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FunctionsFunctionsChapter 7

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Hope Hope youyou can function! can function!

What is R2D2 doing here?

What is his function?

Who is Nibble?

Can he function?

IS he a function?

Who is this nut?

3Top-Down Structured DesignTop-Down Structured Designwith Void Functionswith Void Functions Recall two types of functions Value returning function

– computes a single value– returns value to calling code– uses return command

Void function (procedure)– called as a statement– executes some task

This chapter focuses on the void function

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When to Use FunctionsWhen to Use Functions

To help make the program more understandable

To modularize the tasks of the program– building blocks of the program

Write a module once– those lines of source code are called multiple

times in the program

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Need for FunctionsNeed for Functions

Functions perform a task.Functions perform a task.

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Writing Void FunctionsWriting Void Functions

Declare functions (their names and parameters)– list before main( )– called prototypes

void print_summary (float total);void calculate_rates ( );void find_matching_records (char id[]);

void main ( ) { . . .

void print_summary (float total);void calculate_rates ( );void find_matching_records (char id[]);

void main ( ) { . . .

Function Name Parameter list

Note, must includesemicolon ;

Note, must includesemicolon ;

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Writing Void FunctionsWriting Void Functions

Define functions below main( )– specify source code– include heading again

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

Note no semicolon with

definition

Note no semicolon with

definition

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Flow of ControlFlow of Control

First statement executed in any program is the first statement in the function main( )

When another function called– logical control passed to first statement in that

function’s body– program proceeds through sequence of statements

within the function When last statement of function executed

– control returns to where function was called– control given to next command after the call

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Flow of ControlFlow of Control

void main ( ) { . . . print_summary (rpt_total); revenue = rpt_total * .72675; . . . } void print_summary (int total) { . . . cout << . . . }

void main ( ) { . . . print_summary (rpt_total); revenue = rpt_total * .72675; . . . } void print_summary (int total) { . . . cout << . . . }

- first statement of main- function call, jumps to first statement of that function- proceeds through function- returns to next statement after call

- first statement of main- function call, jumps to first statement of that function- proceeds through function- returns to next statement after call

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Improving FunctionsImproving Functions

We need to communicate to the functions

We need to communicate to the functions

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Improving FunctionsImproving Functions

Sending values to the functionswith value parameters.

Sending values to the functionswith value parameters.

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Function ParametersFunction Parameters

Make functions more versatile Send to the function a value

– tells it how many times to do something– gives it a value to be used in some way (printed,

calculated, etc.)

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Function ParametersFunction Parameters Formal parameter

– declared in the function heading Actual parameter

– variable or expression listed in a call (invocation) of the function

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

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Function Call (Invocation)Function Call (Invocation)

Use the name of the function as if it is a statement

When the program reaches t hat statement, Control is then transferred to the function

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

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Function DeclarationsFunction Declarations

Why the prototypes? All identifiers (including function names)

must be declared before they are used

Compiler must know about the function

void calculate_rates ( );void find_matching_records (char id[]);

void main ( ) { . . . calculate_rates ( ); find_matching_records (emp_id);

void calculate_rates ( );void find_matching_records (char id[]);

void main ( ) { . . . calculate_rates ( ); find_matching_records (emp_id);

ParametersParameters

Function nameFunction nameFunction typeFunction type

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Function DeclarationsFunction Declarations

It is also legal to include the definition (body) of the function with the heading all before main( )

void print_summary (int total) { . . . cout << . . . } void main ( ) { . . . print_summary (rpt_total); revenue = rpt_total * .72675; . . . }

void print_summary (int total) { . . . cout << . . . } void main ( ) { . . . print_summary (rpt_total); revenue = rpt_total * .72675; . . . }

This takes care of informing thecompiler of what it needs

and defining the source code also

This takes care of informing thecompiler of what it needs

and defining the source code also

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Syntax of the Parameter ListSyntax of the Parameter List

