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Data that are naturally grouped are called structures. One can use Scheme's compound data types,
eg, vectors or lists, to represent structures. Eg,
let's say we are dealing with grouped data relevant to
a (botanical) tree. The individual elements of
the data, or fields, could be: height, girth, age, leaf-shape, and leaf-color, making a total of 5 fields. Such data
could be represented as a 5-element vector. The fields
could be accessed using
vector-ref and modified using vector-set!.
Nevertheless, we wouldn't want to be saddled with the
burden of remembering which vector index corresponds to
which field. That would be a thankless and error-prone
activity, especially if fields get excluded or included
over the course of time.
We will therefore use a Scheme macro defstruct to
define a structure data type, which is basically a
vector, but which comes with an appropriate suite of
procedures for creating instances of the structure, and
for accessing and modifying its fields. Thus, our tree
structure could be defined as:
(defstruct tree height girth age leaf-shape leaf-color)
This gives us a constructor procedure named
make-tree; accessor procedures for each field,
named
tree.height, tree.girth, etc; and modifier
procedures for each field, named set!tree.height,
set!tree.girth, etc. The constructor is used as
follows:
(define coconut
(make-tree 'height 30
'leaf-shape 'frond
'age 5))
The constructor's arguments are in the form of
twosomes, a field name followed by its initialization.
The fields can occur in any order, and may even be
missing, in which case their value is undefined.
The accessor procedures are invoked as follows:
(tree.height coconut) => 30
(tree.leaf-shape coconut) => frond
(tree.girth coconut) => <undefined>
The tree.girth accessor returns an undefined value,
because we did not specify girth for the
coconut tree.
The modifier procedures are invoked as follows:
(set!tree.height coconut 40)
(set!tree.girth coconut 10)
If we now access these fields using the corresponding
accessors, we will get the new values:
(tree.height coconut) => 40
(tree.girth coconut) => 10
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