CONCEPT
mappings
LAST UPDATE
Mon, 15 Mar 1999
DESCRIPTION
A step-by-step introduction to mappings:
----------------------------------------
1. What is a mapping?
A mapping is a datatype which allows to store data associated to a key.
In other languages they are also known as 'dictionaries' or 'alists'.
There are also alists in LPC but they are not a separate datatype but are
implemented on top of arrays. Alists are the predecessors of mappings.
The keys and the values can be of any type. But most common datatypes
for keys are strings, integers and objects. Others like arrays, mappings
or closures aren't a good choice because comparision between i.e. arrays
often returns false even if they equal in content. This is because the
driver compares i.e. two arrays by their internal pointers and not by
their content. The reason for this is simple: speed.
Mappings are allways treated as references when passing them to
functions. This means when you pass a mapping to another object and this
object modifies the mapping the modification will take place in a global
scope - visible to all objects holding this mapping in a variable.
2. What are mappings good for?
The term 'dictionary' probably describes the use of a mapping best.
Opposed to arrays mappings don't have a specific order. They provide a
mechanism to create a set of associations between values. Such an
association consists of a unique key and data that is identified by the
key. Think of a dictionary where you have a word and a definition of
it. You use the word to lookup its definition.
Mappings can be used i.e. to hold aliases for commands. The key would
then be the name of the alias and the data the command(s) behind an
alias. Or they can be used for the exits of a room. The keys would be
the directions where one can go to and the associated data would be the
file names of the rooms. But mappings can also be used as a kind of a
sparse array. A sparse array is an array where most of the elements
aren't used (occupied by 0). I.e. if you want to store values at the
position 0, 13 and 37642 of an array you would have to create an array
with a size of at least 37643. This costs a lot of memory so a mapping
would be more useful because you would then use the numbers 0, 13 and
37642 as a key and not as an index to a position (actually the keys of a
mapping are sometimes called indices but this is just because the way
data is accessed in a mapping is similar to arrays: by the [] operator).
This also allows to query all occupied positions of a sparse array by
querying for all the keys of the mapping opposed to an array where you
have to iterate over all elements.
3. How do I create a mapping?
There are several ways to do so. The most convenient is the following:
mapping map;
map = ([ key0: value00; ...; value0n,
... : ... ; ...; ... ,
keyn: valuen0; ...; valuenn ]);
As you can see, a key may have more than one value assigned. But the
amount of values per key must always be equal. It is even possible to
have mappings without any values!
Another method is to use the efun mkmapping(). This efun gets two
arguments with the first beeing an array of keys and the following beeing
arrays of values:
mapping map;
map = mkmapping (({ key0 , ..., keyn }),
({ value00, ..., value0n }),
({ ... , ..., ... }),
({ valuen0, ..., valuenn }));
If the efun only gets one argument, then this argument will be taken as
an array of keys and a mapping without values will be returned.
An empty mapping can be created by using the above described methods by
simply ommitting the keys and values:
mapping map;
map = ([]);
or:
map = mkmapping(({}), ({}));
Or by using the efun m_allocate(). This efun gets as first
argument the amount of keys which will be added soon and an optional
second argument specifying the width of the mapping:
map = m_allocate(n, width);
The value <n> may be a bit confusing since mappings shrink and grow
dynamically. This value just tells the driver how 'long' this mapping is
going to be so proper memory allocations will be performed to reduce
the overhead of memory reallocation. I.e. if you want to read in a file
and store the read data in a mapping you probably know the amount of
keys. So you allocate a mapping with this efun and tell the driver how
much memory should be allocated by specifing a proper <n> value.
Thus causing a speedup when adding the read data to the mapping
afterwards. The <width> just specifies how many values per key this
mapping is going to have. If no width is given, 1 will be taken as
default.
An empty mapping created with '([])' will always have a width of 1. To
create empty mappings with other widths, write it as
map = ([:width ]);
<width> can be any expression returning an integer value (including
function calls), and in fact this notation is just a fancy way of
writing
map = m_allocate(0, width);
4. How can I modify the data of a mapping?
Adding a new key is similiar to modifying the associated data of an
existing key:
map += ([ key: value0; ...; valuen ]);
Or in case only a single value should be modified:
map[key, n] = valuen;
If <n> is out of range or if <key> doesn't exists and <n> is greater
than 0 an "Illegal index" error will be reported. If <n> is equal to 0 or
the mapping only has a single value per key one can abbreviate it with:
map[key] = value;
If there is no <key> (and <n> is equal to 0 or not specified at all) a
new one will be added automatically.
Deletion of a key is done with the -= operator or the efun
m_delete(). A mapping can only be substracted by one without any values:
map -= ([ key ]);
or:
map -= ([ key0, ..., keyn ]);
The efun takes a mapping as first and a key as second argument:
m_delete(map, key);
The efun m_delete() returns the mapping but because mappings are
handled as references there is no need of an assignment like:
map = m_delete(map, key);
5. How can I access the data stored in a mapping?
This can be done by:
valuen = map[key, n];
Or in case of a mapping with just one value per key:
value0 = map[key];
If there is no <key> in the mapping and <n> is 0 or not specified at
all (which is the same) a 0 will be returned or if <n> is greater than 0
an "Illegal index" error will be reported.
