• a function is a block of

8/14/2019 a Function is a Block Of

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FUNCTIONS


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Functions

A function is a block of instructions that isexecuted when it is called from some otherpoint of the program.

Advantages

Reusability

Data Abstraction

Modularity


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C++ comes with libraries of predefinedfunctions.

How do we use predefined (or library)functions?

Everything must be declared before it is used.The statement #include brings thedeclarations of library functions into yourprogram, so you can use the library functions.

Predefined FunctionsLibraries


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Predefined FunctionsLibraries

#include // include declarations of math

// library functions

#include // include definitions of// stdio.h objects

int main()

{

printf( %d\n, sqrt(3.0)) // call math library function

} // sqrt with argument 3.0, send

// the returned value to cout

4


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Function call

A function call is an expression consisting of a functionname followed by arguments enclosed in parentheses.Multiple arguments are separated by commas.

Syntax:FunctionName(Arg_List)

where Arg_List is a comma separated list of arguments.

Examples:

side = sqrt(area);

printf(2.5 to the power 3.0 is %d, pow(2.5, 3.0));


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A Function call

//Computes the size of a dog house that can be purchased//given the users budget.#include #include int main( ){

const double COST_PER_SQ_FT = 10.50;double budget, area, length_side;

printf( "Enter the amount budgeted for your dog house $);scanf( %f, &budget);

area = budget/COST_PER_SQ_FT;length_side = sqrt(area); // the function call


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A Function call

printf( "For a price of $ %d \n" , budget);printf( "I can build you a luxurious square dog house\n);printf("that is %d " ,length_side);printf(" feet on each side.\n);

return 0;}


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Programmer Defined Functionsfunction prototypes

A function prototype tells you all the information you need tocall the function. A prototype of a function (or its definition)must appear in your code prior to any call to the function.

Syntax: Dontforget the semicolon

Type_of_returned_value Function_Name(Parameter_list); Place prototype comment here.

Parameter_list is a comma separated list of parameterdefinitions:

type_1 param_1, type_2 param_2, . type_N param_N

Example:

double total_weight(int number, double weight_of_one);

// Returns total weight ofnumberof items that

// each weigh weight_of_one


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Programmer Defined Functions

#include

double total_cost(int number_par, double price_par);

//Computes the total cost, including 5% sales tax,

//on number_par items at a cost of price_par each.

int main( )

{

double price, bill;

int number;

printf( "Enter the number of items purchased: );

scanf( %d, &number);

printf( "Enter the price per item $);

scanf(%d, &price);

bill = total_cost(number, price); The function call

printf(%d items at$ %d each.\n, number, price);

printf( "Final bill, including tax, is $%d\n, bill);

return 0;


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3.3 Programmer Defined FunctionsDisplay 3.3 A function Definition (Slide 2 of 2)

double total_cost(int number_par, double price_par) The function

{ headingheading

const double TAX_RATE = 0.05; //5% sales tax The function

double subtotal; The function definition

bodysubtotal = price_par * number_par;

return (subtotal + subtotal*TAX_RATE);

}


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Programmer Defined FunctionsCall-by-value Parameters

Consider the function call:

bill = total_cost(number, price);

The values of the arguments number and price are plugged in

for the formal parameters . This process is (A precise definitionof what plugged means will be presented later. For now, wewill use this simile.)

A function of the kind discussed in this chapter does not sendany output to the screen, but does send a kind of output back

to the program. The function returns a return-statement insteadof cout-statement for output.


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Programmer Defined FunctionsAlternate form for Function Prototypes

The parameter names are not required:

double total_cost(int number, double price);

It is permissible to write:

double total_cost(int, double );

Nevertheless, code should be readable to programmers as wellas understandable by the compiler, so check for readability andchose to use parameter names when it increases readability.


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PITFALLArguments in the wrong order

When a function is called, C++ substitutes the first argumentgiven in the call for the first parameter in the definition, thesecond argument for the second parameter, and so on.

There is no check for reasonableness. The only things checked

are: i) that there is agreement of argument type with parametertype and ii) that the number of arguments agrees with the

number of parameters.

