Understanding Conditionals and Procedures in Programming: A Comprehensive Overview

1. CS 106, Winter 2009Class 10, Section 4 Slides by: Dr. Cynthia A. Brown, cbrown@cs.pdx.edu Instructor section 4: Dr. Herbert G. Mayer, herb@cs.pdx.edu 1

2. Conditionals • We’ve learned about several types of conditionals: • If/Else statements • Ifs with no Else • If/ElseIf statements • Select Case blocks • Nested conditionals • We learned how to write Boolean expressions and analyze them using truth tables

3. Case Block example Assume position is a variable with a value between 1 and some number from 2 to 20 SelectCasepostion Case 1 txtOutcome.Text = “Win” ‘several lines of code could go here Case 2 txtOutcome.Text = “Place” Case 3 txtOutcome.Text = “Show” Case 4,5 txtOutcome.Text = “Close but no cigar” Case Else ‘action of last resort txtOutcome.Text = “Out of the money” End Select

4. Flowchart for Select Case Evaluate Selector In Value List 1? yes Action 1 no In Value List 2? yes Action 2 ~ no ~ ~ ~ In Value List n? yes Action n no Perform action of last resort

5. Conditional Controls • We’ve learned about radio buttons and check boxes. • These controls let us specify options to our program. • Only one radio button in a group can be clicked. • Any number of check boxes in a group can be clicked.

6. Assignment 4 • The complete specification of Assignment 4 is on the website. • Assignment 4 builds on Assignment 3, using conditionals. • As before, you will • design the interface, • write the use cases and tests, • list the objects and their events, • name and describe the global variables you will use, • create flowcharts for the nontrivial events, and • write and test the program. • Submit everything in your zipped project folder

7. Example for Assignment 4 • We’ll look at an example that performs similar operations as Assignment 4. • The project for this example is on the website in the IceCream2 zip file.

8. Ice Cream Store Enhancement • We can still enter number of scoops • We can choose the flavor (one flavor) • We can choose toppings (as many as we want) • At the end we get an itemized bill in a list box, with our total and the tip amount.

9. BREAK 10 minutes

10. General Procedures • The main idea: encapsulate some code in its own procedure (Sub or Function) • We’ve seen this in event procedures already • Why create our own procedures? • If we are repeatedly doing the same task, we can write the code in one place and then call the procedure repeatedly • If a procedure is too long and complex, we can break it into understandable parts

11. Problem-Solving Strategy • Break a large, complex problem into smaller, more manageable parts • Master a particular task that comes up repeatedly so you don’t have to think about how it works each time it occurs

12. SubProcedures • A subProcedure has a name, possible parameters, and a body of code. • Example procedure definition: Sub DisplaySum( ByVal num1 as Double, ByVal num2 as Double ) ‘Display two numbers and their sum Dim sum as Double sum = num1 + num2 lstResult.Items.Add( “The sum of “ & num1 &“ and “ & _ num2 & “ is “ & sum & “.” End Sub

13. Procedure Name • Let’s consider the parts of the header: Sub DisplaySum( ByVal num1 as Double, ByVal num2 as Double) • DisplaySum is the name of the procedure. We’ll start procedure names with capital letters and variable names with lower-case letters. • The name is used when use (call) the procedure elsewhere in the program

14. Parameters Sub DisplaySum(ByVal num1 as Double, ByVal num2 as Double) • The parameters are num1 and num2 • A ByVal parameter is very similar to a local variable • Its type is declared in the header (these are type Double) • Its initial value is set in the procedure call • Changes to ByVal parameters have no effect on variables elsewhere in the program; still, it is best as a programming practice not set them to new values inside the procedure

15. Procedure Body Sub DisplaySum( ByVal num1 as Double, ByVal num2 as Double ) ‘Display two numbers and their sum Dim sum as Double sum = num1 + num2 lstResult.Items.Add( “The sum of “ & num1 & “ and “ & _ num2 &“ is “ & sum & “.” End Sub • Note the local variable sum • The result of calling this procedure is to make some text appear in list box lstResult

16. Procedure Call • A procedure is called from elsewhere in the program, for example from within some event procedure • Here’s what it might look like: Dim aVar, bVar As Double aVar = 2 bVar = 3 DisplaySum( 4, 5 ) ‘parameter values are literal numbers DisplaySum( aVar, bVar – 2 ) ‘parameter values are expressions

17. What Happens in a Procedure Call • The expressions for the parameter values are evaluated, and the parameters are set to those values • The Dim statements for the procedure are used to create any local variables • The code for the procedure is executed • Control returns to the line after the procedure call

18. A Picture of the Control Flow Some program code Set parameter values Procedure call Procedure code More program code

19. Two Aspects of Procedures • The Defintion: a separate piece of code where the procedure is defined, as we do with event procedures • The Procedure Call: A piece of code that invokes a procedure • Event procedures are invoked when the event happens. We’ll see examples of code with procedures in our next class.