1. MODULE 3: FUNCTIONS
CO3: Decompose a
problem into
functions and
synthesize a
complete program.

2. TOPICS TO BE COVERED
Introduction to functions
Function prototype/declaration
Function definition
Accessing a function
Parameter passing
Recursion

3. INTRODUCTION
A function is a group of statements that together perform a task. Every C
program has at least one function, which is main()
You can divide up your code into separate functions. How you divide up your
code among different functions is up to you, but logically the division is
such that each function performs a specific task.
A function declaration tells the compiler about a function's name, return
type, and parameters. A function definition provides the actual body of the
function.
A function is a block of code that performs a specific task.
Suppose, you need to create a program to draw a circle and color it. You can
create two functions to solve this problem:
 Create draw circle function
 create a color function
Dividing a complex problem into smaller chunks makes our program easy to
understand and reuse.

4. INTRODUCTION
Types of function
There are two types of function in C programming:
Standard library functions
User-defined functions

5. INTRODUCTION
Standard library functions
The standard library functions are built-in functions in C
programming.
These functions are defined in header files. For example,
•printf()
•It is a standard library function to send formatted output to
the screen (display output on the screen).
•This function is defined in the stdio.h header file.
•Hence, to use the printf() function, we need to include the
stdio.h header file using #include <stdio.h>.
• sqrt()
•This function calculates the square root of a number. The
function is defined in the math.h header file.

6. INTRODUCTION
Library functions are those functions which are already defined
in C library such as strcat(), printf(), scanf() etc. You just need to
include appropriate header files to use these functions.
The prototype and data definitions of these functions are
present in their respective header files. To use these functions
we need to include the header file in our program. For example,
If you want to use the printf() function, the header file <stdio.h>
should be included.
If you try to use printf() without including the stdio.h header file,
you will get an error

7. INTRODUCTION
User-defined function
User-defined functions are those functions which are defined
by the user. These functions are made for code reusability and
for saving time and space.
You can also create functions as per your need. Such functions
created by the user are known as user-defined functions.
How user-defined function works?
#include <stdio.h>
void functionName()
{
... .. ... ... .. ...
}
int main()
{
... .. ... ... .. ...
functionName();
... .. ... ... .. ...
}

8. INTRODUCTION
The execution of a C program begins from the main() function.
When the compiler encounters functionName();, control of the
program jumps to void functionName()
And, the compiler starts executing the codes inside
functionName().
The control of the program jumps back to the main()
function once code inside the function definition is executed
.

9. INTRODUCTION
Advantages of user-defined function
The program will be easier to understand, maintain (add features
later on) ,change and debug.
Reusable codes that can be used in other programs
A large program can be divided into smaller modules. Hence, a large
project can be divided among many programmers.

10. FUNCTION DECLARATION
Function Declaration:
Syntax of function declaration:
return_type function_name (data_type arg1, data_type arg2);
int add(int x, int y); //function declaration
In Function Declaration, we specify the name of the function,
the number of input parameter, their data types & the return
type of the function.
Function declaration tells compiler about the list of arguments
the function is expecting with their data types & the return type
of the function.
In function declaration, specifying the names of the parameter
is optional, but specifying their data types is mandatory.

11. FUNCTION DEFINITION
Function Definition

12. FUNCTION DEFINITION
int add(int, int);
//function declaration
return_type function_name(parameters)
{
Function body
}
a function definition consists of two parts i.e. function header & function
body

13. FUNCTION DEFINITION
Function Header: function header is same as function
declaration without the semicolon. Function header contains
function name, parameter & return type.
 Return Type: Return type is the data type of the value which
will be returned by the function. The function may or may not
return a value. If it does then the data type of the retuning
value should be specified, otherwise the return type needs to
be void.
 Function Name: This is the name of the function using which
we can call the function when & where needed.
 Parameters: The parameters are the input values which will be
passed to the function. It tells about the data types of the
arguments, their order and the number of arguments that will
be passed to the function. The parameters are optional. You
can also have functions without parameters.

14. FUNCTION DEFINITION
Function Body: The function body is the set of
statement which performs a specific task. It defines
what the function does.
int add(int x, int y)
{
int sum = x+y;
return(sum);
}
It is recommended to declare a function before we
define & use it. In C, we can declare & define the
function at the same place.