If you do not put correct arguments in call in the correct order,C++ will happily assign the wrong arguments to the rightparameters.


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Programmer Defined FunctionsSummary of Syntax for a Function that Returns a Value.

Function Prototype:

Type_Returned Function_Name(Parameter_List);

Prototype Comment function header

Function DefinitionsType_Returned Function_Name(Parameter_List)

{Declaration_1Declaration_2

. . . Must include one or

Declaration_Last; more return statements.

body Executable_1;Executable_2;. . .

Executable_Last

}


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Functions with no return value


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General format

void function_name ([parameters])

{

}


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Declaring Void Functions

Similar to functions returning a value

Return type specified as "void"

Example: Function declaration/prototype:

void showResults( double fDegrees,double cDegrees);

Return-type is "void"

Nothing is returned


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Declaring Void Functions

Function definition:void showResults(double fDegrees,double cDegrees)

{statements;

statements;

} Notice: no return statement


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Calling Void Functions

Same as calling predefined void functions

From some other function, like main(): showResults(degreesF, degreesC);

showResults(32.5, 0.3); Notice no assignment, since no

value returned

Actual arguments (degreesF, degreesC)

Passed to function Function is called to "do its job" with the

data passed in


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Returning more than one value

#include

void prevnext (int x, int &prev, int &next)

{ prev = x-1;

next = x + 1;}

int main {int x = 100,y,z;

prevnext(x,y,z);

printf(previous %d next %d,y, z);

return 0;}


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Default Values in arguments

#include

int divide(int a, int b=2) {

int r; r=a/b; return(r);}int main(){

printf(%d\n,divide(2));

printf((%d\n, divide(20,4));return 0; }


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Overloading Function Names

C++ distinguishes two functions by examining thefunction name and the argument list fornumberandtype of arguments.

The function that is chosen is the function with thesame number of parameters as the number ofarguments and and that matches the types of theparameter list sufficiently well.

This means you do not have to generate names forfunctions that have very much the same task, buthave different types.


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//Illustrates overloading the function name ave.#include

double ave(double n1, double n2);//Returns the average of the two numbers n1 and n2.

double ave(double n1, double n2, double n3);//Returns the average of the three numbers n1, n2, and n3.

int main( ){

using namespace std;

printf( "The average of 2.0, 2.5, and 3.0 is %f \n, ave(2.0, 2.5,3.0));

printf( "The average of 4.5 and 5.5 is %f\n, ave(4.5, 5.5));

return 0;

}


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double ave(double n1, double n2) Both these functions have the{ same name, but have parameter

return ((n1 + n2)/2.0); lists that are have different} numbers of parameters.

double ave(double n1, double n2, double n3){

return ((n1 + n2 + n3)/3.0);}


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Overloading Function NamesAutomatic Type Conversion

We pointed out that when overloading function names, the C++compiler compares the number and sequence of types of thearguments to the number and sequence of types for candidatefunctions.

In choosing which of several candidates for use when overloadingfunction names, the compiler will choose an exactmatch if one ifavailable.

An integral type will bepromotedto a larger integral type ifnecessary to find a match. An integral type will be promoted to a

floating point type if necessary to get a match.


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printf( "Round pizza: Diameter = %d inches\n, diameter);printf( "Price = $ %d" ,price_round);printf( " Per square inch = $%f \n, price_round);printf( "Rectangular pizza: length = %d inches\n" , length );

printf( "Rectangular pizza: Width = %d inches\n" , widht );printf( "Price = $ %d, price_rectangular);printf( " Per square inch = $ %d \n" ,unitprice_rectangular);

if (unit_price_round < unitprice_rectangular)printf( "The round one is the better buy.\n);

elseprintf( "The rectangular one is the better buy.\n);

printf( "Buon Appetito!\n);

return 0;}


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Function Template

Is a single, complete function that serves asa model for a family of functions

Generic, flexible

Data type of the arguments of the functionfollow the data type of the actual value thatis passed unto it.


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Function Template

#include template

void showabs(T number){if (number


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Reference

Slides by David B Teague, WesternCarolina University,

• a function is a block of

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