1. Decision Making in Code Logical Tests & Truth Logical Expressions
ISM 3005 Course Introduction
Topics
Decision Making in Code
Logical Tests & Truth
Logical Expressions
Logical Operators & Precedence
Range Testing
The Not Operator
Boolean Properties
“Home computers are being called upon to perform many new functions, including the consumption of homework formerly eaten by the dog”
Doug Larson 2

2. Decision Making Overview
Decision making in code is one of the three fundamental programming constructs
Sequence
Selection or Choice
Iteration
Allows program to execute designated code when certain conditions are met
If dblTotal <= 5000 Then
sglTax = dblTotal * .06
Else
sglTax = (5000 * .06) + (sglTotal ) * .05
End If
What policy is implemented
in this code?

3. Beauty is Truth, Truth Beauty (John Keats)
Programming often depends on testing a condition to control program behavior
Whether specific code will execute
How many times a loop executes
Condition testing is accomplished by evaluation whether an expression is True or False
True and False are literal Boolean values
Dim blnProgrammingIsEasy as Boolean blnProgrammingIsEasy = True
Logical tests are performed using rules of logic and precedence

4. Logical Tests
Logical tests always evaluate to a result of either True or False
In the test If dblTotal <= 5000 Then the current value of the variable dblTotal is tested to see if it is less than or equal to the literal value 5000
The entire expression dblTotal <= 5000 will be replaced with True or False depending on the value of dblTotal
If the expression is True the following code will execute

5. Logical Expressions & Evaluation
Logical expressions are tests
Made with logical operators
Evaluate to True or False
Logical operators
< Less than
<= Less than or equal to
= Equal to
> Greater than
>= Greater than or equal to
<> Not equal to
Require two
characters to
write

6. Logical Expressions & Evaluation (cont.)
Evaluating numeric expressions
If intAge <= 100 Then
If intAge <= Val(txtAge.Text) Then
Either term in the test may be provided by a
Literal • Variable
Function result • Constant
Boolean property • Calculation result
Dissimilar numeric data types can be compared but the results may be unexpected
Best to convert before comparing

7. Logical Expressions & Evaluation (cont.)
Evaluating string data types
String evaluations are made according to the ASCII codes of each value in the strings
“abc” is less than “abc “
“ABC” is less than “abc”
“abcd” is greater than “abc”
UCase(“abc”) is equal to “ABC”
“123” is less than “234”
“45” is greater than “123”
Space
???

8. Logical Expressions & Evaluation (cont.)
Logical operators
And works on multiple conditions and requires both tests to be true for the overall test to be true
If intX >= 3 And intX <= 8 Then
Or will have the overall test true if either test is true
If intX < 3 Or intX > 8 Then
Not reverses the test result to which it is applied
If Not intX = 2 Then
The line continuation character can be useful when constructing complex logical tests

9. Logical Expressions & Evaluation (cont.)
Compound tests with the “And” operator
If x >= 3 And x <= 8 Then
First Second
x Test Test Overall
2 False True False
3 True True True
10 True False False

10. Logical Expressions & Evaluation (cont.)
Compound tests with the “Or” operator
If x >= 3 Or x <= 8 Then
First Second
x Test Test Overall
2 False True True
3 True True True
10 True False True
If x <= 3 Or x >= 8 Then
5 False False False

11. Logical Expressions & Evaluation (cont.)
Precedence of Logical Operators
Not
And Or
Left to right in case of ties
x >= 0 And x < 3 Or x > 8 And x <= 10
__________ Or __________
__________

12. Logical Expressions & Evaluation (cont.)
Parentheses can suppress natural evaluation order
Natural evaluation sequence
(x >= 0 And x < 3) Or x = 10
Changing the precedence of evaluation
x >= 0 And (x < 3 Or x = 10)
Use parentheses to visually group tests
(x >= 0) Or (x <> y And x < 3) Or (x = 10)
Use parentheses to group tests for the Not operator
Not (x = y Or y < 5)

13. It is common to test a value against a range Value in a range
Range Testing
It is common to test a value against a range
Value in a range
intAge >= 18 And intAge <= 30
Value not in a range
intAge < 18 And intAge > 30
Not (intAge >= 18 And intAge <= 30)
Value in a complex range
intAge >= 18 And intAge <= 30 Or intAge > 60
Pay careful attention to the inequality operators and end point values when testing ranges
Avoid overlap of range segments

14. Range tests can be combined with other tests
Range Testing (cont.)
You cannot do this:
18 <= intAge <= 30
Test must be performed as two independent tests of intAge with an appropriate operator (And or Or)
Range tests can be combined with other tests
If intAge < 18 _ And sglTotHoursWorked > 20 _ And (Now.Month < 6 _ Or Now.Month > 8)

15. The Not Operator
Not reverses the Boolean status of whatever immediately follows it
Not True = False – Not False = True
Not 4 <= 5 = Not True = False
4 <= 5 And Not 5 > 4 = True And Not True = True And False = False
Parentheses can extend the scope of Not
Not (4 < 5 Or 5 < 4) = Not (True Or False) = Not (True) = False
Not (4 < 5 Or Not 5 < 4) = Not (True Or Not False) = Not (True Or True) = Not (True) = False

16. Logical tests must evaluate to True or False
Boolean Properties
Logical tests must evaluate to True or False
Many properties are Boolean in that they have values of True or False
The following tests are equivalent
If chkIsValid.Checked = True Then
If chkIsValid.Checked Then
The same applies to Boolean variables and constants
If blnIsValid = True Then
If blnIsValid Then

17. Exercises
Write out using the logical operators but English expressions for values the logic to determine if:
You passed all of your classes last semester
You passed any of your classes last semester
You are ready to graduate this semester
Evaluate the following:
True And False OR False Or Not False
(True And False) OR False Or Not False