Chapter_1.__Functions_in_C++[1].pdf

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CHAPTER 1
Functions in C++
1

➢Basic concept and need of function
➢Declaring and defining a function
➢Function components (parameters and
arguments)
➢Calling /invoking function by value and
reference parameters
➢Functions Recursion
Course Content
2

 Functions are building blocks of the programs.
 A function in the broader sense may be considered as a
component of a big program. A large program may be
divided into suitable small segments and each segment may
be treated and defined as a function. These segments can
be separately compiled, tested and verified.
 Function is a self-contained routines within a larger
program that carry out specific tasks.
 One entity in programming which have a set of– One entity
in programming which have a set of command to carry out
specific task
 Sub-routine
 We can call these function every time we need (in
programming sequences) – reusable.
Function
3

 All C++ programs must contain the function main( ).
 The execution of the program starts from the
function main( ).
Function
4

⇒ More manageable
⇒ Code are easier to maintain
⇒ Reduction in program size.
⇒ Code duplication is avoided.
⇒ Code reusability is provided.
⇒ Functions can be called repetitively.
⇒ Increase program readability.
⇒ Divide a complex problem into many simpler problems.
⇒ Reduce chances of error.
⇒ Makes modifying a program becomes easier.
⇒ Makes unit testing possible.
Why we need function?
5

 When you need to repeat the same process over and
over in a program.
 The function can be called many times but appears in–
The function can be called many times but appears in
the code once.
When we need function?
6

1. Built in functions/pre-defined function
 are part of compiler package.
 Part of standard library made available by compiler.
 Can be used in any program by including respective
header file.
 User needs to include pre-defined header file
(i.e. math.h, time.h)
 functions such as
swap (), sqrt (), exit(), pow(), strlen() etc.
Types of function
7

#include<iostream>
#include<cstring>
using namespace std;
int main()
{
int len; char str[80];
cout<<"Enter the string: ";
cin.getline(str, 80);
len = strlen(str);
cout<<"nLength = "<<len;
cout<<endl;
return 0;
}
Types of function Cont’d… Example
8

2. User defined function
 The functions which are declared and defined by a programmer are
called user defined functions or programmer defined functions.
 Created as per requirement of the program.
Types of function Cont’d…
9

#include<iostream>
int stringLength(char []);
int main()
{
int len;
char str[80];
cout<<"Enter the string: ";
cin.getline(str, 80);
len = stringLength(str);
cout<<"nLength = "<<len;
cout<<endl;
return 0;
}
Types of function Cont’d… Example
10
int stringLength(char x[])
{
int i=0, count=0;
while(x[i] )
{
count++;
i++;
}
return count;
}

❑ user-defined functions can be categorized as:
▪ Functions with no parameters and no return value
• void add(void);
▪ Functions with no parameters and with return value
• int add(void);
▪ Functions with parameters and with no return value
• void add(int,int);
▪ Functions with parameters and with a return value
• int add(int,int);
Types of User defined function
11

❑Three main elements in function :
1. Prototype of Function / Declaration
2. Define the function
3. Called the function
Function - Elements
12

❑ A function prototype is a declaration of a function that
tells the program about the type of value returned by the
function, name of function, number and type of arguments.
Syntax: Return_type function_name (parameter list/argument);
Example: int add(int,int);
void add(void);
int add(float,int);
❑ The names of parameters may or may not be given in the
prototype, however the names are necessary in the head of
function in the function definition.
❑ No function can be declared within the body of another
function.
Function Declaration/Prototype
13

❑ Function definition consists of a function header that
identifies the function, followed by the function body
containing the executable code for that function.
❑ Syntax:
return_type function_name(data_type variable1, data_type variable2,..)
{ //function body
.............
Statements
.............
return( variable);
}
Function Definition
14

❑ When a function defined, space is allocated for that
function in the memory.
❑ The number of and the order of arguments in the
function header must be same as that given in
function declaration statement.
Function Definition Cont’d…
15

