference to the associative array; it is 4 in 32-bit builds and 8 on 64-bit builds.
.length | Returns number of values in the associative array. Unlike for dynamic arrays, it is read-only.
.dup | Create a new associative array of the same size and copy the contents of the associative array into it.
.keys | Returns dynamic array, the elements of which are the keys in the associative array.
.values | Returns dynamic array, the elements of which are the values in the associative array.
.rehash | Reorganizes the associative array in place so that lookups are more efficient. rehash is effective when, for example, the program is done loading up a symbol table and now needs fast lookups in it. Returns a reference to the reorganized array.
.clear | Removes all remaining keys and values from an associative array. The array is not rehashed after removal, to allow for the existing storage to be reused. This will affect all references to the same instance and is not equivalent to destroy(aa) which only sets the current reference to null
.byKey() | Returns a forward range suitable for use as a ForeachAggregate to a ForeachStatement which will iterate over the keys of the associative array.
.byValue() | Returns a forward range suitable for use as a ForeachAggregate to a ForeachStatement which will iterate over the values of the associative array.
.byKeyValue() | Returns a forward range suitable for use as a ForeachAggregate to a ForeachStatement which will iterate over key-value pairs of the associative array. The returned pairs are represented by an opaque type with .key and .value properties for accessing the key and value of the pair, respectively. Note that this is a low-level interface to iterating over the associative array and is not compatible with the Tuple type in Phobos. For compatibility with Tuple, use std.array.byPair instead.
.get(Key key, lazy Value defVal) | Looks up key; if it exists returns corresponding value else evaluates and returns defVal.
.require(Key key, lazy Value value) | Looks up key; if it exists returns corresponding value else evaluates value, adds it to the associative array and returns it.
.update(Key key, Value delegate() create, Value delegate(Value) update) | Looks up key; if it exists applies the update delegate else evaluates the create delegate and adds it to the associative array

## Examples

### Associative Array Example: word count

`too many cooks too many ingredients`

    import std.algorithm;
    import std.stdio;

    void main()
    {
        ulong[string] dictionary;
        ulong wordCount, lineCount, charCount;

        foreach (line; stdin.byLine(KeepTerminator.yes))
        {
            charCount += line.length;
            foreach (word; splitter(line))
            {
                wordCount += 1;
                if (auto count = word in dictionary)
                    *count += 1;
                else
                    dictionary[word.idup] = 1;
            }

            lineCount += 1;
        }

        writeln("   lines   words   bytes");
        writefln("%8s%8s%8s", lineCount, wordCount, charCount);

        const char[37] hr = '-';

        writeln(hr);
        foreach (word; sort(dictionary.keys))
        {
            writefln("%3s %s", dictionary[word], word);
        }
    }

See wc for the full version.

### Associative Array Example: counting pairs

An Associative Array can be iterated in key/value fashion using a foreach statement. As an example, the number of occurrences of all possible substrings of length 2 (aka 2-mers) in a string will be counted:

    import std.range : slide;
    import std.stdio : writefln;
    import std.utf : byCodeUnit; // avoids UTF-8 auto-decoding

    int[string] aa;

    // The string `arr` has a limited alphabet: {A, C, G, T}
    // Thus, for better performance, iteration can be done _without_ decoding
    auto arr = "AGATAGA".byCodeUnit;

    // iterate over all pairs in the string and count each pair
    // ('A', 'G'), ('G', 'A'), ('A', 'T'), ...
    foreach (window; arr.slide(2))
        aa[window.source]++; // source unwraps the code unit range

    // iterate over all key/value pairs of the Associative Array
    foreach (key, value; aa)
    {
        writefln("key: %s, value: %d", key, value);
    }

    > rdmd count.d
    key: AT, value: 1
    key: GA, value: 2
    key: TA, value: 1
    key: AG, value: 2

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