PSPC-UNIT-4.pdf

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UNIT - IV
MODULAR PROGRAMMING
Function and Parameter Declarations: Function definition, types of functions, declaration
and definition of user defined functions, its prototypes and parameters, calling a function.
Variable Scope, Storage classes, recursive functions, Array as function arguments
S.NO TOPIC PAGE NUMBER
1. Function definition 2
2. Types of functions 2
3. Components of function 3
4. Categories of functions 7
5. Storage class 9
6. Variable Scope 12
7. Recursive function 14
8. Arrays as function
arguments
16

2
FUNCTION DEFINITION:
• A function is a group of statements/self-defined block of statements to perform a
specific task.
• Every C program has at least one function i.e main()
• The above figure shows a main function calling func1(), func1 in turn calls func2() and
it goes on. A single function can call as many functions as needed.
• The terminologies to be known while using functions are:
calling function – A function that uses another function
called function – the function which is being called by another function
return – a called function returns some value back to the calling function/ output of a
function
paramters/arguments – the input to the function
TYPES OF FUNCTIONS:
o Standard library functions
o User-defined functions
Built-in functions or Library functions:Build-in functions are predefined functions are
supplied along with the complier whose functionality has been already defined.
Example: printf(), scanf(), getch(), sqrt() etc.
User defined functions:These functions are defined by the user. In these functions the
programmer or developer writes his own logic to perform particular task. These functions are
called from the main() function by using the function call.
Advantages of Functions:
• The major advantages of functions are code reusability. The same code is executed
repeatedly.
• Reduces length of the program
• Debugging is simple to perform
• Functions generates modularity
• Takes less time to write a program
• It saves memory.
Disadvantages of Functions:

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• It is less portable
• Function doesn’t return more than one value at a time. Its needs pointer for such case
• The only disadvantage to a function is that it incurs a function call
• There may not be any speed advantage.
Function Definition :
Syntax:
Return-type function-name (parameters or arguments list)
{
Local declarations;
Executable statements;
Return statement;
}
• Return-type is a valid data type
• Function-name is a valid variable-name which satisfied the rules of declaring a variable
• List of arguments are separated by a comma and they are called formal parameters.
int add (int a, int b)
{
int c;
c=a+b;
return(c);
}
FUNCTION COMPONENTS:
Every function is associated with the following
• Function Declaration or prototype
• Function Parameters
• Function Definition (Function Header + Function Body)
• Return Statement
• Function Call
FUNCTION PROTOTYPE (OR) FUNCTION DECLARATION:
Before using a function, the compiler must know about the number of parameters and the type
of parameters that the function expects to receive and the data type of the value that it’ll return
to the calling function. Function must be declared before using it in a program.
Syntax:
return_data_type function_name (data_type variable1, data_type variable2,..);
Example:
int add(int a, int b);

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float average(float x, float y);
void display(void); //function with no arguments and no return type
• Function_name – must be a meaningful name that will specify the task the function
should perform.
• return_data_type – data type of the result that will be returned to the calling function
• arguments/parameters – the variables that are passed to the calling function along
with its data types
Rules for function declaration:
1. ends with a semicolon
2. function declaration is global
3. use of variable names in function declaration is optional (ex. int add(int, int);)
4. a function can’t be declared within the body of another function
5. function with void in parameter list doesn’t accept any inputs
6. function with void as the return type doesn’t return any values
7. if we do not specify any return type, then by default int is the return type
FUNCTION DEFINITION:
A function definition describes what a function does, how actions are achieved and how it is
used. It comprises two parts:
• Function header
• Function body
Syntax:
return_data_type function_name(data_type variable1, data_type variable2,…)
{
…..
Statements
….
return (variable);
}
• Function header - return_data_type function_name(data_type variable1, data_type
variable2,…)
• Function body –the portion of the program within { } which contains the code to
perform the specific task
• The list of variables in the function header is known as formal parameters.
• The programmer may skip the function declaration in case the function is defined
before being used.
Example:
int a=2, b=3;

