Notes

Outline

Parameters Procedures allow tasks to be encapsulated for use at another time. Parameters provide a technique for providing inputs to procedures and receiving outputs from them.

Body Mass Computation The body mass index is used to determine if a person is overweight: BMI = 4.89weight/height2 where the weight is in pounds, the height is in feet Converting it to a procedure is straightforward … so volunteer to write it, letting your friend build the GUI

The GUI Built By A Friend

Incompatibility of Names A problem with names … Procedure Assumes     Quantity      GUI Assumes heightIN          height        BMIheight weightLBS          weight        BMIweight bodyMass          bmi        BMIndex Though in this case better communication might have saved this case, the need to associate different names is fundamental – it is essential in making procedures reusable.

Adding Parameters The body mass problems can be fixed without dieting Introduce parameters ... Formal parameters are part of the formal definition Formal parameters are “declared” in the parenthesized list following the procedure name To call the procedure, give the actual parameters

Body Mass Index Program

Formal Parameters The formal parameters are “declared” within the parentheses … the syntax is just like DIM statements As with other variables, any names can be chosen Each variable must be given a type: Integer, String, Double Formal parameter variables are “known” only within the procedure, i.e. they are local to a procedure They never conflict with variables in the calling context Different procedures could use the same formal parameter names without confusion or conflict The technical term for this is “scope”: the scope of the formal parameter is local to the procedure.

Input vs Output Many programming languages (including VB6) provide several different ways of passing values back and forth between the actual and the formal parameters. The default in Visual Basic, and the only kind we’ll use in CSE/IMT 100, is pass by reference. Pass by reference allows information to flow in both directions. Formal parameters can be used as inputs or outputs or both Any changes made to a formal parameter will make a change to the corresponding actual parameter.

Actual Parameters The actual parameters must fulfill these requirements known as the formal/actual correspondence rules There must be the same number of actual parameters in the call, as there are formal parameters in the proc declaration The order of the parameters matters -- The 1st actual parameter corresponds to the 1st formal The 2nd actual parameter corresponds to the 2nd formal The types of the actuals must match the types of the formals Any formal used as a procedure output must have a variable as an actual

Review -- Control Flow for Procedures When we call a procedure, Visual Basic jumps to the code for the procedure.  It runs this code, then returns back to where the procedure was called, and continues on. x = 5 Call squid() x = x+1 Private sub squid()      Print “hi there” End Sub

Information Flow for Procedures When we call a procedure, the formal parameter temporarily becomes another name for the actual parameter. In other words, in Visual Basic the formal parameter temporarily becomes an alias for the actual parameter, for as long as the procedure is executing. Aliases in real life: “The Sundance Kid” was an alias for Harry Longabaugh Two names; one person.

Input parameter example Remember … the formal parameter becomes an alias for the actual parameter

Output parameter example

Both Input and Output

Expressions as Actual Parameters

Expressions as Parameters -- Caution

Mini-Exercise #1 What does the program print?

Mini-Exercise #1 -- Answer What does the program print?

Mini-Exercise #2 What does the program print?

Mini-Exercise #2 -- Answer What does the program print?

Mini-Exercise #3 What does the program print?

Mini-Exercise #3 -- Answer What does the program print?

Mini-Exercise #4 What does the program print?

Mini-Exercise #4 -- Answer What does the program print?

Surgeon General’s Warning! The “Fluency” book uses a different way of explaining parameter passing (as assignment statements into the formal parameters). For straightforward programs, this always gives the same results as pass by reference. However, for some messy cases it gives different results. Ugh!  We’re never going to give you such programs in CSE/IMT 100 (in homework or quizzes). If you go on to further study of programming, however, you will probably run into this. The way described in the lecture is how it’s actually done.

Summary Discussion of parameters for procedures Parameters link the variables in the calling context with the variables in the procedure context There is a 1-to-1 relationship between the formal parameters of the procedure definition and the actual parameters of the actual procedure call The default way of passing parameters in Visual Basic is “pass by reference”.  The formal parameter becomes an alias for the actual parameter.