Computer Programming Concepts and Visual Basic David I. Schneider

3. Statements of the form
picBox.Print recVar
are invalid, where recVar is a record variable. Each field of a record must appear
separately in a picBox.Print statement. Also, comparisons involving records
using the relational operators , =, , < =, and > = are valid only with the
record fields, and not with the records themselves.
4. In addition to being declared as numeric, Boolean, or fixed-length string data
types, the elements of a user-defined variable type can also be declared as other
types of records. However, we do not use such structures in this text.
5. An array of fixed-length strings is declared by a statement of the form
Dim arrayName (a To b) As String * n
6. An array of records would be declared with a statement such as
Dim colleges(1 to 8) As collegeData
and information would be accessed with statements such as
picBox.Print colleges(1).nom
7. When fixed-length strings are passed to and from procedures, the corresponding
parameter in the Sub or Function statement must be an ordinary (variablelength)
string.
8. Most data types can be used as field types appearing in a Type block, including
(variable-length) strings. However, the String data type is not allowed in Type
blocks that will be used with random-access files.
8.2 RANDOM-ACCESS FILES
A random-access file is like an array of records stored on a disk. The records are numbered
1, 2, 3, and so on, and can be referred to by their numbers. Therefore, a random-access file resembles
a box of index cards, each having a numbered tab. Any card can be selected from the
box without first reading every index card preceding it; similarly, any record of a random-access
file can be read without having to read every record preceding it.
One statement suffices to open a random-access file for all purposes: creating, appending,
writing, and reading. Suppose a record type has been defined with a Type block and a
record variable, called recVar, has been declared with a Dim statement. Then after the statement
Open “filespec” For Random As #n Len = Len(recVar)
is executed, records may be written, read, added, and changed. The file is referred to by
the number n. Each record will have as many characters as allotted to each value of recVar.
Suppose appropriate Type, Dim, and Open statements have been executed. The two-step
procedure for entering a record into the file is as follows.
1. Assign a value to each field of a record variable.
2. Place the data into record r of file #n with the statement
Put #n, r, recVar
where recVar is the record variable from Step 1.
Random-Access Files 251