15. FUNCTION ARGUMENTS
There are two types of parameter:
Actual Parameter: Those parameters which are passed to functions
while calling them is are known as actual parameter. For example, 23
& 31 in the above example are the actual parameters.
Formal Parameter: Those parameters which are received by the
functions are known as formal parameters. For example, x & y in the
above example are the formal parameters.

16. TYPES OF USER-DEFINED
FUNCTIONS
1. No arguments passed and no return value
2. No arguments passed but a return value
3. Argument passed but no return value
4. Argument passed and a return value

17. TYPES OF USER-DEFINED
FUNCTIONS
1. No arguments passed and no return value
Function with no return no argument, neither returns
a value nor accepts any argument.
This type of function does not communicate with the
caller function. It works as an independent working
block.
Use this type of function when you have some task to
do independent of the caller function.
You must add void as function return type for this
type of function.

18. TYPES OF USER-DEFINED
FUNCTIONS
1. No arguments passed and no return value
Syntax to define function with no return no argument
void function_name()
{
// Function body
}

19. TYPES OF USER-DEFINED
FUNCTIONS
1. No arguments passed and no return value
#include <stdio.h>
void add();
void add()
{
int x = 20;
int y = 30;
int sum = x+y;
printf("sum %d", sum);
}
int main()
{
add();
return 0;
}

20. TYPES OF USER-DEFINED
FUNCTIONS
2. No arguments passed but a return value
Function with return but no arguments, returns a value but
does not accept any argument. This type of function
communicates with the caller function by returning value
back to the caller.
Syntax to define function with return but no arguments
return_type function_name()
{
// Function body
return some_value;
}
Where return_type and some_value must be declared with
same type.

21. TYPES OF USER-DEFINED
FUNCTIONS
2. No arguments passed but a return value
#include <stdio.h>
int add();
int add()
{
int x = 20;
int y = 30;
int sum = x+y;
return(sum);
}
int main()
{
int sum;
sum = add();
printf("sum %d", sum);
return 0;
}

22. TYPES OF USER-DEFINED
FUNCTIONS
3. Argument passed but no return value
Function with no return but with arguments, does not return a
value but accepts arguments as input. For this type of function
you must define function return type as void.
Syntax to define function with no return but with arguments
void function_name(type arg1, type arg2, ...)
{
// Function body
}
Where type is a valid C data type and argN is a valid C identifier.

23. TYPES OF USER-DEFINED
FUNCTIONS
3. Argument passed but no return value
#include <stdio.h>
void add(int, int);
void add(int x, int y)
{
int sum = x+y;
return(sum);
}
int main()
{
add(23, 31);
return 0;
}

24. TYPES OF USER-DEFINED
FUNCTIONS
4. Argument passed and a return value
Function with return and arguments, returns a value and may accept
arguments. Since the function accepts input and returns a result to
the caller, hence this type of functions are dependent on each other.
Syntax to define function with return and arguments
return_type function_name(type arg1, type arg2, ...)
{
// Function body
return some_variable;
}

25. TYPES OF USER-DEFINED
FUNCTIONS
4. Argument passed and a return value
#include <stdio.h>
int add(int, int);
int add(int x, int y)
{
int sum = x+y;
return(sum);
}
int main()
{
int sum = add(23, 31);
printf("%d", sum);
return 0;
}

26. PARAMETER PASSING
Function arguments are the inputs passed to a function.
A function must declare variables to accept passed arguments.
A variable that accepts function argument is known as function
parameter.
A function argument is commonly referred as actual parameter and
function parameter is referred as formal parameter.

27. PARAMETER PASSING
In C programming you can pass value to a function in
two ways.
1. Call by value
2. Call by reference
These two ways are generally differentiated by the type
of values passed to them as parameters.
The parameters passed to function are called actual
parameters whereas the parameters received by
function are called formal parameters.

28. PARAMETER PASSING

29. PARAMETER PASSING

30. PARAMETER PASSING – CALL BY
VALUE
Call by value is the default mechanism to pass arguments to a
function.
In Call by value, during function call actual parameter value is copied
and passed to formal parameter.
Changes made to the formal parameters does not affect the actual
parameter.
In this parameter passing method, values of actual parameters are
copied to function’s formal parameters and the two types of
parameters are stored in different memory locations.
So any changes made inside functions are not reflected in actual
parameters of the caller.