❑ The function call statement invokes the function.
❑ When a function is invoked the compiler jumps to the
called function to execute the statements that are a
part of that function.
❑ Once the called function is executed, the program
control passes back to the calling function.
Syntax: function_name(variable1, variable2, ...)
Function Call
16

❑ Names (not the types) of variables in function
declaration, function call and function definition may
vary.
❑ Arguments may be passed in the form of expressions
to the called function.
❑ In such a case, arguments are first evaluated and
converted to the type of formal parameter and then
the body of the function gets executed.
❑ If the return type of the function is not void, then
the value returned by the called function may be
assigned to some variable as given below.
variable_name = function_name(variable1, variable2, ...);
Function Call Cont’d…
18

❑ The terms calling function and called function are derived
from the telephone communication. The one who rings the
number is the calling person and one who receives the call is
the one called. The same terminology applies to functions.
❑ Thus the function which calls another function is the calling
function and the function which is called is the called
function.
❑ The different data variables that a function accepts for
processing are called parameters of the function.
❑ The values that represent the parameters and are passed on
to the function when it is called, are the arguments of the
function.
❑ Some functions return a numeric value to the calling function.
These are called return type functions. If a function does
not return a numeric value we call it void function.
19

➢ Given the next program, which function is the calling function and
which is the called function?
#include<iostream.h>
#include<conio.h>
void nextMsg();
int main()
{
cout<< “Hello!n”;
nextMsg();
cout<<“Worldn”;
return 0;
}
void nextMsg() {
cout<< “GoodBye!n”;
return; }
20

➢ The return statement is used to terminate the
execution of a function and return control to the
calling function.
➢ When the return statement is encountered, the
program execution resumes in the calling function at
the point immediately following the function call.
➢ A return statement may or may not return a value to
the calling function.
Syntax: return <expression> ;
➢ The value of expression, if present, is returned to the
calling
➢ function. However, in case expression is omitted, the
return value of the function is undefined.
Return Statement
21

➢ The return statement is used to terminate the
execution of a function and return control to the
calling function.
➢ When the return statement is encountered, the
program execution resumes in the calling function at
the point immediately following the function call.
➢ A return statement may or may not return a value to
the calling function.
Syntax: return <expression> ;
➢ The value of expression, if present, is returned to the
calling
➢ function. However, in case expression is omitted, the
return value of the function is undefined.
Return Statement
22

➢ There are two ways in which arguments(parameters)
can be passed to the called function.
➢ Call by value in which values of the variables are passed
by the calling function to the called function.
➢ Call by reference in which address of the variables are
passed by the calling function to the called function.
Passing Parameters To The Function
23

➢ The called function creates new variables (formal
arguments) to store the value of the arguments passed
to it. Therefore, the called function uses a separate
copy of the actual arguments (formal arguments) to
perform its intended task.
➢ If the called function is supposed to modify the value
of the parameters passed to it, then the change will be
reflected only in the called function. In the calling
function no change will be made to the value of the
variables.
Call by value
24

 For instance:
#.....
void Foo(int num)
{
Num = 0;
cout<< “num = ” << num << “ n”;
}
int main(void)
{
int x = 10;
Foo(x);
cout<< “x = ”<<x<< “n”;
getch();
return 0;
}
Call by value Cont’d…
25

▪ When the function is called and x passed to it, num
receives a copy of the value of x. As a result, although num
is set to 0 by the function, this does not affect x.
▪ The program produces the following output:
Num = 0
x = 10
▪ Passing arguments in this way, where the function creates
copies of the arguments passed to it is called passing by
value.
26

27
/* program to illustrate the concept of call by value */
#include<iostream.h>
#include<conio.h>
void add(int,int);
int main()
{
int a,b;
cout<<“enter values of a and b”<<endl;
cin>>a>>b;
add(a,b);
return 0;
}
void add(int x,int y)
{
int c;
c=x+y;
cout<<“addition is”<<c;
}

