CS 106, Winter 2009 Class 11, Section 4

1. CS 106, Winter 2009Class 11, Section 4 Slides by: Dr. Cynthia A. Brown, cbrown@cs.pdx.edu Instructor section 4: Dr. Herbert G. Mayer, herb@cs.pdx.edu 1

3. Individual Vote Impact Chart (from NY Times) White number = registered voters Black number = electoral votes

4. 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

5. General Procedures • The main idea: encapsulate some code in its own procedure (Sub or Function) • 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

6. Two Aspects of Procedures • The Definition: 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.

7. Sub Procedures • A subprocedure definition has a name, possibleformalparameters, and a body of code. • Example procedure definition: Private SubDisplaySum(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

8. Function Procedures • Function procedures have one extra element: they return a value • We’ve seen examples of functions built into VB: for example, CDbl or FormatCurrency • A function can have parameters • A function returns a result that has a particular data type: Double for CDbl, String for FormatCurrency

9. Function Procedures • Afunction procedure definition has a name, possibleformalparameters, a type, and a body of code. • Example procedure definition: Private FunctionComputeSum(ByVal num1 as Double, ByVal num2 as Double) as Double ‘Add two numbers and return their sum Dim sum as Double sum = num1 + num2 Return(sum) End Sub

10. Note on Names • Our convention is to name procedures starting with a capital letter • I like to use a verb in the name, since a procedure carries out an action or computes a value

11. Procedure Call • A procedure is called from elsewhere in the program, for example from within some event procedure • It uses the procedure name and actual parameters (the book calls actual parameters arguments) • Here’s what it might look like: DimaVar, bVar, cVarAs Double aVar = 2 bVar = 3 DisplaySum(aVar, bVar – 2) cVar = ComputeSum(aVar, bVar) ‘silly example since we could just add

12. What Happens in a Procedure Call • The expressions for the actual parameter values are evaluated, and the formal 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 (sub procedure), or to the line of the call with the returned value (function)

13. A Picture of the Control Flowfor a Procedure Call Some program code Set parameter values Procedure call Procedure code More program code

14. Let’s look at some examples.

15. BREAK 10 minutes