31. PARAMETER PASSING – CALL BY
VALUE
#include<stdio.h>
void swap(int a, int b)
{
int temp;
temp=a;
a=b;
b=temp;
}
void main()
{
int a=100, b=200;
swap(a, b); // passing value to function
printf("nValue of a: %d",a);
printf("nValue of b: %d",b);
}
If you change the value of function parameter, it is changed for the current
function only but it not change the value of variable inside the caller method

32. PARAMETER PASSING - CALL BY
REFERENCE
In call by reference, original value is changed or modified because we
pass reference (address).
Here, address of the value is passed in the function, so actual and
formal arguments shares the same address space.
Hence, any value changed inside the function, is reflected inside as
well as outside the function.

33. PARAMETER PASSING - CALL BY
REFERENCE
#include<stdio.h>
void swap(int *a, int *b)
{
int temp;
temp=*a;
*a=*b;
*b=temp;
}
void main()
{
int a=100, b=200;
swap(&a, &b); // passing value to function
printf("nValue of a: %d",a);
printf("nValue of b: %d",b);
}

34. PARAMETER PASSING – CALL BY
VALUE AND CALL BY REFERENCE
Difference between call by value and call by reference.
CALL BY VALUE CALL BY REFERENCE
This method copy original value into
function as a arguments.
This method copy address of arguments
into function as a arguments.
Changes made to the parameter inside
the function have no effect on the
argument.
Changes made to the parameter affect
the argument. Because address is used to
access the actual argument.
Actual and formal arguments will be
created in different memory location
Actual and formal arguments will be
created in same memory location

35. SUMMARY
•The basic purpose of the function is code reuse.
•From any function we can invoke (call) any another functions.
•Always compilation will be take place from top to bottom.
•Always execution process will starts from main() and ends with main() only.
•In implementation when we are calling a function which is define later for
avoiding the compilation error we need to for forward declaration that is
prototype is required.
•In function definition first line is called function declaration or function
header.
•Always function declaration should be match with function declaratory.
•In implementation whenever a function does not returns any values back to
the calling place then specify the return type.
•Void means nothing that is no return value.
•In implementation whenever a function returns other than void then specify
the return type as return value type that is on e type of return value it is
returning same type of return statement should be mentioned.
•Default return type of any function is an int.

36. #include<stdio.h>
Void takevalue();
Void sum();
Void takevalue()
{
Int a,b;
Printf(“enter 2 values”);
Scanf(“%d %d”,&a,&b);
}
Void sum()
{ takevalue();
Int c;
C=a+b;
Return c;
}
Void main()
{ sum();
Printf(“The sum is %d”, c);
}

37. RECURSION
Recursion is expressing an entity in terms of itself.
In C programming, recursion is achieved using functions known as
recursive function. Recursive functions are very powerful in solving
and expressing complex mathematical problems.
The process in which a function calls itself directly or indirectly is
called recursion and the corresponding function is called as recursive
function.
Using recursive algorithm, certain problems can be solved quite
easily.

38. RECURSION VS ITERATION
Basis For
Comparison
Recursion Iteration
Basic
The statement in a body of function calls the
function itself.
Allows the set of instructions to be repeatedly
executed.
Format
In recursive function, only termination condition
(base case) is specified.
Iteration includes initialization, condition, execution of
statement within loop and update (increments and
decrements) the control variable.
Termination
A conditional statement is included in the body of
the function to force the function to return without
recursion call being executed.
The iteration statement is repeatedly executed until a
certain condition is reached.
Condition
If the function does not converge to some condition
called (base case), it leads to infinite recursion.
If the control condition in the iteration statement never
become false, it leads to infinite iteration.
Infinite Repetition Infinite recursion can crash the system. Infinite loop uses CPU cycles repeatedly.
Applied Recursion is always applied to functions. Iteration is applied to iteration statements or "loops".
Stack
The stack is used to store the set of new local
variables and parameters each time the function is
called.
Does not uses stack.
Overhead
Recursion possesses the overhead of repeated
function calls.
No overhead of repeated function call.
Speed Slow in execution. Fast in execution.
Size of Code Recursion reduces the size of the code. Iteration makes the code longer.

