Computer Programming Concepts and Visual Basic David I. Schneider

444 Computer Programming Concepts and Visual Basic
OCT If n is a whole number between 0 and 2,147,483,647, Oct(n) is the octal (that is, base
8) representation of n.
ON ERROR The statement On Error GoTo lineLabel sets up error-trapping. An error then
causes a jump to the error-handling routine beginning with the first statement following the
specified line label. The On Error statement and its target must be in the same procedure.
[line label]
ON . . . GOSUB and ON . . . GOTO The statement On expression GoSub lineLabel1, lineLabel2,
. . . causes a GoSub to lineLabel1, lineLabel2, . . . depending on whether the value of
the expression is 1, 2, .20.20.20. Similarly, the GoTo variation causes an unconditional jump
to the appropriate line label. The GoSub or GoTo statement and its target must be in the same
procedure. [line label]
OPEN The statement Open “filespec”For mode As #n allows access to the file filespec in
one of the following modes: Input (information can be read sequentially from the file), Output
(a new file is created and information can be written sequentially to it), Append (information
can be added sequentially to the end of a file), or Binary (information can be read or
written in an arbitrary fashion). The statement Open “filespec”For Random As #n Len = g
allows random-access to the file filespec in which each record has length g. Throughout the
program, the file is referred to by the reference number n (from 1 through 255). Another variation
of the Open statement is Open “LPT1”For Output As #n, which allows access to the
printer as if it were a sequential file.
In a network environment, two enhancements to the Open statement are available. Visual
Basic accesses data files in two ways: it reads from them or writes to them. When several
processes may utilize a file at the same time, accurate file handling requires that certain
types of access be denied to anyone but the person who has opened the file. The statement
Open “filespec”For mode Lock Read As #n or Open “filespec”For Random Lock Read As #n
Len = g opens the specified file and forbids any other process from reading the file as long
as the file is open. Lock Write forbids any other process from writing to the file as long as
the file is open. Lock Read Write forbids any other process from reading or writing to the
file as long as the file is open. Lock Shared grants full access to any other process, except
when a file is currently opened and locked by a process for a certain access mode, then
another process attempting to open the file for the same mode will receive the message “Permission
denied”and be denied access. [filespec] [binary file]
OPTION BASE After the statement Option Base m is executed, where m is 0 or 1, a statement
of the form Dim arrayName(n) defines an array with subscripts ranging from m to n.
Visual Basic’s extended Dim statement, which permits both lower and upper subscript
bounds to be specified for each array, achieves a wider range of results, making its use
preferable to Option Base.
OPTION COMPARE The statement Option Compare Text, placed in the (Declarations) section
of (General), causes string comparisons to be case-insensitive. Thus, if Option Compare
Text is in effect, the comparison “make”= “MaKe”will be true. The statement Option Compare
Binary placed in the (Declarations) section produces the default comparison rules,
which are case-sensitive and use the character order given in the ANSI/ASCII character
tables.
OPTION EXPLICIT If the statement Option Explicit appears in the (Declarations) section
of (General), each variable must be declared before it is used. A variable is declared by
appearing in a Const, Dim, Global, ReDim, or Static statement, or by appearing as a parameter
in a Sub or Function definition.
OR (Bitwise Operator) The expression byte1 Or byte2 is evaluated by expressing each byte
as an 8-tuple binary number and then Oring together corresponding digits, where 1 Or 1, 1
Or 0, and 0 Or 1 are all equal to 1, while 0 And 0 is equal to 0. For example, the expression
37 Or 157 translated to binary 8-tuples becomes 00100101 Or 10011101. Oring together corresponding
digits gives the binary 8-tuple 10111101 or decimal 189. Thus, 37 Or 157 is 189.