Computer Programming Concepts and Visual Basic David I. Schneider

106 Computer Programming Concepts and Visual Basic
COMMENTS
1. A condition involving numeric variables is different from an algebraic truth.
The assertion (a + b) < 2 * a, considered in Example 2, is not a valid algebraic
truth because it isn’t true for all values of a and b. When encountered in a Visual
Basic program, however, it will be considered true if it is correct for the current
values of the variables.
2. Conditions evaluate to either true or false. These two values often are called the
possible truth values of the condition.
3. A condition such as 2 < n < 5 should never be used, because Visual Basic will
not evaluate it as intended. The correct condition is (2 < n) And (n < 5).
4. A common error is to replace the condition Not (2 < 3) by condition (3 > 2).
The correct replacement is (3 >= 2).
4.2 IF BLOCKS
An If block allows a program to decide on a course of action based on whether a certain condition
is true or false. A block of the form
If condition Then
action1
Else
action2
End If
causes the program to take action1 if condition is true and action2 if condition is false. Each
action consists of one or more Visual Basic statements. After an action is taken, execution
continues with the line after the If block. Figure 4.1 contains the pseudocode and flowchart
for an If block.
If condition is true Then
Processing step(s) 1
Else
Processing step(s) 2
End If
FIGURE 4-1 Pseudocode and Flowchart for an If Block
EXAMPLE 1
Write a program to find the larger of two numbers input by the user.