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int c;
c=a+b;
return(c);
FUNCTION CALL:
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 part of that function. Once the
called function is executed, the program control passes back to the calling function.
Syntax:
variable=function_name(variable1, variable2,….);//function that returns a value
(or)
function_name(variable1, variable2,….); //function that doesn’t return any value
Example:
d =add(10,20);
avg = average(a,b);
display(); //function with no arguments and no return type
Rules for function call:
1. Function name and type of arguments must be same as that of declaration and definition
2. If the parameters passed is more than specified, the extra parameters will be discarded
3. If the parameters passed are less than expected, then the unmatched arguments will be
initialized to some garbage value
4. Names of variables in function declaration, definition and function call may vary
5. If the data types does not match then it’ll be initialized to some garbage value or a
compile time error will be generated
6. Arguments may be passed in the form of expression. (ex. increment(x+y);)
7. The list of variables in the function call is known as actual parameters.
RETURNING A VALUE:
The return statement is used to terminate the execution of a function and return the control to
the calling function. A return statement may or may not return a value to the calling function.
Syntax:
return <expression>;
For functions that have no return statement, the control automatically returns to the calling
function after the last statement of the called function is executed.A function that does not
return anything is indicated by the keyword void.
Function parameters:
The parameters are classified into two types;
• Actual Parameters
• Formal Parameters or dummy parameters
Actual parameters - The parameters specified in the function call are called actual parameters.
c= add(10,20); //10 and 20 are actual parameters.

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Formal parameters - The parameters specified in the function declaration are called formal
parameters.
int add (int a, int b); //a and b are called formal parameters.
Rules:
• Number of arguments in the actual parameters and formal parameters must be
equal.
• The data type of each argument in actual and formal parameters must be same.
• The order of actual and formal parameter must be important.
Sample program for addition of 2 numbers:
#include<stdio.h>
//FUNCTION DECLARATION
int sum(int a, int b);
int main()
{
int a, b, c;
printf(“Enter the values of a & b:”);
scanf(“%d %d”,&a,&b);
//FUNCTION CALL
c = sum(a,b);
printf(“Sum is %d”, c);
return 0;
}
//FUNCTION DEFINITION
int sum(int x, int y) //FUNCTION HEADER
{ //FUNCTION BODY
int z;
z = x+y;
return z;
}
Sample program with multiple return statements:
#include<stdio.h>
//FUNCTION DECLARATION
int compare(int a, int b);
int main()
{
int a, b, res;
//FUNCTION CALL

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res = compare(a,b);
if(res==0)
printf(“EQUAL”);
else if (res==1)
printf(“a is greater than b”);
else
printf(“a is less than b”);
return 0;
}
//FUNCTION DEFINITION
int compare(int x, int y) //FUNCTION HEADER
{ //FUNCTION BODY
if(x==y)
return 0;
else if(x>y)
return 1;
else
return -1;
}
CATEGORIES OF FUNCTIONS:
User-defined functions can be categorized as:
• Function with no arguments and no return value
• Function with no arguments and return value
• Function with arguments and no return value
• Function with arguments and return value
Functions with no arguments and no return value:
#include <stdio.h>
void sum(void);
int main()
{
sum();
return 0;
}
void sum()
{
int a,b,c;

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c = a+b;
printf(“sum is %d”, c);
}
The sum() function takes input from the user, finds the sum of the 2 numbers and displays it
on the screen.The empty parentheses in sum(); statement inside the main() function indicates
that no argument is passed to the function.The return type of the function is void. Hence, no
value is returned from the function.
Function with no arguments and return value:
#include <stdio.h>
int sum(void);
int main()
{
int c;
c = sum();
return 0;
}
int sum()
{
int a,b,c;
c = a+b;
return c;
}
The empty parentheses in c=sum(); statement indicates that no argument is passed to the
function. And, the value returned from the function is assigned to c.Here, the sum() function
takes input from the user and returns the result.
Function with arguments and no return value:
#include <stdio.h>
void sum(int, int);
int main()
{
int a,b,c;

