# LookupTable

A LookupTable is an unordered collection of values; each value indexed by a "key," which is a value of any type that's used to look up a value stored in the collection. In effect, this class provides what some programming languages call an "associative array," because it allows a value to be associated with an arbitrary key, and then efficiently found given the same key.

You must #include <lookup.h> in your source code to use the LookupTable class.

## Creating a LookupTable

You create a lookup table with the new operator:

    local tab = new LookupTable();

You can optionally specify two parameters to fine-tune the table's efficiency: the "bucket count," and the initial capacity. When you create a LookupTable with no arguments as shown above, the object uses default values for these parameters; the default values will always work, but you might be able to improve a table's performance by overriding the default values, especially if the table will contain an especially large or small number of entries. Note that it is never necessary to specify the parameters, since a lookup table will always work properly with the defaults; the only thing the parameters do is tune the object's performance.

    // override the default creation parameters
    // use 256 hash slots and an initial capacity of 1024 entries
    local tab = new LookupTable(256, 1024);

The first parameter, the bucket count, specifies the size of the hash table that the object uses to organize the keys. See below for more information about what the bucket count means and how to select this value.

Note that the bucket count is fixed over the life of the object.

The second parameter, the initial capacity, is purely advisory. This parameter sets the amount of memory the table initially allocates so that it can accommodate the given number of stored values. If you eventually add more values to the table than the initial entry count, the object will automatically expand to accommodate more entries. For maximum efficiency, you should choose a value that's about the same as the number of entries you ultimately expect; this will ensure that you don't use excessive memory by allocating many more initial slots than you end up using, while at the same time avoiding repeated expansion of the table.

### Creating from a key/value list

You can also create a LookupTable that's pre-loaded with an initial set of values. To do this, pass a list, Vector, or list-like object as the argument to the constructor:

    local tab = new LookupTable(['red', 0xff0000,
                                 'green', 0x00ff00,
                                 'blue', 0x0000ff]);

This creates a table and initializes it with the keys and values from the list. The constructor interprets the list elements as pairs of Key and Value elements. So for our example above, we'd get the same result by setting the values explicitly like this:

    tab['red'] = 0xff0000;
    tab['green'] = 0x00ff00;
    tab['blue'] = 0x0000ff;

When you create a table from a list, the bucket count and overall size will both be initialized according to the length of the list. The system chooses a size that's a little larger than the list; this leaves room for modest expansion without having to allocate more memory, but not so much that a lot of extra memory will be tied up if the list doesn't end up expanding.

You can also specify the default value for the table, which is the value returned when you index the table with a key that doesn't exist in the table. To do this, simply add the default value as the last element, without a corresponding key value. The constructor knows that this odd-numbered last element is the default value because it's not paired with another element.

### Shorthand notation

The compiler provides a special short-form syntax that lets you create a LookupTable and load it with values. It works exactly like new LookupTable() with a list argument, but it's less typing. It looks like this:

    local tab = ['red' -> 0xff0000, 'green' -> 0x00ff00, 'blue' -> 0x0000ff];

The notation looks a lot like an ordinary list, but for each element of the list, you provide a key and a value, separated by an arrow symbol, -> (that's a hyphen followed by a greater-than sign, without any spaces between the two).

This creates the same LookupTable as the earlier example that called constructor with a list argument. In fact, it really is equivalent code - the compiler internally converts the shorthand notation into the same new LookupTable() call as before.

You can also specify the default value, which is the value returned when you index the table by a key that doesn't exist in the table. To do this, include a final item in the list using the notation *->Value - an asterisk, followed by an arrow, followed by the desired default value. (The syntax is meant to suggest a "wildcard" key, to match any other key that's not defined in the table. The asterisk isn't literally stored as a key, though; the default value has no key, since it's specifically for use when a key isn't present.) The default value must be the last item in the list.

    local tab = ['red' -> 0xff0000, 'green' -> 0x00ff00, 'blue' -> 0x0000ff,
                 * -> 'undefined'];

Even though the list-style shorthand notation might look like a "constant" value, it's not. It's just a compact way of writing the equivalent new LookupTable expression. Each time you evaluate the expression, it'll create a new object. If the code is part of a routine that will be executed repeatedly, you might want to create the table once and save it somewhere (in an object property, say), to avoid the overhead of creating many copies of the table.

## Storing and retrieving values

You use a lookup table as though it were a list or Vector, except that you can use any type of value as an "index." For example, we could use a lookup table to associate objects with strings:

    symtab['book'] = book;
    symtab['ball'] = ball;
    symtab['table'] = table;

Now, if we want to look up the object associated with one of these words, we simply index the lookup table:

    obj = symtab['ball'];

If you store a value in the lookup table, and a value was already stored at the same key, the old value at the key is discarded and replaced by the new value. If you look up a key that doesn't exist in the table, the result is the "default value" - initially nil, but you can change this to another value if desired with the setDefaultValue() method.

