Lecture 4. Functions

1Functions

Lecture 4

Computer Programming(MEng 1052)

Mechanical Engineering

Department

Prepared by: Addisu D. & Beza T. March, 2015

Introduction A function is defined as a group of instruction used to achieve a

specific task.

When we write larger programs we have to have a way of

breaking problems down into smaller sub-problems.

C++, like most programming languages, has facilities to include

separate subparts inside of a program. In C++ these subparts are

called functions.

Functions decrease the complexity of a program. A program

having multiple functions can be easily debugged than a program

that is large and runs without a single function. So a function

reduces the size of a program and increases the program

modularity.

Functions would make the program easier to understand, write,

test, and debug.

Every C++ program has at least one function, main().2

Advantages of functions

Increase program modularity

Reduction in the amount of work & development time

Program and function debugging is easier

Division of work is simplified

Reduction in the size of the program due to code

reusability

Functions can be accessed repeatedly without

redevelopment, which in turn promotes reuse of code

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Introduction

Types of function

Functions are of two types:

1. Predefined functions (Library/built-in function)

2. User defined function

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Predefined functions

C++ comes with libraries of predefined functions that you can

use in your programs.

Library functions are pre-written functions or built in

functions. Many activities in C++ use library functions.

Use of all library functions requires the inclusion of a header

file, which contains function prototypes and constant

definitions.

We have already seen the use of #include in all of

the examples so far.

The name inside the angular brackets < > is the name of a file

known as a header file. A header file for a library provides the

compiler with certain basic information about the library, and

an include directive delivers this information to the compiler.

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This enables the linker to find object code for the functions in

the library so that it can correctly link the library to your

program.

For example, the library iostream contains the definitions of

cin and cout, and the header file for the iostream library is

called iostream.

The other most commonly used library in C++ is the math

library which contains the definition of many mathematical

functions, and the header file for this library is cmath.

If your program uses a predefined function from some library,

then it must contain a directive that names the header file for

that library, such as the following:

#include

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Some Predefined Functions

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8Some Predefined Functions

9Some Predefined Functions

Notice that the absolute value functions abs and labs are in the

library with header file cstdlib, so any program that uses either of

these functions must contain the following directive:

#include

If you want to produce the absolute value of a number of type

int, you use abs; if you want to produce the absolute value of a

number of type long, you use labs; and if you want to produce

the absolute value of a number of type double, you use fabs.

abs and labs are in the library with header file cstdlib, while fabs

is in the library with header file cmath.

fabs is an abbreviation for floating-point absolute value.

Recall that numbers with a fraction after the decimal point, such

as numbers of type double, are often called floating-point

numbers.10


Another example of a predefined function is pow, which is

in the library with header file cmath. The function pow can

be used to do exponentiation in C++.

For example, if you want to set a variable result equal to xy,

you can use the following:

result = pow(x, y);

Hence, the following three lines of program code will

output the number 9.0 to the screen, because (3.0)2.0 is 9.0:

double result, x = 3.0, y = 2.0;

result = pow(x, y);

cout

Function Call

A function call is an expression consisting of the function name

followed by arguments enclosed in parentheses. If there is more

than one argument, the arguments are separated by commas.

A function call is an expression that can be used like any other

expression of the type specified for the value returned by the

function.

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Example

PROGRAMMER-DEFINED FUNCTIONS

Declaring and Defining Functions

Using functions in your program requires that you first declare

the function and then define the function.

Function declaration tells the compiler the name, return type,

and parameters of the function.

The definition tells the compiler how the program works.

No function can be called from any other function that hasnt

first been declared.

The declaration of a function is called its prototype.

Many of the built-in functions you use will have their function

prototypes already written in the files you include in yourprogram by using#include.

For functions you write yourself, you must include the

prototype.15

Function Declaration

Function declarations are normally placed before the main part

of your program.

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Example: A Function Definition

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The description of the function is given in two parts that are

called the function declaration and the function definition.

The function declaration (also known as the function

prototype) describes how the function is called.

C++ requires that either the complete function definition or the

function declaration appears in the code before the function is

called.

