Visual Basic.NET How to Program (PDF)

Chapter 5 Control Structures: Part 2 153
19 ' calculate amount after each year
20 For year = 1 To 10
21 amount = principal * (1 + rate) ^ year
22 output &= year & vbTab & _
23 String.Format("{0:C}", amount) & vbCrLf
24 Next
25
26 ' display output
27 MessageBox.Show(output, "Compound Interest", _
28 MessageBoxButtons.OK, MessageBoxIcon.Information)
29
30 End Sub ' Main
31
32 End Module ' modInterest
Fig. Fig. 5.8 5.8 For/Next structure used to calculate compound interest (part 2 of 2).
The For/Next structure executes its body 10 times, varying control variable year
from 1 to 10 in increments of 1. Line 21 performs the calculation from the problem statement
a = p (1 + r) n
where a is amount, p is principal, r is rate and n is year.
Lines 22–23 append additional text to the end of String output. The text includes
the current year value, a tab character (vbTab) to position to the second column, the
result of the method call String.Format("{0:C}", amount) and, finally, a newline
character (vbCrLf) to start the next output on the next line. The first argument passed to
Format is the format string. We have seen Strings containing {0}, {1} and so on,
where the digit within the braces indicates the argument being displayed. In Chapter 4, we
used a more complicated format string to print a floating-point number with two digits after
the decimal. In these more complicated format strings, such as "{0:C}", the first digit (0)
serves the same purpose. The information specified after the colon (:) is called the formatting
code. The C (for “currency”) formatting code indicates that its corresponding argument
(amount) should be displayed in monetary format. Figure 5.9 shows several formatting
codes; a complete list can be found in the MSDN documentation “Standard Numeric
Format Strings.” All formatting codes are case insensitive. Note that format codes D and X
can be used only with integer values.