cmpt 128: introduction to computing science for engineering students

1 © Janice Regan, CMPT 128, Jan 2007

CMPT 128: Introduction to Computing Science for Engineering Students

Functions (2)

2© Janice Regan, CMPT 128, 2007-2012

3 steps: User defined functions Function Declaration/prototype

Information than the compiler needs to properly interpret calls to the function

Function Definition Actual implementation of the function

Function Call Using the function in the calling function


Declaring a user’s function A function prototype tells us (and the compiler)

the name and type of the function The identifier of the function The type of each parameter The identifier of each parameter (optional) The order of the parameters

The function prototype is usually placed after the #include statements before the int main () statement The function can then be called by any other functions


Writing a function prototype The function declaration or function prototype always

has a similar form. <return type> <function identifier> ( <parameter list >);

int myMean ( int a, int b, int c, int d);

double sumOfSquares ( double in1, double in2);

bool isReady ( int , double , int ); // In a prototype identifiers in the parameter list are optional


Understanding the prototype (1)

The function declaration or function prototype always has a similar form.

<return type> <function identifier> ( <parameter list >); The function identifier indicates the name of the function The return type indicates the type of the value returned by

the function When the function is called the statement calling the function can

be considered to be an expression The value of this expression (the call) in the calling function has a

type, return type




<return type> <function identifier> ( <parameter list >);

A series of parameters (or arguments) follow the function identifier. These parameters indicate the data that is supplied to (and

sometimes under special conditions returned from) the function Each parameter is of a particular data type that is specified within

the declaration




<return type> <function identifier> ( <parameter list >); each parameter in the parameter list includes a type and an

optional identifier The function can have any number of parameters Each parameter of the function can have any type Different parameters can have different types Order of the parameters is important

Parameters in the declaration, definition, and call must always be in the same order


Prototypes: an example As an example consider

double sinc ( double x ); OR double sinc ( double );

The identifier (name) of the function is sinc The function returns a value of type double to the calling function

The value of the function call, the expression sinc(inputvalue) in the calling function is of type double

The parameter list contains a single parameter of type double An identfier for the parameter may be given (optional)


Function Definition Two main parts

Function Head Gives the compiler information to match the function with

the function declaration

Function Body Actual C++ statements implementing the function

Function definitions are placed after or before the main function (not inside the main function)


Placement of Function Definition

Int main (){ // body of main function}

double sinc(double x) // function head{

//function body}


Sample Function Definitiondouble sinc(double x)

{

if (fabs(x) < 0.0001)

{

return(1.0);

}

else

{

return( sin(x)/x);

}

}

Function head

Function body

Function call to library function sin


Function Head (examples) The first line of a function definition is the

function head

The head for the main function is int main ( )

The head for our example function is double sinc (double x)

There is no ; at the end of a function head

14

The function head indicates the type of the function double sinc( double x )

The function head indicates a function name or identifier double sinc( double x )

The function head indicates a list of parameters for the function, each parameter includes a type and an identifier

double sinc (double x)© Janice Regan, CMPT 128, 2007-2012

Function Head (content)


Function head: formal parameters The parameters in the function head are

referred to as formal parameters The function has 0 or more formal parameters

double sinc( double x ) Each of a function’s formal parameters have types

double sinc( double x ) A function may have formal parameters of more than

one type Each parameter must be given its own type Multiple parameters are separated by commas int sample( double x, int y, char z, double a)


The body of a function After the function head the body of the function is

enclosed in {}

double sinc(double x){ //variable declarations double y; //declaring variable //for return value //calculations to determine y return(y);}

Function body


Parts of the function body Local variables are declared Calculate the return value of the function Return the value to the calling function

return (returnValue); In a more complicated function you must assure all

possible paths to completion of a function end with a return statement

For a void function return statements are not required Return statements without arguments may be used to return

from the function to the calling program


Calling a function float fabs(float x); … limitValue = fabs(-9.7);

fabs(-9.7) is an expression known as a function call, or function invocation

The arguments in the brackets ( ) of a function call are called actual arguments or actual parameters.

An actual argument in a function call can be A literal (like -9.7) any variable whose value is of the correct type any expression whose value is of the correct type


Calling a function Consider a void function (returns no value) A function call to a void function does not have a value

(a void function does not return a value) A function call to a void function cannot be used in an

expression

Consider calling a function that is not void A function call to a non-void function has a value so it

can be used as part of a more complicated expressions bonus = fabs(mylimit) * myfactor; A function call to a function of any non void type is allowed

wherever it’s legal to use an expression


Returning a function’s value (1)

A non-void function will take the supplied values of the actual parameters, calculate a result, then return that result to the calling program

The function has a type. The type of the function is the type of the value returned by that function to the calling program

A function is invoked using a function call, the function call expression is given the value that is returned by the function


Sample Function Definitiondouble sinc(double x)

{

if (fabs(x) < 0.0001)

{

return(1.0);

}

else

{

return( sin(x)/x);

}

}

Function head

Function body

Function call to library function sin


Using our sample function // declare functions used

double sinc(double x);

int main (void){

// declare variables double a, b;

// obtain input data a, call function, print resultscout << “ enter value for which sinc is to be determined “);cin >> a;b= sinc(a);cout << "sinc( " << a << " ) = " << b;

}

Function prototype or function declaration

Function call


Returning a function’s value (2)

Our sample function determines the value of sin(x)/x when we supply a value for the parameter x

The function sinc(x) will take the value of x, calculate the value of sin(x)/x and return the resulting value to the calling program

To return the value of the function to the calling program following command is used

return(ValueToBeReturned); The type of variable or expression ValueToBeReturned

should match the type of the function returning the value A function of any type other than void must contain at least

one return statement. It may contain more. There must be 1 return statement ending each flow of control through the function


Calling Void Functions For a function declared as

void showResults(double x, double y); Can call the function in a calling function as follows

showResults(degreesF, degreesC); showResults(32.5, 0.3);

A function call to a void function does not have a value, because a void function does not return a value. A = showResults(32.5,0.3) * 3.0; //this is an invalid statement A function call to a void function has no value to be used in an

arithmetic expression ( or any other kind of expression) A function call to a void function cannot be assigned to a variable

since it has no value to place in that variable

cmpt 128: introduction to computing science for engineering students

Documents

function identifier

sincthe function

calling function janice

type janice regan

int main statementthe

value of type double

declaration janice regan

functionthe type