Computer Programming Concepts and Visual Basic David I. Schneider

38 Computer Programming Concepts and Visual Basic
looks into this memory location for the value of the variable and displays the value in the picture
box.
A combination of constants, variables, and arithmetic operations that can be evaluated
to yield a number is called a numeric expression. Expressions are evaluated by replacing
each variable by its value and carrying out the arithmetic. Some examples of expressions are
2 * distance + 7, n + 1, and (a + b) / 3.
EXAMPLE 3
The following program displays the value of an expression.
Private Sub cmdCompute_Click()
picResults.Cls
a = 5
b = 4
picResults.Print a * (2 + b)
End Sub
[Run, and then click the command button. The following is displayed in the picture box.]
30
If var is a variable, then the statement
var = expression
first evaluates the expression on the right and then assigns its value to the variable. For instance,
the event procedure in Example 3 can be written as
Private Sub cmdCompute_Click()
picResults.Cls
a = 5
b = 4
c = a * (2 + b)
picResults.Print c
End Sub
The expression a * (2 + b) is evaluated to 30 and then this value is assigned to the variable c.
Because the expression on the right side of an assignment statement is evaluated before
an assignment is made, a statement such as
n = n + 1
is meaningful. It first evaluates the expression on the right (that is, it adds 1 to the original
value of the variable n), and then assigns this sum to the variable n. The effect is to
increase the value of the variable n by 1. In terms of memory locations, the statement
retrieves the value of n from n’s memory location, uses it to compute n + 1, and then places
the sum back into n’s memory location.
■ PRINT METHOD
Consider the following event procedure.
Private Sub cmdDisplay_Click()
picResults.Cls
picResults.Print 3
picResults.Print -3
End Sub
[Run and then click the command button.]