A LookupTable matches key values the same way the == operator does.

## Iterating over a LookupTable's contents

LookupTable is a subclass of Collection, so you can use the createIterator() method to create an Iterator to iterate over the elements of the lookup table. The Iterator that a LookupTable creates is called a LookupTableIterator; it visits the values stored in the table in an arbitrary order.

Because a LookupTable is a Collection, you can also use the foreach statement to iterate over its values. When you use foreach with a LookupTable, the iteration variable is assigned a value from the table (not a key) on each iteration.

## LookupTable methods

LookupTable is a subclass of Collection, and thus includes all Collection methods. In addition, LookupTable defines the methods listed below.

applyAll(func)

For each element in the table, this method invokes the callback function (func)(key, value), and then changes the element's value to the return value of func. This allows you to modify all of the values in the table according to the given function. The keys are not changed. The entries are visited in arbitrary order. No return value.

forEach(func)

For each element in the table, invokes the function (func)(value). Any return value from func is ignored; the values and keys stored in the table are not changed. The entries are visited in arbitrary order. No return value.

forEachAssoc(func)

For each element in the table, invokes the callback function (func)(key, value). Any return value from func is ignored; the values and keys stored in the table are not changed. The entries are visited in arbitrary order. This argument is the same as forEach(), except that this method provides the callback with the key in addition to the value for each element. No return value.

getBucketCount()

Returns the number of "hash buckets" in the table. Since the number of buckets is fixed over the life of the object, this simply returns the bucket count that was specified when the object was created.

getDefaultValue()

Returns the table's default value. This is the value returned when you index the table with a key that doesn't exist in the table. The default value is initially nil, but you can change it with setDefaultValue().

getEntryCount()

Returns the number of key/value entries in the table. This returns the number of entries actually stored in the table, which is unrelated to the initial capacity that was specified when the object was created.

isKeyPresent(key)

Checks to see if an entry with the given key is present in the table. Returns true if the key is present, nil if not.

keysToList()

Returns a list consisting of all of the keys in the table. The order of the keys is arbitrary, so callers must not assume any particular ordering.

nthKey(n)

Returns the key at the given index in the table. This returns the key that appears at the given index in the keysToList() list, but if you just want one key it's more efficient to use this method, since it doesn't construct a list. If the index is out of range, an error occurs.

nthVal(n)

Returns the value at the given index in the table. This returns the value that appears at the given index in the keysToList() list, but if you just want one value it's more efficient to use this method, since it doesn't construct a list. If the index is out of range, an error occurs.

removeElement(key)

Removes the element with the given key, if any. If there is no element with the given key in the table, this makes no change to the table and simply returns nil. If an element is found, the element is removed, and the value associated with the key (before the removal) is returned.

setDefaultValue(val)

Sets the default value for the table. The default value is returned when you index the table with a key that doesn't exist in the table. The default value is initially nil, but you can use this method to change it to any other value.

valsToList()

Returns a list consisting of all of the values in the table. The order of the values is arbitrary.

## Performance considerations

A LookupTable is implemented as a special data structure known as a hash table. You can find more on the theory of hash tables on the Web (for example, the Wikipedia page), so we won't go into that here. You don't really need to know anything about hash tables to use LookupTable objects, but if you do happen to be familiar with how they work, you can take advantage of a couple of parameters to optimize their performance.

When you create a LookupTable, you can specify two sizes that control how the internal hash table is organized: the number of "buckets," and the initial capacity. These parameters affect performance in several ways.

First, the larger the bucket count and initial capacity, the more memory the hash table consumes. It's best to choose sizes that aren't wildly greater than the actual needs of the table, to avoid wasting memory.

Second, it's generally faster to look up a value in a table when the table has more hash buckets. This trend continues up to the point where the number of buckets is about equal to the number of entries in the table; beyond that, there's usually no benefit to adding more buckets. If you have a rough idea of the number of elements the table will ultimately have, you can use this to pick a bucket count when creating the table. It's not important to figure the number exactly; if there are only half as many hash buckets as entries, you'll still get pretty close to ideal performance. And keep in mind that you need to balance this against the memory usage of more buckets.

Third, it's be faster to add new values to the table if the table doesn't have to be reallocated too frequently. The system must allocate new memory for the table every time you exceed its current capacity. The table starts with the initial capacity you specify when you create it, and it automatically expands as needed when you add new values. Each expansion operation takes a little extra time. If you know roughly the number of values you'll ultimately be adding to the table, you should ideally make the initial capacity large enough to hold all of them, since that will make it unnecessary to add more memory later.

For the most part, you shouldn't spend a lot of time worrying about any of this. The LookupTable object manages its memory and internal structure automatically, and it will work fine if you always use the defaults. The only time you're likely to notice any difference at all by tweaking the creation parameters is for large tables (with hundreds or thousands of elements) that you use frequently.

TADS 3 System Manual
Table of Contents | The Intrinsics > LookupTable
Prev: List Next: Object