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sum(a, b);
return 0;
}
void sum(int a, int b)
{
int c;
c = a+b;
printf(“sum is %d”, c);
}
The 2 numbers entered by the user is passed to sum() function.Here, the sum() function adds
the 2 numbers and displays the appropriate result.
Function with arguments and return value:
#include <stdio.h>
int sum(int, int);
int main()
{
int a,b,c;
c = sum(a, b);
return 0;
}
int sum(int a, int b)
{
int c;
c = a+b;
return c;
}
The input from the user is passed to sum() function.The sum() function adds the 2 numbers and
the result is returned to the main() function in which the result is displayed.
STORAGE CLASSES:
A storage class defines the scope (visibility) and life-time of variables and/or functions within
a C Program. They precede the type that they modify. We have four different storage classes
in a C program:
• auto
• register

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• static
• extern
Storage Default value Scope Life
Auto Memory An unpredictable
value, which is often
called a garbage
value
Local to the block in
which the variable is
defined
Till the control remains
within the block in which
the variable is defined
Static Memory Zero Local to the block in
defined
Value of the variable
persists between different
function calls
External Memory Zero Global As long as the program’s
execution. Doesn’t came to
an end
Register CPU Registers Garbage Value Local to the block in
defined
Till the control remains
within the block in which
the variable is defined
The auto Storage Class:
• The auto storage class is the default storage class for all local variables.
{
int mount;
auto int month;
}
The example above defines two variables with in the same storage class. 'auto' can only be used
within functions, i.e., local variables.
The register Storage Class:
• The register storage class is used to define local variables that should be stored in a
register instead of RAM. This means that the variable has a maximum size equal to the
register size (usually one word) and can't have the unary '&' operator applied to it (as it
does not have a memory location).
{
register int miles;
}

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• The register should only be used for variables that require quick access such as
counters. It should also be noted that defining 'register' does not mean that the variable
will be stored in a register. It means that it MIGHT be stored in a register depending on
hardware and implementation restrictions.
The static Storage Class:
• The static storage class instructs the compiler to keep a local variable in existence
during the life-time of the program instead of creating and destroying it each time it
comes into and goes out of scope. Therefore, making local variables static allows them
to maintain their values between function calls.
• The static modifier may also be applied to global variables. When this is done, it causes
that variable's scope to be restricted to the file in which it is declared.
#include <stdio.h>
void func(void);
static int count = 5; /* global variable */
main() {
while(count--) {
func();
}
return 0;
}
void func( void ) {
static int i = 5; /* local static variable */
i++;
printf("i is %d and count is %dn", i, count);
}
Output:
i is 6 and count is 4
The extern Storage Class:
• The extern storage class is used to give a reference of a global variable that is visible to
ALL the program files. When you use 'extern', the variable cannot be initialized
however, it points the variable name at a storage location that has been previously
defined.

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• When you have multiple files and you define a global variable or function, which will
also be used in other files, then extern will be used in another file to provide the
reference of defined variable or function. Just for understanding, extern is used to
declare a global variable or function in another file.
The extern modifier is most commonly used when there are two or more files sharing
the same global variables or functions as explained below.
First File: main.c
#include <stdio.h>
int count ;
extern void write_extern();
main() {
count = 5;
write_extern();
}
Second File: support.c
#include <stdio.h>
extern int count;
void write_extern(void) {
printf("count is %dn", count);
}
Here, extern is being used to declare count in the second file, where as it has its definition in
the first file, main.c. Now, compile these two files as follows −
$gcc main.c support.c
It will produce the executable program a.out. When this program is executed, it produces the
following result −
count is 5
VARIABLE SCOPE:
Variable Scope is a region in a program where a variable is declared and used. So, we can have
three types of scopes depending on the region where these are declared and used:
• Local variables are defined inside a function or a block
• Global variables are outside all functions
• Formal parameters are defined in function parameters