In the above sample function program given the function

declaration is

double total_cost(int number_par, double price_par);

The function declaration tells you everything you need to know

in order to write a call to the function.

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It tells you the name of the function, in this case total_cost.

It tells you how many arguments the function needs and

what type the arguments should be; in this case, the

function total_cost takes two arguments, the first one of

type int and the second one of type double.

The identifiers number_par and price_par are called formal

parameters.

The names of the formal parameters can be any valid

identifiers.

The first word in a function declaration specifies the type

of the value returned by the function.

Thus, for the function total_cost, the type of the value

returned is double.

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Function call

In the above sample program the function call is in the

following line:

bill = total_cost(number, price);

The function call is the expression on the right-hand side of

the equal sign.

The function name is total_cost, and there are two arguments:

The first argument is of type int, the second argument is of

type double.

The compiler does not care whether theres a comment along

with the function declaration, but you should always include a

comment that explains what value is returned by the function.

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A function definition describes how the function computes

the value it returns.

If you think of a function as a small program within your

program, then the function definition is like the code for this

small program. In fact, the syntax for the definition of a

function is very much like the syntax for the main part of a

program.

A function definition consists of a function header followed

by a function body.

The function header is written the same way as the function

declaration, except that the header does not have a semicolon

at the end. This makes the header a bit repetitious, but thats

OK.

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Function definition

The function body consists of declarations and executable

statements enclosed within a pair of braces.

Thus, the function body is just like the body of the main part of

a program.

When the function is called, the argument values are plugged

in for the formal parameters and then the statements in the

body are executed.

The value returned by the function is determined when the

function executes a return statement.

A return statement consists of the keyword return followed by

an expression.

return (subtotal + subtotal*TAX_RATE);

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Function definition

When this return statement is executed, the value of the

following expression is returned as the value of the function

call:

(subtotal + subtotal*TAX_RATE)

The parentheses are not needed. The program will run

exactly the same if the return statement is written as

follows:

return subtotal + subtotal*TAX_RATE;

However, on larger expressions, the parentheses make the

return statement easier to read.

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Function definition

Function Prototype and Function Definition

The prototype for the FindArea ( ) function is on line 4.

Compare the prototype with the definition of the function on

line 25. Note that the name, the return type, and the parameter

types are the same. If they were different, a compiler error

would have been generated.

In fact, the only required difference is that the function

prototype ends with a semicolon and has no body.

Also note that the parameter names in the prototype are lengthand width, but the parameter names in the definition are l and w.

The names in the prototype are not used; they are there as

information to the programmer. When they are included, they

should match the implementation when possible. This is a

matter of good programming style and reduces confusion, but it is not required, as you see here.

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The body of the function is always enclosed in braces, even when

it consists of only one statement, as in this case.

The definition of a function consists of the function header and its

body. The header is exactly like the function prototype, except

that the parameters must be named, and there is no terminating

semicolon.

The body of the function is a set of statements enclosed in braces.

A function prototype tells the compiler the return type, name, and

parameter list.

Functions are not required to have parameters, and if they do, the

prototype is not required to list their names, only their types.

A prototype always ends with a semicolon (;).

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A function definition must agree in return type and parameter

list with its prototype. It must provide names for all the

parameters, and the body of the function definition must be

surrounded by braces.

All statements within the body of the function must be

terminated with semicolons, but the function itself is not ended

with a semicolon; it ends with a closing brace.

If the function returns a value, it should end with a return

statement, although return statements can legally appear

anywhere in the body of the function. Every function has a return type

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Analysis

On lines 7 and 8, two float variables are declared, one to hold the temperature in Fahrenheit and one to hold the temperature in

degrees Celsius.

The user is prompted to enter a Fahrenheit temperature on line 10, and that value is passed to the function Convert ( ).

Execution jumps to the first line of the function Convert ( ) on line 20, where a local variable, also named TempCel, is declared.

Note that this local variable is not the same as the variable TempCel on line 8. This variable exists only within the function

Convert ( ). The value passed as a parameter, TempFer, is also just a local copy of the variable passed in by main ( ).

This function could have named the parameter FerTemp and the local variable CelTemp, and the program would work equally

well. You can enter these names again and recompile the program to see this work.

