Computer Programming Concepts and Visual Basic David I. Schneider

40 Computer Programming Concepts and Visual Basic
COMMENTS
1. Numbers must not contain commas, dollar signs, or percent signs. Also, mixed
numbers, such as 8 1/2, are not allowed.
2. Some people think of the equal sign (=) in an assignment statement as an arrow
pointing to the left. This stresses the fact that the value on the right is assigned
to the variable on the left.
3. Parentheses should be used when necessary to clarify the meaning of an expression.
When there are no parentheses, the arithmetic operations are performed in
the following order: (1) exponentiations; (2) multiplications and divisions;
(3) additions and subtractions. In the event of ties, the leftmost operation is carried
out first. Table 2-1 summarizes these rules.
TABLE 2-1
Level of Precedence for Arithmetic Operations
( ) Inner to outer, left to right
^ Left to right in expression
*/ Left to right in expression
+ – Left to right in expression
4. Restricted keywords cannot be used as names of variables. For instance, the
statements print = 99 and end = 99 are not valid. Some other common restricted
keywords are Call, If, Let, Select, and Sub. If a keyword is used as a variable
name, you will soon be warned that something is wrong. As soon as the cursor
is moved from the line, an error message will appear, and the line will turn red.
The use of some other keywords (such as Error, Height, Name, Rate, Time, Val,
Width, and Year) as variable names does not trigger an immediate warning, but
generates an error message when the program is run. Although there is a way to
get Visual Basic to accept this last group of keywords as variable names, we
will never use keywords as variable names. Most of the items in Appendix C,
other than properties, are reserved words. You can tell immediately when you
inadvertently use a reserved word as a variable in an assignment statement
because Visual Basic automatically capitalizes the first letter of keywords. For
instance, if you type “rate = 50” and press the Enter key, the line will change to
“Rate = 50”.
5. Grammatical errors, such as misspellings or incorrect punctuations, are called
syntax errors. Certain types of syntax errors are spotted by the smart editor
when they are entered, whereas others are not detected until the program is executed.
When Visual Basic spots an error, it displays a dialog box. Some incorrect
statements and their errors are given below.
Statement Reason for Error
picBox.Primt 3 Misspelling of keyword
picBox.Print 2 + No number follows the plus sign
9W = 5 9W is not a valid variable name
6. Errors detected while a program is running are called run-time errors.
Although some run-time errors are due to improper syntax, others result from
the inability of the computer to carry out the intended task. For instance, if the
value of numVar is 0, then the statement
numVarInv = 1/numVar
interrupts the program with the run-time error “Division by zero.” If the file
DATA.TXT is not in the root folder of the C drive, then a statement that refers