In parentheses For each parameter

– specify type then name– separate type-name pairs with commas

void print_max_value (int value_1, int value_2, int value_3) { . . . }

void print_max_value (int value_1, int value_2, int value_3) { . . . }

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Local VariablesLocal Variables

A variable declared within a block It is not accessible outside of that block Memory allocated when statement reached

within the block Memory de-allocated when block finished

void print_max_value (int value_1, int value_2, int value_3) { int hold_value; hold_value = value_1; if (value_2 > hold_value)

hold_value = value_2; . . . }

void print_max_value (int value_1, int value_2, int value_3) { int hold_value; hold_value = value_1; if (value_2 > hold_value)

hold_value = value_2; . . . }

Block

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Local VariablesLocal Variables What if local variable (in the block) has same

name as a variable outside the block? Inside the block

– the local variable takes precedence

– the exterior variable not accessible

Outside the block– the local variable

is not accessible

void print_sum (int n1, int n2) { int sum; sum = n1 + n2; cout << sum; }

void main( ){ float sum, x, y; . . . print_sum (x, 34); . . .

void print_sum (int n1, int n2) { int sum; sum = n1 + n2; cout << sum; }

void main( ){ float sum, x, y; . . . print_sum (x, 34); . . .

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The Return StatementThe Return Statement A void function does not return a function

value Two ways to have the function return

control to calling routine– when the function finishes all statements– place the command return in the function

void print_odd_even (int x){

if (x % 2) // no remainder => even { cout << "even";

return; }cout << "odd";

}

void print_odd_even (int x){

if (x % 2) // no remainder => even { cout << "even";

return; }cout << "odd";

}

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Naming Void FunctionsNaming Void Functions

Remember … void functions are used as statements

The name should suggest what task it is performing– implies use of a verb

It helps the program self document

void print_the_sum ( );void max_value (int a, int b, int c);void calculate_deductions (float hours);void cube_root (float num);

void print_the_sum ( );void max_value (int a, int b, int c);void calculate_deductions (float hours);void cube_root (float num); Which are

good names?Which are

good names?

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Header FilesHeader Files

Recall our use of #include <xxxx.h> Header files contain ... named constants like

const int INT_MAX = 32767; new type definitions (besides int, char, etc.) function prototypes like

float pow( float, float );float sqrt( float );char toupper( char );

int islower( char );

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Header FilesHeader Files

Note the help window from Turbo C++ concerning header files

We most often use

string.hiostream.hfstream.hiomanip.hmath.h

We most often use

string.hiostream.hfstream.hiomanip.hmath.h

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Value ParametersValue Parameters

Defn => a formal parameter that receives a copy of the contents of the corresponding actual parameter

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

void main ( ) { . . . print_summary (rpt_total); . . . } void print_summary (int total) { . . . cout << . . . }

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Value ParameterValue Parameter

Acts much like an assignment of a value to a variable

The formal parameter is considered local to the function

The actual parameter may be an expression or a constant

void main ( ) { print_summary (0.5*rpt_total); . . . print_summary (200); } void print_summary (int total) { . . . cout << . . . }

void main ( ) { print_summary (0.5*rpt_total); . . . print_summary (200); } void print_summary (int total) { . . . cout << . . . }

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Value ParametersValue Parameters

Consider … When we change the contents of the value parameter in the function …– What (if anything) happens to the actual

parameter?

No, nothing happens.The actual parameter remains unchanged

No, nothing happens.The actual parameter remains unchanged

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Reference ParametersReference Parameters

What if we wanted the actual parameter to change?

C++ allows us to do this with reference parameters.

What is different in thisversion of the function?What is different in thisversion of the function?

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Reference ParametersReference Parameters

It would be helpful tobe able to have the functions

communicate back to thecalling module.

It would be helpful tobe able to have the functions

communicate back to thecalling module.