6. How can I test for the existance of a key?
A return value of 0 is sufficient for most applications but sometimes
the ambiguity between an existing value of 0 and a nonexisting key can
lead to a problem. Therefore one can use the efun member() or
mapping_contains() to check if there actually is a key in the mapping:
if (member(map, key)) {
...
}
or:
if (mapping_contains(&value0, ..., &valuen, map, key)) {
...
}
This also shows how one can retrieve all values associated to a key
from a mapping in a single step. The '&' is the reference operator which
is neccesary to let the efun store the values in the variables.
In case of mappings with no values, the efun member() and
mapping_contains() are equal in their behaviour and their way of calling
because mapping_contains() won't get any reference variables to store the
values in (obviously, because there aren't any).
Also normally member() is known to return the postion of an element in
a list (i.e. a character in a string or data in an array) and if an
element couldn't be found -1 is returned. But in the case of mappings
there are no such things as order and postion. So member() only returns 0
or 1.
7. How can I copy a mapping?
A mapping can be copied with the + operator or by the efun
copy_mapping():
newmap = ([]) + map;
or:
newmap = copy_mapping(map);
A mapping should only be copied when it is neccesary to get an own copy
of it that must not be shared by other objects.
8. How can I get all keys of a mapping?
The efun m_indices() gets a mapping as argument and returns an array
holding all keys defined in this mapping:
keys = m_indices(map);
9. How can I get all the values of a mapping?
The efun m_values() gets a mapping as argument and returns an array
holding all the first (second, ...) values of it.
values0 = m_values(map); returns the first values
values0 = m_values(map, 0); dito
values1 = m_values(map, 1); returns the second values
etc
10. How can I determine the size of a mapping?
Because a mapping is a kind of rectangle it has two sizes: a length and
a width. There are three different efuns to query these values. The first
two are the efuns sizeof(), which returns the amount of key-value
associations (the length of a mapping), and widthof(), which returns the
number of values per key (the width). The third is the efun get_type_info().
get_type_info() is meant to be a function to identify a datatype. Its
return value is an array of two numerical values. The first specifies
the datatype of the argument and the second is a datatype dependend
value. In the case of a mapping the first value is T_MAPPING (which is a
value defined in <lpctypes.h>) and the second the amount of values per
key (a.k.a. columns or the width of the mapping - actually it would be
correct to say that the width of a mapping is the amount of columns plus
one for the keys but this is uncommon).
11. What is the best method to iterate over a mapping?
First of all the main purpose of a mapping is not meant to be a set of
data to iterate over. Afterall the keys in a mapping have no specific but
a random order (at least on the LPC side). But still it is possible and
sometimes even neccesary to do so.
If all key-value associations should be processed then one should use
walk_mapping(). If all keys of a mapping should be processed to create a
new mapping being a subset of the given one, then filter_mapping() should
be used. If all keys are going to be processed and to create a new
mapping with the same set of keys as the given mapping, then one would
use map_mapping(). But in the case of an iteration that should/can stop
even if not all data is processed it is probably wise to iterate over the
mapping by first querying for the keys and then to iterate over them with
a for() or a while() loop and querying the values by 'hand'.
The efun walk_mapping() gets a mapping as first argument and the name
of a function as second one. All the following arguments are treated as
extras which will be passed to the function specified with the 2nd
argument. Instead of a string for the name of a function a closure can be
used, too. Nothing will be returned:
...
walk_mapping(map, "func", xarg0, ..., xargn);
...
void func(mixed key, mixed value0, ..., mixed valuen,
mixed xarg0, ..., mixed xargn) {
...
}
func() will be called for all key-value associations and gets as first
argument the key. The next arguments are the values behind the key and
are passed as references. The rest of the passed arguments are those
specified as extras. Because the values are passed as references (opposed
to copies) it is possible to modify them from inside func() by simply
assigning new value to the variables <value0>, ..., <valuen>.
The efun filter_mapping() calls a function for each key in a mapping
and creates a new mapping which only contains key-value associations for
which the called function returned true (not equal 0 that is). The first
argument is the mapping to iterate over and the second is a function name
given as a string or a closure:
...
submap = filter_mapping(map, "func", xarg0, ..., xargn);
...
int func(mixed key, mixed xarg0, ..., mixed xargn) {
...
}
func() gets as first argument the key and the others are those passed
as extras to filter_mapping().
The efun map_mapping() gets a mapping as first argument and a string as
a function name (or again a closure) as second argument. Any additional
arguments are again used as extras that will be passed to the iteration
function. This efun returns a new mapping with the same keys as the given
one. The values returned by the function that is invoked for each key
will be used as the associated data behind each key of the new mapping:
...
newmap = map_mapping(map, "func", xarg0, ..., xargn);
...
mixed func(mixed key, mixed xarg0, ..., mixed xargn) {
...