▪ In call by value method, the only way to return the
modified value of the argument to the caller is
explicitly using the return statement.
▪ The better option when a function can modify the
value of the argument is to pass arguments using call
by reference technique.
▪ In call by reference, we declare the function
parameters as references rather than normal
variables.
▪ Any changes made by the function to the arguments it
received are visible by the calling program.
▪ To indicate that an argument is passed using call by
reference, an ampersand sign (&) is placed after the
type in the parameter list.
Call by Reference
28

▪ In call by value method, the only way to return the
modified value of the argument to the caller is
explicitly using the return statement.
▪ The better option when a function can modify the
value of the argument is to pass arguments using call
by reference technique.
▪ In call by reference, we declare the function
parameters as references rather than normal
variables.
▪ Any changes made by the function to the arguments it
received are visible by the calling program.
▪ To indicate that an argument is passed using call by
reference, an ampersand sign (&) is placed after the
type in the parameter list.
Call by Reference
29

Call by Reference Cont’d…
30

void Foo(int & num)
{
num = 0;
cout<< “num = ” << num << “ n”;
}
int main(void)
{
int x = 10;
Foo(x);
cout<< “x = ”<<x<< “n”;
getch();
return 0;
}
31

➢ Any change made on num will be effected to x. Thus,
the program produces the following output:
num = 0
x = 0
32

➢ C++ allows to call a function without specifying all its
arguments.
➢ In such cases, the function assigns a default value to
a parameter which does not have a matching
arguments in the function call.
➢ Default values are specified when the function is
declared. The complier knows from the prototype how
many arguments a function uses for calling.
Function With Default Arguments
33

Example : float result(int marks1, int marks2, int marks3=75);
▪ A subsequent function call
average = result(60,70);
passes the value 60 to marks1, 70 to marks2 and lets
the function use default value of 75 for marks3.
▪ The function call
average = result(60,70,80);
passes the value 80 to marks3.
Function With Default Arguments Cont’d…
34

➢ Example
void fun(int a=10, int b=20); //Declaration
OUTPUT
int main()
{
fun(3,5);//Function Calling
fun(3);//Function Calling
fun();//Function Calling
return 0;
}
void fun(int a, int b)//Definition
{
cout<<"a:"<<a<<endl;
cout<<"b:"<<b<<endl;
}
Function With Default Arguments Cont’d…
35
a: 3
b: 5
a: 3
b: 20
a: 10
b: 20

Local Variable
➢ A variable defined inside a function (defined inside
function body between braces) is called a local
variable or automatic variable.
➢ Its scope is only limited to the function where it is
defined. In simple terms, local variable exists and can
be accessed only inside a function.
➢ The life of a local variable ends (It is destroyed)
when the function exits.
➢ Each block in a program defines a local scope. Thus
the body of a function represents a local scope. The
parameters of a function have the same scope as the
function body.
Global versus Local variables
36

➢ Variables defined within a local scope are visible to
that scope only. Hence, a variable need only be unique
within its own scope.
➢ Local scopes may be nested, in which case the inner
scope overrides the outer scopes.
Eg: int xyz;//xyz is global
void Foo(int xyz)//xyz is local to the body of Foo
{
if(xyz > 0)
{
Double xyz;//xyz is local to this block
}
}
Global versus Local variables Cont’d…
37

Global Variable
➢ If a variable is defined outside all functions, then it
is called a global variable.
➢ The scope of a global variable is the whole program.
This means, It can be used and changed at any part of
the program after its declaration. Likewise, its life
ends only when the program ends.
38

➢ Everything defined at the program scope level
(outside functions) is said to have a global scope,
meaning that the entire program knows each variable
and has the capability to change any of them.
Eg. int year = 1994;//global variable
int max(int,int);//gloabal funcrion
int main(void)
{
//…
}
39

Example:-
#include<iostream>
using namespace std; //Declaring a global variable
int a = 5;
int main()
{
//Printing the value of a
cout<<"The value of a is "<<a<<"n";
//Modifying the value of a
a--;
//Printing the value of a
cout<<"The value of a is "<<a<<"n";
return 0;
}
40

