Computer Programming Concepts and Visual Basic David I. Schneider

48 Computer Programming Concepts and Visual Basic
6. Most major programming languages require that all variables be declared
before they can be used. Although declaring variables with Dim statements is
optional in Visual Basic, you can tell Visual Basic to make declaration mandatory.
The steps are as follows:
(a) From any code window, click on the down-arrow to the right of the Object
box and click on (General).
(b) Type
Option Explicit
and press Enter.
Then, if you use a variable without first declaring it in a Dim statement, the
message “Variable not defined” will appear as soon as you attempt to run the
program. One big advantage of using Option Explicit is that mistypings of variable
names will be detected. Otherwise, malfunctions due to typing errors are
often difficult to detect.
7. You can have Visual Basic automatically place Option Explicit in every program
you write. The steps are as follows:
(a) Press Alt/T/O and click on the Editor tab to invoke the editor options.
(b) If the square to the left of “Require Variable Declaration” does not contain
a check mark, click on the square and press the OK button.
8. Variables that are not (explicitly) declared with Dim statements are said to be
implicitly declared. Such variables, which have a data type called Variant, can
hold strings, numbers, and several other kinds of information.
9. You can display the type of a variable with the following steps—position the
cursor over the word, press the right mouse button, and click on Quick Info.
10. Val can be applied to strings containing nonnumeric characters. If the beginning
of the string str represents a number, then Val(str) is that number; otherwise, it
is 0. For instance, Val(“123Blastoff ”) is 123, and Val(“ab3”) is 0.
11. The KeyPress event also applies to command buttons and picture boxes.
12. Concatenation of strings also can be represented by a plus sign (+). However,
restricting the plus sign to operations on numbers eliminates ambiguity and provides
self-documenting code.
13. If Val is omitted from the statement
numVar = Val(txtBox.Text)
or Str is omitted from the statement
txtBox.Text = Str(numVar)
Visual Basic does not complain, but simply makes the conversion for you. However,
errors can arise from omitting Val and Str. For instance, if the contents of
txtBox1.Text is 34 and the contents of txtBox2.Text is 56, then the statement
numVar = txtBox1.Text + txtBox2.Text
assigns the number 3456 rather than 90 to numVar. (This is because Visual
Basic does not perform the conversion until just before the assignment.) If
txtBox1 is empty, then the statement
3 * txtBox1.Text
will stop the program and produce the error message “Type mismatch.” We follow
the standards of good programming practice by always using Val and Str to
convert values between text boxes and numeric variables. Similar considerations
apply to conversions involving label captions.