}
func() gets as first argument the key and the others are those passed
as extras to map_mapping().
Because a function can only return a single value (even when it is an
array) it restricts the use of map_mapping() to only allow creation of
mappings with a single value per key.
12. Is it possible to join/intersect/cut mappings with another?
Joining mappings is only possible, if they have the same width (amount
of values per key). One can use the + and += operator:
map = map1 + map2 + ... + mapn;
map += map1 + map2 + ... + mapn;
Intersection of two mappings is only possible by using
filter_mapping(). There is no efun or operator which features this. The
'easiest' way may be the following function:
mapping intersect_mapping(mapping map1, mapping map2) {
closure cl;
cl = lambda(({ 'key }), ({ #'member, map2, 'key }));
return filter_mapping(map1, cl, map2);
}
This function returns a new mapping which consists of all key-value
associations of <map1> for which an equal key could be found in
<map2>. This function uses a closure which returns 0 or 1 depending on
wether a key from <map1> is contained in <map2> or not.
Cutting out all key-value associations of a mapping for which a key
could be found in another mapping can be done by using the - and -=
operator:
mapping cut_mapping(mapping map1, mapping map2) {
return map1 - mkmapping(m_indices(map2));
}
Because a maping can only be substracted by one without any values we
first have to create such by using m_indices() and mkmapping().
13. What are those mappings without any values (besides keys) good for?
Because the way how the driver searches for a key in a mapping is
rather fast, those mappings can be used as a set of elements with a fast
method for testing if an element is contained in the set. This technique
is called hashing (further explanation would lead too far) which is
faster than searching for values in array (which is done in a linear
fashion).
Another (maybe more pratical) use of these mappings are to create a
array of unique values out of an array with several equal values:
uniques = m_indices(mkmapping(array));
mkmapping() uses <array> to create a mapping without any values but
just keys. And because a mapping can only have unique keys all multiple
values in <array> are taken as one. The call of m_indices() then returns
an array of these unique keys. Actually we only make use of those
mappings temporarily.
14. How can I convert an alist into a mapping and vice versa?
There are no special efuns which handle such conversions. But it can be
done by the following functions:
mapping alist_to_mapping(mixed *alist) {
return apply(#'mkmapping, alist);
}
The efun apply() takes a closure and an array of values and passes each
element of the array as an argument to the closure. Because an alist
consists of an array of arrays with the first beeing the list of keys and
the others the values associated to each key passing them as arguments to
the efun closure #'mkmapping via apply() causes the creation of a mapping
out of an alist.
mixed *mapping_to_alist(mapping map) {
mixed *alist;
symbol *vars;
string var;
closure cl;
int width;
width = get_type_info(map)[1];
alist = allocate(width + 1);
vars = allocate(width + 2);
for (var = "a"; width; var[0]++, width--) {
alist[width] = ({});
vars[width] = quote(var);
}
alist[0] = ({});
vars[0] = 'key;
vars[<1] = 'alist;
cl = lambda(vars, ({ #'=, 'alist, ({ #'insert_alist }) + vars }));
walk_mapping(map, cl, &alist);
return alist;
}
This function is a bit more complicated than the other and detailed
description would lead too far of the topic. This function has one
restriction: it can only turn a mappings with up to 26 values per key
into an alist. But this should be sufficient for probably all
applications which use mappings.
And Hyps further comment on this:
The function mapping_to_alist() is also not that
clever because insert_alist() allways creates a new
alist. A second (optional) argument to m_values() to
specify the value column would be better. Besides
this, the conversion of a mapping into an alist could
be done by to_array().
15. Dirty Mappings
'Dirty mappings' are nothing the LPC programmer directly is involved
with, however, as it relates to the way mappings are implemented
internally by the gamedriver. However, as this term shows up in
various driver statistics, it is explained here.
There are two fundamental approaches to implement mappings:
1. Store all data entries in an array-like structure, in sorted order.
2. Store all data in a hashtable, each entry allocaed separately.
Method 1 is very space efficient, as it doesn't need much overhead
per entry; however, insertions and deletions of entries are
relatively slow as all other entries need to be moved.
Method 2 is very fast as nothing needs to be moved in memory,
however it has a large overhead.
The gamedriver uses a hybrid method: at the basis is a mapping
implementation based on arrays. However the driver uses a hash table
in addition to handle all the ongoing insertions and deletions.
Every once in a while, the contents of the hash table are sorted
into the base array, reasoning that any entry surviving for longer
time in the hash table is worth keeping in a more space-efficient
manner. 'Dirty' mappings are such mappings with both an array and a
hash part, 'clean' mappings are those with just an array part.
HISTORY
The ([:width ]) notation was added in LDMud 3.2.9/3.3.208 .
SEE ALSO
alists(LPC), closures(LPC), structs(LPC), mkmapping(E),
walk_mapping(E)
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