ism 3230-ch7

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User-Defined Functions II

ISM 3230 Intro to Comp Systems and Software Development

Dr. Caryn Conley

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Chapter Goals

Chapter 7 Objectives

Learn how to construct and use void functions in a program

Discover the difference between value and reference parameters

Explore reference parameters and value-returning functions

Learn about the scope of an identifier

Examine the difference between local and global identifiers

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Void Functions

What are void fun ct ions?

Void functions and value-returning functions have similarstructures

Both have a heading and a body

In our class, we will place ALL user-defined functions (void and

value-returning) after the function main When we do this, we must place the function prototype

before the function main

Remember, the function prototype is like a declaration of thefunction so that we can compile our program

A void function does not have a return type - DIFFERENT Therefore, our return statement will not have a value:

return;

Formal parameters are optional

A call to a void function is a stand-alone statement - DIFFERENT

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Void Functions (cont)

Void fun ct ion syntax No parameters

Function definition syntax:

void is a reserved word

Function call syntax:

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Void fun ct ion syntax With parameters

Function definition syntax:

Formal parameter list syntax:

Function call syntax:

Actual parameter list syntax:

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Types of parameters

We use parameters to allow our functions to communicate witheach other

There are two types of parameters to enable different types ofcommunication: value (what we learned last week) and

reference (new concept for today) Value parameter: a formal parameter that receives a copy

of the content of corresponding actual parameter

Reference parameter: a formal parameter that receivesthe location (memory address) of the corresponding actualparameter

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Value Parameters

What are value parameters?

If a formal parameter is a value parameter

The value of the corresponding actual parameter is copiedinto it

The value parameter has its own copy of the data

During program execution The value parameter manipulates the data stored in its own

memory space

This means the value stored in the actual parameter CANNOTbe changed by the function

This is how we talked about parameters last week

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Reference Variables as Parameters

What are reference variables?

If a formal parameter is a reference parameter

It receives the memory address of the corresponding actualparameter

A reference parameter stores the address of the corresponding

actual parameter During program execution to manipulate data

The address stored in the reference parameter directs it tothe memory space of the corresponding actual parameter

This means the value stored in the actual parameter CAN be

changed by the function!!

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Reference Variables as Parameters (cont)

What are reference variables? (con t)

Reference parameters can:

Pass one or more values from a function

Change the value of the actual parameter

Reference parameters are useful in three situations:

Returning more than one value

Changing the actual parameter

When passing the address would save memory space andtime

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Reference vs Value Parameters

Syntax

How do we know if it is a value or a reference parameter?

Look at the function heading!

Value parameters:

In the list of formal parameters, list the data type and

the parameter name Example: int absoluteValue(int nNumber)

Reference parameters :

In the list of formal parameters, list the data type

immediately followed by & and the parameter name

Example: void absoluteValue(int&nNumber)

Tiny symbol. HUGE

implications.

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Parameter Types and Memory Allocation

Memo ry impl icat ions

When a function is called

Memory for its formal parameters and variables declared inthe body of the function (called local variables) is allocated inthe function data area

In the case of a value parameter The value of the actual parameter is copied into the memory

cell of its corresponding formal parameter

In the case of a reference parameter

The address of the actual parameter passes to the formal

parameter Content of formal parameter is an address

During execution, changes made by the formal parameterpermanently change the value of the actual parameter

NOTE: Stream variables (e.g., ifstream) should be passed by

reference to a function

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#include

using namespace std;

int absoluteValue (int nNumber); //Function prototype

int main()

{

int nAbs, nAbsolute;

cout > nAbs;nAbsolute = absoluteValue(nAbs); //Call absoluteValue

cout

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User-Defined Functions (cont)

Pseudocode for main funct ion

Declare constants (if necessary)

Declare variables

Number

Absolute

Prompt user Enter an integer:

Get Number from user

Set Absolute = Call absoluteValue passing Number

Print The absolute value of the number entered: Absolute

Call to a value-returning function

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Pseudocode for absoluteValue fun ct ion

Value parameters:

Number

Reference parameters:

None

Return:

Number


Declare variables (if necessary)

If Number < 0

Calculate Number = -Number

Return Number

NEW!