➢ Because a local scope overrides the global scope,
having a local variable with the same name as a global
variable makes the latter inaccessible to the local
scope.
➢ Eg
int num1;
void fun1(int num1)
{
//…
}
➢ The global num1 is inaccessible inside fun1(), because
it is overridden by the local num1 parameter.
Scope Operator
41

➢ This problem is overcome using the scope operator
“::” which takes a global entity as argument.
int num1 = 2;
void fun1(int num1)
{
//… num1=33;
cout<<num1; // the out put will be 33
cout<<::num1; //the out put will be 2 which is the
global
if(::num1 != 0)//refers to global num1
//…
}
Scope Operator
42

➢ The terms automatic and static describe what happens to
local variables when a function returns to the calling
procedure.
➢ By default, all local variables are automatic, meaning that
they are erased when their function ends. You can
designate a variable as automatic by prefixing its
definition with the term auto.
Eg. The two statements after main()‘s opening brace
declared automatic local variables:
main()
{
int i;
auto float x;
…
}
Automatic versus static variables
43

➢ The opposite of an automatic is a static variable. All
global variables are static and, as mentioned, all
static variables retain their values. Therefore, if a
local variable is static, it too retains its value when its
function ends-in case this function is called a second
time.
➢ To declare a variable as static, place the static
keyword in front of the variable when you define it.
44

➢ Static variables can be declared and initialized
within the function, but the initialization will be
executed only once during the first call.
➢ If static variables are not declared explicitly, they
will be declared to 0 automatically.
➢ Eg.
void my_fun()
{
static int num;
static int count = 2;
count=count*5;
num=num+4;
}
45

➢ In the above example:
✓ During the first call of the function my_fun(), the static
variable count will be initialized to 2 and will be multiplied by 5
at line three to have the value 10. During the second call of the
same function count will have 10 and will be multiplied by 5.
✓ During the first call of the function my_fun(), the static
variable num will be initialized to 0 (as it is not explicitly
initialized) and 4 will be added to it at line four to have the value
4. During the second call of the same function num will have 4
and 4 will be add to it again.
✓ N.B. if local variables are static, their values remain in case the
function is called again.
46

➢ Functions with the same name are called overloaded
functions. C++ requires that each overloaded
functions differ in its argument list. Overloaded
functions enable you to have similar functions that
work on different types of data.
➢ N.B: if two or more functions differ only in their
return types, C++ can‘t overload them. Two or more
functions that differ only in their return types
must have different names and can‘t be overloaded.
Overloading Function
47

➢ Example
#include<iostream>
void fun();
void fun(int);
void fun(int,int);
int main()
{
fun();
fun(2);
fun(5,6);
return 0;
}
void fun()
{
cout<<"n Function with 0
parameter:"<<endl;
}
void fun(int a);
{
parameter:"<<endl;
}
void fun(int a,int b)
{
parameter:"<<endl;
}
Overloading Function
48

➢ A function which calls itself is said to be recursive.
Recursion is a general programming technique
applicable to problems which can be defined in terms
of themselves.
➢ Take the factorial problem, for instance which is
defined as:
• factorial of 0 is 1
• factorial of a positive number n is n time the
factorial of n- 1.
Recursion Function
49

➢ The second line clearly indicates that factorial is
defined in terms of itself and hence can be expressed
as a recursive function.
int Factorial(unsigned int n )
{
return n = = 0 ? 1 : n * factrial(n-1);
}
Recursion Function
50

➢ For n set to 4, the following figure shows the
recursive call:
➢ A recursive function must have at least one termination condition
which can be satisfied. Otherwise, the function will call itself
indefinitely until the runtime stack overflows.
➢ The three necessary components in a recursive method are:
o A test to stop or continue the recursion
o An end case that terminates the recursion
o A recursive call(s) that continues the recursion
Recursion Function
51

Example
#include<iostream>
void sum(int);
int main()
{
int n;
cout<<"Enter Numebr:";
cin>>n
int result=sum(n);
cout<<"The sum of
Numbers 0-"<<n<<"is:";
cout<<result;
return 0;
}
int sum(int n)
{
if(n==0)
return 0;
else
return n+sum(n-1);
}
Recursion Function
52