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The local function variable TempCel is assigned the value that

results from subtracting 32 from the parameter TempFer, multiplying by 5, and then dividing by 9..

This value is then returned as the return value of the function,

and on line 12 it is assigned to the variable TempCel in the main ( ) function. The value is printed on line 14.

As an exercise, try entering the program again with other

variable names as illustrated below:

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Analysis

Local Variables

Variables that are declared within the body of a function

definition are said to be local to that function or to have that

function as their scope.

Variables that are declared within the main part of the program

are said to be local to the main part of the program or to

have the main part of the program as their scope.

When we say that a variable is a local variable without any

mention of a function and without any mention of the main

part of the program, we mean that the variable is local to some

function definition.

If a variable is local to a function, then you can have another

variable with the same name that is declared in the main part of

the program or in another function definition, and these will be

two different variables, even though they have the same name.

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Example-Function with local variables

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Global Variables

Variables defined outside of any function have global scope and thus are available from any function in the program,

including main ( ).

Local variables with the same name as global variables do not change the global variables. However, a local variable

with the same name as a global variable hides the global

variable.

If a function has a variable with the same name as a global variable, the name refers to the local variable-not the global--

when used within the function.

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Example-Global variable

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More Sample programs and

Exercises

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The following program finds the larger of two accepted numbers from

the user.

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Exercise

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Exercise

Write a function declaration and a function definition for a

function that takes three arguments, all of type int, and that

returns the sum of its three arguments.

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1. Write a function declaration and a function definition for a

function that takes one argument of type int and one argument

of type double, and that returns a value of type double that is

the average of the two arguments.

2. Write a function declaration and a function definition for a

function that takes one argument of type double. The function

returns the character value 'P' if its argument is positive and

returns 'N' if its argument is zero or negative.

3. Write an int function cube ( ) that returns the cube of its

single int formal parameter.

4. Write a float function triangle ( ) that computes the area of a

triangle using its two formal parameters h and w, where h is

the height and w is the length of the bases of the triangle.

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Exercises

void FUNCTIONS

The functions discussed so far always return a single value.

In C++, a function must either return a single value or return

no values at all.

A function that returns no value is called a void function.

Definitions of void Functions

In C++ a void function is defined in a way similar to the way

that functions that return a value are defined.

For example, the following is a void function that outputs the

result of a calculation that converts a temperature expressed in

Fahrenheit degrees to a temperature expressed in Celsius

degrees.

The actual calculation would be done elsewhere in the program.

This void function implements only the subtask for outputting

the results of the calculation.51

there are only two differences between a function definition

for a void function and the function definitions of functions

that return a value.

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void FUNCTIONS

One difference is that we use the keyword void where we

would normally specify the type of the value to be returned.

This tells the compiler that this function will not return any

value. The name void is used as a way of saying no value is

returned by this function.

The second difference is that the return statement does not

contain an expression for a value to be returned, because,

after all, there is no value returned.

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void FUNCTIONS

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void FUNCTIONS

Recursion Functions

Sometimes, the best way to solve a problem is by solving a smaller version of the exact same problem first.

Recursion is a technique that solves a problem by solving a smaller version of the same type.

When you turn this into a program, you end up with functions that call themselves (recursion functions).

There are many problems whose solution can be defined recursively

Example: n factorial

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1 if n = 0n!= (recursive solution)

(n-1)!*n if n > 0

1 if n = 0n!= (closed form solution)

1*2*3**(n-1)*n if n > 0

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Recursion Functions

1. Write a function called isPrime ( ) that accepts a

number and determine whether the number is prime

or not.

2. Write a function called isEven ( ) that uses the

remainder operator(%) to determine whether an

integer is even or not.

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Quiz_Sec. C (5%)

Quiz_Sec. D (5%)

1. Write a function called SumofEven ( ) that

calculates and displays the sum of even numbers

from 2 to 100.

2. Write a function called isOdd ( ) that uses the

remainder operator(%) to determine whether an

integer is odd or not.

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End of Lecture 4

Next Lecture

Lecture 5: Arrays and Strings

66

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Lecture 4. Functions