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#include


void absoluteValue (int&nNumber); //Function prototype

int main()

{

int nAbs = 0;

cout > nAbs;absoluteValue(nAbs); //Call absoluteValue

cout

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A fun ct ion w ith reference parameters in a f low c hart

Number

Start

Prompt

user forumber

Getnumber

from user

Callabsolute

Value

EndPrint

number

main

NEW!

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A fun ct ion w ith reference parameters in a f low c hart (cont)

number

Start

End

absoluteValue

ifnumber

< 0

Calculatenumber

yes

no

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Pseudocode for main funct ion


Declare variables

Number

Prompt user Enter an integer:

Get Number from user

Call absoluteValue passing Number

Print The absolute value of the number entered: Number

Call to a void function

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Pseudocode for absoluteValue fun ct ion

Value parameters:

None

Reference parameters:

Number

Return:

None


Declare variables (if necessary)

If Number < 0

Calculate Number = -Number

Return

NEW!

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Scope of an Identifier

What is scope?

The scope of an identifier refers to where in the program anidentifier is accessible

Local identifier: identifiers declared within a function (or block)

Global identifier: identifiers declared outside of every function

definition C++ does not allow nested functions

The definition of one function cannot be included in the bodyof another function

This will cause a compiler error!

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Scope of an Identifier (cont)

#include


const double RATE = 10.50;

int nGuests = 0;

void calculateCharge(int nNites, double& dCharge);

int main()

{

int nNights = 0;

double dAmountDue = 0.0;

return 0;

} //End main method

void calculateCharge(int nNites, double& dCharge)

{

double dTemp = 0.0;

return;

} //End main method

Global identifiers

Local identifiers Canonly see in main

Local identifiers Can only see incalculateCharge

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Imp l icat ions of p revious sl ide

Global identifiers:

Can use these identifiers in either main or calculateCharge:

RATE

nGuests

Local identifiers:

Can use these identifiers ONLY in main:

nNights

dAmount

Can use these identifiers ONLY in calculateCharge:

dTemp

dCharge

nNites

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Some C++ rules

Global identifiers (such as variables) are accessible by a functionif:

The function name is different from the identifier

All parameters of the function have names different than the

name of the identifier, and All local identifiers (such as local variables) have names

different than the name of the identifier

The scope of a function name is the same as the scope of aglobal variable

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Som e ISM 3230 rules

The only global identifiers you may use in your program include

Constants (always declared before any function)

User-defined functions (function prototype always declaredbefore any function definition)

Declaring local variables All local variables declared at the beginning of the function

body (i.e. at the top of the function)

All identifiers in your program, including functionparameters, should have unique names (i.e. do not usenNum1

inmain

and anothernNum1

inabsoluteValue

) Using the return statement

All functions must include one and only one return

statement

Value returning: return;

Void functions: return;

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Class Exercise

No coding tod ay

Refer to the handout, and identify the following information ineach of the examples

Function prototype, function heading, function body andfunction definitions

Function call statements, formal parameters, and actualparameters

Value parameters and reference parameters

Local variables and global variables

Mark the order in which the statements will execute

Identify the final values of each variable at the end ofprogram execution

Identify the output of each program

For the last example only, create a flow chart and

pseudocode for each function

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Summary

Summary o f Chapter 7

Void function: does not have a data type

A return statement without any value can be used in a voidfunction to exit it early

The heading starts with the word void

To call the function, you use the function name together withthe actual parameters in a stand-alone statement

Two types of formal parameters:

Value parameters

Reference parameters

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Summary (cont)

Summary of Chapter 7 (con t)

A value parameter receives a copy of its corresponding actualparameter

A reference parameter receives the memory address of itscorresponding actual parameter

If a formal parameter needs to change the value of an actualparameter, you must declare this formal parameter as areference parameter in the function heading

Variables declared within a function are called local variables

Variables declared outside of every function definition are globalvariables

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