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CS 106, Winter 2009Class 11, Section 4

Slides by: Dr. Cynthia A. Brown, cbrown@cs.pdx.eduInstructor section 4: Dr. Herbert G. Mayer, herb@cs.pdx.edu

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Individual Vote Impact Chart (from NY Times)

White number = registered votersBlack number = electoral votes

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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

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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

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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.

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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 sumDim sum as Double

sum = num1 + num2lstResult.Items.Add(“The sum of “ & num1 &“ and “ & _

num2 &“ is “ & sum &“.”End Sub

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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

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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 sumDim sum as Double

sum = num1 + num2Return(sum)End Sub

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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

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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 DoubleaVar = 2bVar = 3DisplaySum(aVar, bVar – 2)cVar = ComputeSum(aVar, bVar) ‘silly example since we could just add

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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)

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A Picture of the Control Flowfor a Procedure Call

Some program code

Procedure call

More program code

Procedure code

Set parameter values

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Let’s look at some examples.

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BREAK

10 minutes