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Local variables:
• Variables that are declared inside a function or a block are called localvariables and are
said to have localscope.
• These local variables can only be used within the function or block in which these are
declared.
• We can use (or access) a local variable only in the block or function in which it is
declared. It is invalid outside it.
• Local variables are created when the control reaches the block or function containing
the local variables and then they get destroyed after that.
#include <stdio.h>
void fun1()
{
int x = 4; /*local variable of function fun1*/
printf("%dn",x);
}
int main()
{
int x = 10;/*local variable of function main*/
{
int x = 5;/*local variable of this block*/
printf("%dn",x);
}
printf("%dn",x);
fun1();
}
Output:
5
10
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Global variables:
• Variables that are defined outside of all the functions and are accessible throughout the
program are global variables and are said to have global scope.
• Once declared, these can be accessed and modified by any function in the program.
• We can have the same name for a local and a global variable but the local variable gets
priority inside a function.
#include <stdio.h>

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int x = 10;/*Global variable*/
void fun1()
{
int x = 5; /*local variable of same name*/
printf("%dn",x);
}
int main()
{
printf("%dn",x);
fun1();
}
Output:
10
5
You can see that the value of the local variable ‘x’ was given priority inside the function ‘fun1’
over the global variable have the same name ‘x’.
Formal parameters:
• Formal Parameters are the parameters which are used inside the body of a function.
• Formal parameters are treated as local variables in that function and get a priority over
the global variables.
#include <stdio.h>
int x = 10;/*Global variable*/
void fun1(int x)
{
printf("%dn",x); /*x is a formal parameter*/
}
int main()
{
fun1(5);
}
Output:
5
Recursive function:
In C programming language, function calls can be made from the main() function, other
functions or from the same function itself. The recursive function is defined as follows...

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A function called by itself is called recursive function.
The recursive functions should be used very carefully because, when a function called by itself
it enters into the infinite loop. And when a function enters into the infinite loop, the function
execution never gets completed. We should define the condition to exit from the function call
so that the recursive function gets terminated.
When a function is called by itself, the first call remains under execution till the last call gets
invoked. Every time when a function call is invoked, the function returns the execution control
to the previous function call.
1.Factorial of a number using recursion
#include<stdio.h>
#include<conio.h>
int factorial( int ) ;
int main()
{
int fact, n ;
printf("Enter any positive integer: ") ;
scanf("%d", &n) ;
fact = factorial( n ) ;
printf("nFactorial of %d is %dn", n, fact) ;
return 0;
}
int factorial( int n )
{
int temp ;
if( n == 0)
return 1 ;
else
temp = n * factorial( n-1 ) ; // recursive function call
return temp ;
}
Output:

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In the above example program, the factorial() function call is initiated from main() function
with the value 3. Inside the factorial() function, the function calls factorial(2), factorial(1) and
factorial(0) are called recursively. In this program execution process, the function call
factorial(3) remains under execution till the execution of function calls factorial(2), factorial(1)
and factorial(0) gets completed. Similarly the function call factorial(2) remains under execution
till the execution of function calls factorial(1) and factorial(0) gets completed. In the same way
the function call factorial(1) remains under execution till the execution of function call
factorial(0) gets completed. The complete execution process of the above program is shown in
the following figure...

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Array as function arguments:
Just like variables, array can also be passed to a function as an argument .
#include <stdio.h>
void disp(char);
void main()
{
int x;
char arr[] = {'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j'};

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for ( x=0; x<10; x++)
{
/* I’m passing each element one by one using subscript*/
disp (arr[x]);
}
}
void disp( char ch)
{
printf("%ct", ch);
}
Output: a b c d e f g h I j
Passing Array using call by reference:
When we pass the address of an array while calling a function then this is called function call
by reference. When we pass an address as an argument, the function declaration should have
a pointer as a parameter to receive the passed address.
#include <stdio.h>
Void disp(int *);
void main()
{
int arr[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
for (int i=0; i<10; i++)
{
/* Passing addresses of array elements*/
disp (&arr[i]);
}
}
void disp( int *num)
{
printf("%d ", *num);
}
Output:
1 2 3 4 5 6 7 8 9 0

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