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Reference ParametersReference Parameters

Reference Parameters providethat capability

Reference Parameters providethat capability

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Reference ParametersReference Parameters

Use the ampersand & between the parameter type and the identifier

What actually happens is that this causes the address of the actual parameter to be sent to the formal parameter

Then anything that happens to the formal parameter is happening to the actual parameter

31Contrast Contrast Value & Reference ParametersValue & Reference Parameters

Receives copy of value Actual parameter can be

constant, variable, expression

Value travels one way only (in)

Exact match of types (formal & actual) not critical

Receives address of actual parameter

Actual parameter must be a variable

Value can be thought of as traveling both ways (in and out)

Formal & actual parameters must be of same type

ValueValue ReferenceReference

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Reference ParametersReference Parameters

5 10

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Recall our previous model for a function with value parameters(values go in only)

Now consider a new version for reference parametersValues go bothin & out

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Another Look at Value Another Look at Value ParametersParameters

CALLINGBLOCK

FUNCTION CALLED

“incoming”

value ofactual parameter

Sometimes called“Pass-by-value”

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Another Look at Reference Another Look at Reference ParametersParameters

CALLINGBLOCK FUNCTION

CALLED

“incoming”

original value ofactual parameter

“outgoing”

changed value ofactual parameter OR,Sometimes called

“Pass by reference”Sometimes called

“Pass by reference”

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Another Version ofAnother Version ofPass-by-referencePass-by-reference

CALLINGBLOCK FUNCTION

CALLED

actual parameter has no value yet when call occurs

“outgoing”

new value ofactual parameter

36Matching Actual & Formal Matching Actual & Formal ParametersParameters Must be same number of parameters in call

as in declaration/definition Parameters are paired off left to right as they

occur void print_max (int n1, int n2, int n3) { if ( … … cout << … }

int main(){ … print_max (x, 17, y + z); …

void print_max (int n1, int n2, int n3) { if ( … … cout << … }

int main(){ … print_max (x, 17, y + z); …

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Actual & Formal ParametersActual & Formal Parameters

Actual (in the call) value parameters may be variables, constants, or expressions

void print_max (int n1, int n2, int n3) { if ( … … cout << … }

int main(){ … print_max (x, 17, y + z); …

void print_max (int n1, int n2, int n3) { if ( … … cout << … }

int main(){ … print_max (x, 17, y + z); …

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Actual & Formal ParametersActual & Formal Parameters

Actual (in the call) reference parameters must be variables

void swap (int &n1, int &n2) { int hold = n1; n1 = n2; n2 = hold}

int main(){ int x = 5; y = 3; z = 12 swap (x, z); swap (z, y); …

void swap (int &n1, int &n2) { int hold = n1; n1 = n2; n2 = hold}

int main(){ int x = 5; y = 3; z = 12 swap (x, z); swap (z, y); …

What ends up stored where?

Or … who’s on first?

What ends up stored where?

Or … who’s on first?

x = y = z = x = y = z =

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Reusing FunctionsReusing Functions

A programmer can use reuse functions– from other programs you have written– utility functions from other sources

Such functions can be considered “black boxes”– you merely call the function– you need not know how it does what it does– this is called “encapsulation”

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Reusing FunctionsReusing Functions

To call a function a programmer must know the interface– the name of the function

– the names and types of the parameters

– any pre- or post-conditions Pre-conditions -- any requirements of the function

– max or min values of parameters

– a certain file must be open, etc. Post-conditions -- guaranteed results if pre-

conditions met

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The Assertion StatementThe Assertion Statement A command that declares a certain condition

– if the condition is not met, the program is halted with an error message

Helps programmer catch pre-conditions or other program module requirements

The assertions can also help comment or document the program

Note #include <assert.h>Note #include <assert.h>

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Preconditions and PostconditionsPreconditions and Postconditions

The precondition is an assertion describing everything that the function requires to be true at the moment the function is invoked.

