Page 77 - DCAP201_FUNDAMENTALS_OF_DATA_STRUCTURES

Fundamentals of Data Structures

Notes 2 Test(3)
2 Final
2 Grade
The identifier Student(20) indicates that there are 20 students. The identifier Test (3) indicates
that there are three tests per student. Observe that there are 8 elementary items per Student,
since test is counted three times. Altogether, there are 160 elementary items in the entire Student
structure.

5.3.1 Indexing Items in a Record

Suppose we want to access some data item in a record. In some cases, we cannot simply write the
data name of the item since the same name may appear in different places in the record.

Notes Accordingly, in order to specify a particular item, we may have to qualify the name
by using appropriate group item names in the structure. This qualification is indicated by
using decimal points to separate group items from sub items.

Example:
(a) Consider the record structure Newborn in the example of newborn given above. Sex and
year need no qualification, since each refers to a unique item in the structure. On the other
hand, suppose we want to refer to the age of the father. This can be done by writing
Newborn.Father.Age or simply Father.Age

The first reference is said to be fully qualified. Sometimes one adds qualifying identifiers
for clarity.
(b) Suppose the first line in the record structure in the example of newborn given above is
replaced by
1 Newborn(20)
That is, Newborn is defined to be a file with 20 records. Then every item automatically
becomes a 20-element array. Some languages allow the sex of the sixth newborn to be
referenced by writing
Newborn.Sex[6] or simple Sex[6]
Analogously, the age of the father of the sixth newborn may be referenced by writing

Newborn. Father. Age[6] or simply Father.Age[6]
(c) Consider the record structure Student in the student example given above. Since Student
is declared to be a file with 20 students, all items automatically become 20-element arrays.
Furthermore, Test becomes a two-dimensional array. In particular, the second test of the
sixth student may be referenced by writing
Student. Test[6, 2] or simply Test[6, 2]

The order of the subscripts corresponds to the order of the qualifying identifiers. For
example,
Test[3,1]
does not refer to the third test of the first student, but to the first test of the third student.

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