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Lecture 6:

Functions

So far, you used only the function main; the programming instructions are packed into it. This is good only for short programs.

For large programs, it is not practical to put the entire programming

instructions into one function

You must learn how to break the problem into manageable pieces (functions).

What is meant by a function?

EX:When an automobile is manufactured, it is put together from previously

manufactured parts.

Some parts are made by the company itself; others, by different companies.

Functions are like building blocks.They let you divide complicated programs into

manageable pieces.

Functions have other advantages:

• While working on one function, you can focus on just that part of the program

and construct it, debug it, and perfect it.

• Different people can work on different functions in the same time.

• If a function is needed in more than one place in a program or in different

 programs, you can write it once and use it many times.

• Using functions greatly enhances the program’s readability because it reduces

the complexity of the function main.

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1) Predefined Functions (Cont.)

Examples:

b) The square root function, sqrt(x), calculates the nonnegative square root of x

for x >= 0.0

• sqrt(2.25)

= 1.5

• sqrt(16)

= 4.0

The function sqrt is of type double and has only one parameter.

1) Predefined Functions (Cont.)

Predefined functions are organized into separate libraries.

EX:

the header file iostream contains I/O functions, and

the header file cmath contains math functions

To use predefined functions in a program, you must include the header file

that contains the function’s specification via the include statement.

For example, to use the function pow, the program must include:

#include

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EX:

#include #include //to use pow and abs functions

using namespace std;

int main()

{

int x;

double u, v;

u = 4.2;

v = 3.0;

cout

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2. User-Defined Functions

are classified into two categories:

a. Value-returning functions:

Functions that have a return type.

These functions return a value of a specific data type using return statement

 b. Void functions:

Functions that do not have a return type.

These functions do not use a return statement to return a value.

a. Value-returning functions

Because the value returned by a value-returning function is unique ,قيمة و حد

you can use this value in one of three ways:

• Save the value for further calculation (in an assignment statement).

• Use the value in some calculation (as a parameter in a function call).

• Print the value (in an output statement).

=> That is, a value-returning function is used (called) in an expression.

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The syntax of a value-returning function is:

The syntax to call a value-returning function is:

A function’s formal parameter list can be empty but the parentheses are still needed.

OR: a call to a value-returning function with an empty formal parameter list is:

Notes:

In a function call, the number of actual parameters, must match the formal

 parameters in the given order.

That is, actual and formal parameters have a one-to-one correspondence.

A function call in a program causes the body of the called function to execute.

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return Statement

Once a value-returning function computes the value, the function returns this value via

the return statement.

In other words, it passes this value outside of the function via the return statement.

The return statement has the following syntax:

Notes:

- The data type of the value that expr computes must match the function type.

- When a return statement executes in a function, the function immediately terminates

and the control goes back to the caller.

- The function call statement is replaced by the value returned by the return statement.

- When a return statement executes in the function main, the program terminates.

- The return statement can appear anywhere in the function. Once it executes, all

subsequent statements are skipped. Return the value as soon as it is computed.

return is a reserved word.

EX1:

Write a function that determines the largest of two numbers.

 Note that this function requires that you use an additional variable max

(called a local declaration, in which max is a variable local to this function.

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EX1 (Cont.)

Other solutions without declaring any local variable:

Because the execution of a return statement in a function terminates the function, the

 preceding function larger can also be written (without else) as:

How to use a function in a program?You can call a function in different ways as follows:

double one = 13.00; //or receive it from the user

double two = 36.53;

double maxNum;

cout

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How to use a function in a program?You cancall a function in differentways as follows (Cont.)

Once a function is written, you can use it anywhere in the program.

EX:

The function compareThree calls the preceding  لس بق

function larger:

First, the expression larger(y, z) is evaluated; that is, the inner لد خلي call

executes first, which gives the larger of y and z i.e., either y or z.

 Next, the outerلخ رج

call determines the larger of x and either y or z.

Finally, the return statement returns the largest number.

EX2:

The header of abs function is:

int abs( int number)

The definition of abs function is:

int abs( int number)

{

if (number < 0)

number = - number;

return number;

}

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The order in which user-defined functions should appear in a program

Do you place the function larger before or after the function main?

Following the rule that you must declare an identifier before you can use it and

knowing that the function main uses the identifier larger, logically you must

 place larger before main.

In reality, C++ programmers customarily follow the following order:

1. Place function prototypes before the function main

Then

2. The function main

Then

3. The definition of all other user-defined functions.

Function Prototype

The general syntax of the function prototype of a value-returning function is:

EX:

double larger( double x, double y); OR  double larger( double, double);

ends with a semicolon

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The entire program

Program uses functions larger, compareThree, and main to determine the larger/largest of 2 or 3 numbers:

#include

using namespace std;

double larger( double x, double y); //function prototype

double compareThree( double x, double y, double z);

int main(){

double one, two;

cout two;

cout

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Questions Samples:

1. Consider the following function definition:

double func(double x, int y, string name)

{ //function body }

Which of the following is the correct function prototype?

i. double func();ii. double func(double, int, string);

iii. double func(double x, int y, string name)

iv. func(double x, int y, string name);///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

2. Consider the following statements:

double num1, num2, num3;

int int1, int2, int3, value;

num1 = 5.0; num2 = 6.0; num3 = 3.0;

int1 = 4; int2 = 7; int3 = 8;

and the function prototype:

double cube (double a, double b, double c);

Which of the following statements are valid?

a. value = cube (num1, 15.0, num3); b. cout