The postcondition describes the state at the moment the function finishes executing.

The caller is responsible for ensuring the precondition, and the function code must ensure the postcondition.

FOR EXAMPLE . . .

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Function with PostconditionsFunction with Postconditionsvoid GetRating ( /* out */ char& Letter)// Precondition: None// Postcondition:User has been prompted to enter a char// && Letter == one of these input values: E,G,A, or P{ cout << “Enter employee rating.” << endl;

cout << “Use E, G, A, or P : ” ; do { cin >> Letter; if ( (Letter != ‘E’) && (Letter != ‘G’)

&& (Letter != ‘A’) && (Letter != ‘P’) )

cout << “Rating invalid. Enter again: ”; } while ( (Letter != ‘E’) && (Letter != ‘G’)

&& (Letter != ‘A’) && (Letter != ‘P’) );}

void GetRating ( /* out */ char& Letter)// Precondition: None// Postcondition:User has been prompted to enter a char// && Letter == one of these input values: E,G,A, or P{ cout << “Enter employee rating.” << endl;

cout << “Use E, G, A, or P : ” ; do { cin >> Letter; if ( (Letter != ‘E’) && (Letter != ‘G’)

&& (Letter != ‘A’) && (Letter != ‘P’) )

cout << “Rating invalid. Enter again: ”; } while ( (Letter != ‘E’) && (Letter != ‘G’)

&& (Letter != ‘A’) && (Letter != ‘P’) );}

States function’s guaranteeStates function’s guarantee

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Function with Preconditions Function with Preconditions and Postconditionsand Postconditions

void GetRoots( /* in */ float a, /* in */ float b, /* in */ float c, /* out */ float& Root1, /* out */ float& Root2 )

// Precondition: a, b, and c are assigned// && a != 0 && b*b - 4*a*c != 0// Postcondition: Root1 and Root2 are assigned && Root1 and Root2 // are roots of quadratic with coefficients a, b, c{

float temp;

temp = b * b - 4.0 * a * c;

Root1 = (-b + sqrt(temp) ) / ( 2.0 * a );

Root2 = (-b - sqrt(temp) ) / ( 2.0 * a );

return;}

void GetRoots( /* in */ float a, /* in */ float b, /* in */ float c, /* out */ float& Root1, /* out */ float& Root2 )

// Precondition: a, b, and c are assigned// && a != 0 && b*b - 4*a*c != 0// Postcondition: Root1 and Root2 are assigned && Root1 and Root2 // are roots of quadratic with coefficients a, b, c{

float temp;

temp = b * b - 4.0 * a * c;

Root1 = (-b + sqrt(temp) ) / ( 2.0 * a );

Root2 = (-b - sqrt(temp) ) / ( 2.0 * a );

return;} If preconditions are met,

postconditions areguaranteed

If preconditions are met,postconditions are

guaranteed

Why needed?Why needed?

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Documenting Data Flow DirectionDocumenting Data Flow Direction

Data Flow <=> flow of information ...– from the call of the function – into the function– then from the function (if it is a reference

parameter)– back to where it is called

Specify in a comment in the parameter list

void GetRoots( /* in */ float a, /* in */ float b, /* in */ float c, /* out */ float& Root1, /* out */ float& Root2 )

void GetRoots( /* in */ float a, /* in */ float b, /* in */ float c, /* out */ float& Root1, /* out */ float& Root2 )

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Testing and Debugging HintsTesting and Debugging Hints

Note documentation guidelines, Appendix E (pg A10-A13)

Make sure to have function prototypes near top of program

Use semicolon after prototype, but not after definition

Formal parameter list must specify type for each parameter

Use value parameters unless value must be returned to actual parameter

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Testing and Debugging HintsTesting and Debugging Hints

Reference parameters must have & with the data type

Match sequence of actual parameter list to formal parameter list

Reference parameters must have variables in the call

Remember to use ... – debugger– variable watch window