Control Statements Selection (Conditional Logic)

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Buck Dickerson
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1 Control Statements Selection (Conditional Logic) INTRODUCTION In the last few weeks, you were introduced to the concept of flow of control: the sequence of statements that the computer executes. In procedurally written code, the computer usually executes instructions in the order that they appear. However, this is not always the case. There are different type for controlling the flow of your program. There are three ways to control the flow of your program: Sequence structure o Execute one statement after the other in which they are written Selection structure o The program has to test an expression, if that expression is valid (correct/yes/true), then the program execute a group of statements, otherwise, the program ignore such group of statements. Repetition structure o The program has to test an expression, if that expression is valid (correct/yes/true), then the program execute a group of statements, otherwise, the program ignore such group of statements. Repeat this process until the expression is not valid anymore. We handled the first type (sequence) in the last few weeks; this document handles the selection type. SELECTION BACKGROUND People make decisions on a daily basis. What should I have for lunch? What should I do this weekend? Every time you make a decision, you base it on some criterion. For example, you might decide what to have for lunch based on your mood at the time, or whether you are on some kind of diet. After making this decision, you act on it. Thus decision-making is a two-step process first deciding what to do based on a criterion, and secondly taking an action.

2 Decision-making by a computer is based on the same two-step process too. In VBA there are multiple ways to make a decision (most important one is if statement (also known as the selection statement)). When processing a selection statement, the computer first evaluates some expression (criterion or condition). If it is met, the specified action is performed; otherwise, these actions are ignored. EXPRESSION An expression in a combination of one or more constants, variables, operators, and functions that are interprets (according to its particular rules of precedence and of association) and computes to produce a single value. Expression s result is usually numerical, string, or logical (it could be more advanced than that, but that is outside the boundaries of CS146). Examples: Expression Expression Value Expression Value s Data Type Numerical X=4 Y=10 X * Y 40 Numerical 5 >2 True Logical (Boolean) In the selection structure, we are more interested in the logical (Boolean) expression. EXPRESSION OPERATORS A logical expression is an expression that evaluates to true or false. They are mostly used with selection and Loops. When you create a logical expression, you use logical operators. The following table shows these operators.

3 Operator Description = Equal to <> Not Equal to < Less than > Greater than <= Less than or equal to >= Greater than or equal to These operators are used to compare between two values and they return a single Boolean value: TRUE FALSE. For example: Test = 5 > 3 o In this case, the expression tests if 5 is greater than 3 and it returns TRUE. Then variable called test will become a Boolean variable with TRUE value. Test = 5 >= 13 o In this case, the expression tests if 5 is greater or equal to 13 and it returns FALSE. Then variable called test will become a Boolean variable with FALSE value. Test: = 5 <> 5

4 o In this case, the expression tests if 5 does not equal to 5 and it returns FALSE. Then variable called test will become a Boolean variable with FALSE value. The following table gives more example. Expression/Condition This is TRUE when x < 5 x is less than 5 x <= 5 x is less than or equal to 5 x > 5 x is greater than 5 x >= 5 x is greater than or equal to 5 x = 5 x is equal to 5 x <> 5 x does not equal 5 x > 5 And x < 10 x is greater than 5 AND x is less than 10 x = 2 Or x >10 x is equal to 2 OR x is greater than 10 Range("A1") = "John" Range("A1") <> "John" Cell A1 contains text "John" Cell A1 does not contain text "John" SELECTION STATEMENTS The following tables shows the different types of selection statements used in VBA. More details about each type will be explained later. If Then If If If If [Expression is true] Then If [Expression is true] Then If [Expression 1 is true] Then If [Expression 2 is true] Then If score > 50 Then Debug.Print "Passed"

5 and If ( must come after If's) If without Endif (One line only) Select Case If [Expression 1 is true] Then If [Expression 2 is true] Then If [Expression is true] Then [do something] Select Case (Expression) Case End Select If score <= 50 Then If score > 50 Then Debug.Print "Passed" If score > 30 Then Debug.Print "Yikes" If value <= 0 Then value = 0 Dim score As Integer result As String score = Range("A1").Value Select Case score Case Is >= 80 result = "very good" Case Is >= 70 result = "good" Case Is >= 60 result = "sufficient" Case result = "insufficient" End Select RESOURCES

Programming Logic and Design Sixth Edition

Objectives Programming Logic and Design Sixth Edition Chapter 4 Making Decisions In this chapter, you will learn about: Evaluating Boolean expressions to make comparisons The relational comparison operators