39. RECURSION VS ITERATION
Recursion happens when a function calls a copy of itself to work on a smaller
problem.
Any function which calls itself is called recursive function, and such function
calls are called recursive calls.
Recursion involves several numbers of recursive calls, but it is important to
impose a termination condition of recursion.
Recursion code is shorter than iterative code however it is difficult to
understand.
Recursion cannot be applied to all the problem, but it is more useful for the
tasks that can be defined in terms of similar subtasks.
For Example, recursion may be applied to sorting, searching, and traversal
problems.
Iterative solutions are more efficient than recursion since function call is always
overhead.
Any problem that can be solved recursively, can also be solved iteratively.
But some problems are best suited to be solved by the recursion, for example,
tower of Hanoi, Fibonacci series, factorial finding, etc.

40. EXAMPLE
#include <stdio.h>
int fact (int);
int main()
{
int n,f;
printf("Enter the number whose factorial you want to calculate?");
scanf("%d",&n);
f = fact(n);
printf("factorial = %d",f);
}

41. EXAMPLE
int fact(int n)
{ if (n==0)
{ return 0;
}
else if ( n == 1)
{ return 1;
}
else
{ return n*fact(n-1);
}
}

42. WORKING

43. RECURSIVE FUNCTION
A recursive function performs the tasks by dividing it into the
subtasks.
There is a termination condition defined in the function which is
satisfied by some specific subtask. After this, the recursion stops and
the final result is returned from the function.
The case at which the function doesn't recur is called the base case
whereas the instances where the function keeps calling itself to
perform a subtask, is called the recursive case.
All the recursive functions can be written using this format.
Pseudocode for writing any recursive function is given below.

44. PSEUDOCODE FOR WRITING ANY
RECURSIVE FUNCTION IS GIVEN
BELOW
if (test_for_base)
{
return some_value;
}
else if (test_for_another_base)
{
return some_another_value;
}
else
{
// Statements;
recursive call;
}

45. MEMORY ALLOCATION OF
RECURSIVE METHOD
Each recursive call creates a new copy of that method in the memory.
Once some data is returned by the method, the copy is removed from
the memory.
Since all the variables and other stuff declared inside function get
stored in the stack, therefore a separate stack is maintained at each
recursive call.
Once the value is returned from the corresponding function, the stack
gets destroyed.
Recursion involves so much complexity in resolving and tracking the
values at each recursive call.
Therefore we need to maintain the stack and track the values of the
variables defined in the stack.

46. MEMORY ALLOCATION OF
RECURSIVE METHOD
Let us consider the following example to understand the
memory allocation of the recursive functions.
int display (int n)
{
if(n == 0)
return 0; // terminating condition
else
{
printf("%d",n);
return display(n-1); // recursive call
}
}
recursive function for n = 4. First, all the stacks are maintained which prints the
corresponding value of n until n becomes 0, Once the termination condition is
reached, the stacks get destroyed one by one by returning 0 to its calling stack.
Consider the following image for more information regarding the stack trace for the
recursive functions.

47. MEMORY ALLOCATION OF
RECURSIVE METHOD

48. TYPES OF RECURSION
Recursion are mainly of two types depending on whether a function
calls itself from within itself or more than one function call one
another mutually. The first one is called direct recursion and another
one is called indirect recursion.
Direct Recursion: These can be further categorized into four types:
1. Tail Recursion: If a recursive function calling itself and that
recursive call is the last statement in the function then it’s known as
Tail Recursion.
After that call the recursive function performs nothing. The function
has to process or perform any operation at the time of calling and it
does nothing at returning time.
Tail Recursion occurs if a recursive function calls itself (Direct
Recursion) and the function call is the last statement or step to be
processed in the function before returning having reached the base
case. After processing the call the function returns control back to
the parent function call. It is during the time of function call the

49. TYPES OF RECURSION-TAIL
RECURSION
// Code Showing Tail Recursion
#include <stdio.h>
// Recursion function
void fun(int n)
{
if (n > 0) {
printf("%d ", n);
// Last statement in the function
fun(n - 1);
}
}
// Driver Code
int main()
{
int x = 3;
fun(x);
return 0;
}