➢ Both iteration and recursion are based on control
structure. Iteration uses a repetition structure (such
as for, while, do…while) and recursive uses a selection
structure (if, if else or switch).
➢ Iteration and recursion can occur infinitely: An infinite
loop occurs with iteration if the loop-continuation test
never becomes false; infinite recursion occurs if the
recursion step does not reduce the problem in a manner
that converges on the base case
Recursion versus iteration
53

➢ Recursion has disadvantage as well. It repeatedly
invokes the mechanism, and consequently the overhead
of method calls. This can be costly in both processor
time and memory space.
➢ Each recursive call creates another copy of the
method (actually, only the function‘s variables); this
consumes considerable memory.
➢ N.B: Use recursion if:
✓ A recursive solution is natural and easy to understand
✓ A recursive solution doesn‘t result in excessive duplicate
computation.
✓ the equivalent iterative solution is too complex and
✓ of course, when you are asked to use one in the exam!!
Recursion versus iteration
54

➢ C++ provides inline functions to reduce the function
call overhead.
➢ An inline function is a function that is expanded in
line when it is called. When the inline function is called
whole code of the inline function gets inserted or
substituted at the point of the inline function call.
➢ This substitution is performed by the C++ compiler at
compile time. An inline function may increase
efficiency if it is small.
Inline Function
55

Syntax:
inline return-type function-name(parameters)
{
// function code
}
Inline Function
56

➢ The compiler may not perform inlining in such
circumstances as:
o If a function contains a loop. (for, while and do-
while)
o If a function contains static variables.
o If a function is recursive.
o If a function return type is other than void, and the
return statement doesn’t exist in a function body.
o If a function contains a switch or goto statement.
Inline Function
57

➢ You can pass array as an argument to a function just
like you pass variables as arguments.
➢ When arrays are passed as a parameter to a function,
specify the name of the array without any brackets in
the function call.
➢ For a one-dimensional array, the function definition
should include the brackets, but it is not necessary to
specify the exact length of the array.
Passing Arrays to a Function
58

➢ For example, the following illustrates passing a single
one-dimensional array to a function:
void SomeFunction(int arr[]) // Use empty brackets for 1-d array
{
arr[0]=10;
}
int main()
{
int intArray[100];
SomeFuntion(intArray); // Leave brackets off
cout << arr[0] << endl; // Will output 10 since array is
reference
}
Passing Arrays to a Function Cont’d…
59

➢ If we were to pass in the length of the array, the
function prototype would look like:
void SomeFunction(int arr[], int length);
and would be invoked via:
int intArray[100];
SomeFunction(intArray, 100);
Passing Arrays to a Function Cont’d…
60

// C++ Program to display single array element
#include <iostream>
void arr(int a )
{
cout << "Numbers " << a <<endl;
}
int main()
{
int myArr[] = {1, 2, 3};
arr(myArr[1]); // pass array element myArr[1] only.
return 0;
}
Example: Passing Single array to a function
61

#include <iostream>
int SumValues (int [], int ); //function prototype
void main( )
{
int Array[10]={0,1,2,3,4,5,6,7,8,9};
int total_sum;
total_sum = SumValues (Array, 10); //function call
cout <<”Total sum is “ <<total_sum;
}
int SumValues (int values[], int num_of_values) //function header
{
int sum = 0;
for( int i=0; i < num_of_values; i++)
sum+=values[i];
return sum;
}
Example: Passing 1D arrays to a function
62

// C++ Program to display the elements of 2D array by passing it to a function
#include <iostream>
void display(int a[][2])
{
cout << "The Elements are: " << endl;
for (int i = 0; i < 2; ++i)
{
for (int j = 0; j < 2; ++j)
{
cout << "arr[" << i << "][" << j << "]: " << a[i][j] << endl;
}
}
}
int main()
{
int arr[2][2] = { {1, 2}, {3, 4} };
display(arr);
return 0;
}
Example: Passing 2D arrays to a function
63

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