Computer Programming Concepts and Visual Basic David I. Schneider

EXAMPLE 8
The DC Lottery number is obtained by selecting a Ping-Pong ball from each of three separate bowls. Each
ball is numbered with an integer from 0 through 9. Write a computer program to produce a lottery number.
(Such a program is said to simulate the selection of Ping-Pong balls.)
SOLUTION:
The value of Int(10 * Rnd) will be an integer from 0 through 9, and each of these integers has the same
likelihood of occurring. Repeating the process three times produces the requested digits.
Private Sub cmdDisplayANumber_Click()
‘Display a lottery number
picNumber.Cls
Randomize
picNumber.Print Int(10 * Rnd);
picNumber.Print Int(10 * Rnd);
picNumber.Print Int(10 * Rnd)
End Sub
[Run and then click the command button. One possible output to be displayed in the picture box is as follows.]
8 3 9
Note: Run the program in Example 8 several times and notice that the output changes each
time. Then delete the Randomize statement and run the program several times.
COMMENTS
1. Requesting the square root of a negative number terminates the execution of the
program and gives the error message “Invalid procedure call or argument.”
2. If n is greater than the length of str, then the value of Left(str, n) will be the
entire string str. A similar result holds for Mid and Right.
3. Visual Basic has a function called LCase that is analogous to UCase. LCase
converts all uppercase letters in a string to lowercase letters.
4. Because the values of the functions Left, Mid, Right, UCase, and the Format
functions are strings, they are referred to as string-valued functions.
5. Mid is an important function. It will be used several times in this book to examine
each letter of a string.
6. Trim is useful when reading data from a text box. Sometimes users type spaces
at the end of input. Unless the spaces are removed, they can cause havoc elsewhere
in the program. Also, Trim is useful in trimming the leading spaces from
numbers that have been converted to strings with Str.
7. The InStr function has a useful extension. The value of InStr(n, str1, str2) is the
position of the first occurrence of str2 in str1 in position n or greater. For
instance, InStr(5, “Mississippi”, “ss”) is 6.
8. In Example 4, we found that 4 is the remainder when 256 is divided by 14.
Mathematicians say “4 = 256 modulo 14.” Visual Basic has an operation, Mod,
that performs this calculation directly. If m and n are positive integers, then n
Mod m is the remainder when n is divided by m. Visual Basic also has an operation
called integer division, denoted by \, which gives the quotient portion of
a long division problem. For instance, 14/4 is 3.5 whereas 14\4 is the integer 3.
9. Recall that the function Mid has the form Mid(str, m, n) and returns the substring
of str starting with position m and having length n. Visual Basic does its
best to please for unexpected values of m and n. If m is greater than the length
Built-In Functions 65