Computer Programming Concepts and Visual Basic David I. Schneider

146 Computer Programming Concepts and Visual Basic
[Run and press the command button.]
COMMENTS
1. The body of a For...Next loop need not be indented. However, because indenting
improves the readability of the program, it is good programming style. As
soon as you see the word For, your eyes can easily scan down the program to
find the matching Next statement. You then know two facts immediately: the
number of statements in the body of the loop and the number of passes that will
be made through the loop.
2. For and Next statements must be paired. If one is missing, the program will
generate the error message “For without Next”or “Next without For.”
3. Consider a loop beginning with For i = m To n Step s. The loop will be executed
exactly once if m equals n no matter what value s has. The loop will not be
executed at all if m is greater than n and s is positive, or if m is less than n and
s is negative.
4. The value of the control variable should not be altered within the body of the
loop; doing so might cause the loop to repeat indefinitely or have an unpredictable
number of repetitions.
5. Noninteger step values can lead to roundoff errors with the result that the loop
is not executed the intended number of times. For instance, a loop beginning
with For i = 1 To 2 Step .1 will be executed only 10 times instead of the intended
11 times. It should be replaced with For i = 1 To 2.01 Step .1.
5.4 A CASE STUDY: ANALYZE A LOAN
This case study develops a program to analyze a loan. Assume the loan is repaid in equal
monthly payments and interest is compounded monthly. The program should request the amount
(principal) of the loan, the annual rate of interest, and the number of years over which the loan
is to be repaid. The four options to be provided by command buttons are as follows.
1. Calculate the monthly payment. The formula for the monthly payment is
payment = p * r / (1 – (1 + r) ^ (–n))
where p is the principal of the loan, r is the monthly interest rate (annual rate
divided by 12) given as a number between 0 (for 0 percent) and 1 (for 100 percent),
and n is the number of months over which the loan is to be repaid.
Because a payment computed in this manner can be expected to include fractions
of a cent, the value should be rounded up to the next nearest cent. This corrected
payment can be achieved using the formula