A structure is a heterogeneous container object, i.e., it is an object with elements whose values do not have to be of the same data type. The elements or fields of a structure are named, and one accesses a particular field of the structure via the field name. This should be contrasted with an array whose values are of the same type, and whose elements are accessed via array indices.
A user-defined data type is a structure with a fixed set of fields defined by the user.
The struct keyword is used to define a structure. The syntax
for this operation is:
struct {field-name-1, field-name-2, ... field-name-N};
NULL.
For example,
variable t = struct { city_name, population, next };
t.
Like arrays, structures are passed around via a references. Thus,
in the above example, the value of t is a reference to the
structure. This means that after execution of
variable u = t;
t and u refer to the same structure,
since only the reference was used in the assignment. To actually
create a new copy of the structure, use the dereference
operator, e.g.,
variable u = @t;
The dot (.) operator is used to specifiy the a particular
field of structure. If s is a structure and field_name
is a field of the structure, then s.field_name specifies
that field of s. This specification can be used in
expressions just like ordinary variables. Again, consider
variable t = struct { city_name, population, next };
t.city_name = "New York";
t.population = 13000000;
if (t.population > 200) t = t.next;
t.
One of the most important uses of structures is to create a dynamic data structure such as a linked-list. A linked-list is simply a chain of structures that are linked together such that one structure in the chain is the value of a field of the previous structure in the chain. To be concrete, consider the structure discussed earlier:
variable t = struct { city_name, population, next };
next field is to provide the link to the
next structure in the chain. Suppose that there exists a function,
read_next_city, that reads city names and populations from a
file. Then we can create the list via:
define create_population_list ()
{
variable city_name, population, list_root, list_tail;
variable next;
list_root = NULL;
while (read_next_city (&city_name, &population))
{
next = struct {city_name, population, next };
next.city_name = city_name;
next.population = population;
next.next = NULL;
if (list_root == NULL)
list_root = next;
else
list_tail.next = next;
list_tail = next;
}
return list_root;
}
list_root and list_tail
represent the beginning and end of the list, respectively. As long
as read_next_city returns a non-zero value, a new structure is
created, initialized, and then appended to the list via the
next field of the list_tail structure. On the first
time through the loop, the list is created via the assignment to the
list_root variable.
This function may be used as follows:
variable Population_List = create_population_list ();
if (Population_List == NULL) error ("List is empty");
define get_largest_city (list)
{
variable largest;
largest = list;
while (list != NULL)
{
if (list.population > largest.population)
largest = list;
list = list.next;
}
return largest.city_name;
}
vmessage ("%s is the largest city in the list",
get_largest_city (Population_List)));
get_largest_city is a typical example of how one traverses
a linear linked-list by starting at the head of the list and
successively moves to the next element of the list via the
next field.
Now consider a function that sorts the list according to population. To illustrate the technique, a bubble-sort will be used, not because it is efficient, it is not, but because it is simple and intuitive.
define sort_population_list (list)
{
variable changed;
variable node, next_node, last_node;
do
{
changed = 0;
node = list;
next_node = node.next;
last_node = NULL;
while (next_node != NULL)
{
if (node.population < next_node.population)
{
% swap node and next_node
node.next = next_node.next;
next_node.next = node;
if (last_node != NULL)
last_node.next = next_node;
if (list == node) list = next_node;
node = next_node;
next_node = node.next;
changed++;
}
last_node = node;
node = next_node;
next_node = next_node.next;
}
}
while (changed);
return list;
}
list and node, i.e.,
if (list == node) list = next_node;
A user-defined data type may be defined using the typedef
keyword. In the current implementation, a user-defined data type
is essentually a structure with a user-defined set of fields. For
example, in the previous section a structure was used to represent
a city/population pair. We can define a data type called
Population_Type to represent the same information:
typedef struct
{
city_name,
population
} Population_Type;
variable a = Population_Type[10];
a[0].city_name = "Boston";
a[0].population = 2500000;
Population_Type may also be used with the
typeof function:
if (Population_Type = typeof (a)) city = a.city_name;
@ may be used to create an instance of the
new type:
a = @Population_Type;
a.city_name = "Calcutta";
a.population = 13000000;