50. TYPES OF RECURSION-TAIL
RECURSION

51. TYPES OF RECURSION- HEAD
RECURSION
2. Head Recursion: If a recursive function calling
itself and that recursive call is the first statement
in the function then it’s known as Head
Recursion. There’s no statement, no operation
before the call. The function doesn’t have to
process or perform any operation at the time of
calling and all operations are done at returning
time.
A recursive function is said to be Non-Tail, if the
recursive call to the same function is not the last
statement or the last step processed by the
function. After returning back from the call stack
there is some code left to evaluate. In the case, of
a function, the recursive call is the first statement

52. TYPES OF RECURSION- HEAD
RECURSION
// C program showing Head Recursion
#include <stdio.h>
// Recursive function
void fun(int n)
{
if (n > 0) {
// First statement in the function
fun(n - 1);
printf("%d ", n);
}
}
// Driver code
int main()
{
int x = 3;
fun(x);
return 0;
}

53. TYPES OF RECURSION-HEAD
RECURSION

54. TYPES OF RECURSION-HEAD
RECURSION
Tail-recursive functions are considered far better than non-tail
recursive functions as they can be further optimized by the compiler.
Since the recursive call is the last statement, there is nothing left to
do in the current function, so the compiler does not save the current
function call in the call stack.

55. TYPES OF RECURSION-TREE
RECURSION
3. Tree Recursion: To understand Tree Recursion let’s first understand Linear
Recursion. If a recursive function calling itself for one time then it’s known
as Linear Recursion. Otherwise if a recursive function calling itself for more
than one time then it’s known as Tree Recursion.
fun(n)
{
if(n>0)
{ // some code
fun(n-1); // Calling itself only once
{ //some code
}

56. TYPES OF RECURSION-TREE
RECURSION
The recursion in which the function calling itself calls for one more
than time inside it’s block, is called a Tree Recursion. If the call is
made only once inside the function block then, it is termed as Linear
Recursion. A famous example of this type of recursion is in Nth
Fibonacci Number problem, where given a number we have to find
the nth term value in Fibonacci series.

57. TYPES OF RECURSION-TREE
RECURSION
//Program to generate Nth Fibonacci Number showing Tree-Recursion
#include<stdio.h>
int fib(int n)
{
// Base Case
if (n <= 1)
return n;
return fib(n-1) + fib(n-2);
}
void main()
{
int n = 4;
printf("%d", fib(4));
}

58. TYPES OF RECURSION-TREE
RECURSION

59. TYPES OF RECURSION-TREE
RECURSION
// C program to show Tree Recursion
#include <stdio.h>
// Recursive function
void fun(int n)
{
if (n > 0) {
printf("%d ", n);
// Calling once
fun(n - 1);
// Calling twice
fun(n - 1);
}
}
// Driver code
int main()
{
fun(3);
return 0;
}

60. TYPES OF RECURSION-TREE
RECURSION

61. TYPES OF RECURSION-NESTED
RECURSION
Nested Recursion: In this recursion, a recursive function will
pass the parameter as a recursive call. That means “recursion
inside recursion”.
// C program to show Nested Recursion
#include <stdio.h>
int fun(int n)
{
if (n > 100)
return n - 10;
// A recursive function passing parameter
// as a recursive call or recursion
// inside the recursion
return fun(fun(n + 11));
}
// Driver code
int main()
{
int r;
r = fun(95);
printf("%dn", r);
return 0;
}

62. TYPES OF RECURSION-NESTED
RECURSION

63. TYPES OF RECURSION-INDIRECT
RECURSION
Indirect Recursion: In this recursion, there may be more than one
functions and they are calling one another in a circular manner.
fun(A) is calling for fun(B), fun(B) is calling for fun(C) and fun(C) is
calling for fun(A) and thus it makes a cycle.

64. TYPES OF RECURSION-INDIRECT
RECURSION
// C program to show Indirect Recursion
#include <stdio.h>
void funB(int n);
void funA(int n)
{
if (n > 0) {
printf("%d ", n);
// Fun(A) is calling fun(B)
funB(n - 1);
}
}
void funB(int n)
{
if (n > 1) {
printf("%d ", n);
// Fun(B) is calling fun(A)
funA(n / 2);
}
}
// Driver code
int main()
{
funA(20);
return 0;
}

65. TYPES OF RECURSION-INDIRECT
RECURSION

66. DISADVANTAGES OF RECURSION
What is the disadvantage of recursion?
The memory is incremented by the number of times a recursive
function is called. This is one of the drawbacks/disadvantages of
recursion.
When to (not) use recursion?
If the memory utilization is the concern of if there is limited memory
resource available, it is always a good choice implementing primitive
type recursion with loops.
Mobile has very limited memory space to execute any apps. If you are
developing a mobile application, avoid using recursion.

