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

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 1^st actual parameter corresponds to the 1^st formal

The 2^nd actual parameter corresponds to the 2^nd 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.


	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.


	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


	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.


	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


	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.


	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.


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 1^st actual parameter corresponds to the 1^st formal
		The 2^nd actual parameter corresponds to the 2^nd 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


	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


	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.


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


	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.


	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.