67. STORAGE CLASSES IN C
Storage classes in C are used to determine the lifetime, visibility,
memory location, and initial value of a variable. There are four types
of storage classes in C
Automatic
External
Static
Register

68. STORAGE CLASSES IN C
Storage
Classes
Storage Place Default Value Scope Lifetime
auto RAM Garbage Value Local Within function
extern RAM Zero Global
Till the end of the main program
Maybe declared anywhere in the
program
static RAM Zero Local
Till the end of the main program,
Retains value between multiple
functions call
register Register Garbage Value Local Within the function

69. STORAGE CLASSES - AUTO
Automatic
Automatic variables are allocated memory automatically at runtime.
The visibility of the automatic variables is limited to the block in
which they are defined.
The scope of the automatic variables is limited to the block in which
they are defined. The automatic variables are initialized to garbage by
default.
The memory assigned to automatic variables gets freed upon exiting
from the block.
The keyword used for defining automatic variables is auto.
Every local variable is automatic in C by default.

70. STORAGE CLASSES - AUTO
#include <stdio.h>
int main()
{
int a; //auto
char b;
float c;
printf("%d %c %f",a,b,c); // printing initial default value of automatic variables
a, b, and c.
return 0;
}

71. STORAGE CLASSES - STATIC
The variables defined as static specifier can hold their value between
the multiple function calls.
Static local variables are visible only to the function or the block in
which they are defined.
A same static variable can be declared many times but can be
assigned at only one time.
Default initial value of the static integral variable is 0 otherwise null.
The visibility of the static global variable is limited to the file in which
it has declared.
The keyword used to define static variable is static.

72. STORAGE CLASSES - STATIC
#include<stdio.h>
static char c;
static int i;
static float f;
static char s[100];
void main ()
{
printf("%d %d %f %s",c,i,f); // the initial default value of c, i, and f will be printed.
}

73. STORAGE CLASSES - REGISTER
The variables defined as the register is allocated the memory into the CPU
registers depending upon the size of the memory remaining in the CPU.
We can not dereference the register variables, i.e., we can not use &operator
for the register variable.
The access time of the register variables is faster than the automatic
variables.
The initial default value of the register local variables is garbage value.
The register keyword is used for the variable which should be stored in the
CPU register. However, it is compiler’s choice whether or not; the variables
can be stored in the register.
We can store pointers into the register, i.e., a register can store the address
of a variable.
Static variables can not be stored into the register since we can not use more
than one storage specifier for the same variable.

74. STORAGE CLASSES - REGISTER
#include <stdio.h>
int main()
{
register int a; // variable a is allocated memory in the CPU register.
The initial default value of a is garbage
printf("%d",a);
return 0;
}

75. STORAGE CLASSES - EXTERN
The external storage class is used to tell the compiler that the
variable defined as extern is declared with an external linkage
elsewhere in the program.
The variables declared as extern are not allocated any memory. It is
only declaration and intended to specify that the variable is declared
elsewhere in the program.
The default initial value of external integral type is 0 otherwise null.
We can only initialize the extern variable globally, i.e., we can not
initialize the external variable within any block or method.
An external variable can be declared many times but can be initialized
at only once.
If a variable is declared as external then the compiler searches for
that variable to be initialized somewhere in the program which may

76. STORAGE CLASSES - EXTERN
#include <stdio.h>
int main()
{
extern int a;
printf("%d",a);
}

77. MEMORY MANAGEMENT IN C

78. PROBLEMS
1. Write a recursive function to calculate 1+2+3+...+n
2. Write a function to check whether a given integer is
a square number or not.
Eg. 4 , 9 , 16 , 25 etc
3. Using recursion write a program to find octal ,
binary and hexadecimal equivalent of the natural
(decimal) number entered by the user