CSE

1. C Programming
User Defined
Functions
1

2. Introduction to Functions
b A complex problem is often
easier to solve by dividing it
into several smaller parts,
each of which can be solved by
itself.
b This is called structured
programming.
2

3. Introduction to Functions
b These parts are sometimes made
into functions in C.
b main() then uses these
functions to solve the original
problem.
3

4. Structured Programming
b Keep the flow of control in a
program as simple as possible.
b Use top-down design.
• Keep decomposing (also known as
factoring) a problem into smaller
problems until you have a
collection of small problems that
you can easily solve.
4

5. Top-Down Design Using Functions
b C programs normally consist
of a collection of user-
defined functions.
• Each function solves one of the
small problems obtained using
top-down design.
• Functions call or invoke other
functions as needed.
5

6. 6
Functions
b A function is a block of code that
performs a specific task.
b A function is a black box that gets
some input and produces some
output
b Functions provide reusable code
b Functions simplify debugging

7. Functions
b Functions
• Modularize a program
• All variables declared inside
functions are local variables
–Known only in function defined
• Parameters
–Communicate information between
functions
–Local variables
7

8. Advantages of Functions
b Functions separate the concept
(what is done) from the
implementation (how it is
done).
b Functions make programs easier
to understand.
b Functions can be called several
times in the same program,
allowing the code to be reused.
8

9. Advantages of Functions
b Benefits of functions
• Divide and conquer
–Manageable program development
• Software reusability
–Use existing functions as building
blocks for new programs
–Abstraction - hide internal details
(library functions)
• Avoid code repetition
9

10. Function Definitions
b Function definition format
return-value-type function-
name( parameter-list )
{
declarations and statements
}
10

11. Function Definitions
• Function-name: any valid
identifier
• Return-value-type: data type of
the result (default int)
–void – indicates that the function
returns nothing
• Parameter-list: comma separated
list, declares parameters
–A type must be listed explicitly
for each parameter unless, the
parameter is of type int
11

12. Function Definitions,
Prototypes, and Calls
#include <stdio.h>
void prn_message(void); /* fct prototype */
/* tells the compiler that this */
/* function takes no arguments */
int main(void) /* and returns no value. */
{
prn_message(); /* fct invocation */
}
void prn_message(void) /* fct definition */
{
printf(“A message for you: “);
printf(“Have a nice day!n”);
} 12

13. Demo Program – Using a Function
to Calculate the Minimum of 2 Values
#include <stdio.h>
int min(int a, int b);
int main(void)
{
int j, k, m;
printf(“Input two integers: “);
scanf(“%d%d”, &j, &k);
m = min(j, k);
printf(“nOf the two values %d and %d, “
“the minimum is %d.nn”, j, k, m);
return 0;
}
int min(int a, int b)
{
if (a < b)
return a;
else
return b;
}
13

14. 14
Types of User-defined Functions in C
Programming
•Functions with no arguments and no return
value
•Functions with no arguments and a return
value
•Functions with arguments and no return value
•Functions with arguments and a return value

15. Scope and Lifetime of a variable
Every variable in C programming
has two properties: type and
storage class.
Type refers to the data type of
a variable.
And, storage class determines
the scope and lifetime of a
variable.
15

16. Scope and Lifetime of a variable
There are 4 types of storage
class:
1. automatic
2. external
3. static
4. register
16

17. Local Variable
b The variables declared inside
the function are automatic or
local variables.
b The local variables exist
only inside the function in
which it is declared. When
the function exits, the local
variables are destroyed.
17

18. Global Variable
Variables that are declared
outside of all functions are
known as external variables.
External or global variables
are accessible to any
function.
18

19. Register Variable
b The register keyword is used to
declare register variables.
Register variables were supposed to
be faster than local variables.
b However, modern compilers are very
good at code optimization and there
is a rare chance that using
register variables will make your
program faster.
19

20. Static Variable
Static variable is declared by
using keyword static. For
example;
static int i;
The value of a static variable
persists until the end of the
program.
20

21. Calling Functions: Call by Value
and Call by Reference
b Used when invoking functions
b Call by value
• Copy of argument passed to
function
• Changes in function do not
effect original
• Use when function does not need
to modify argument
–Avoids accidental changes 21

22. Calling Functions: Call by Value
and Call by Reference
b Call by reference
• Passes original argument
• Changes in function effect
original
• Only used with trusted functions
b For now, we focus on call by
value
22

23. Recursion
b Recursive functions
• Functions that call themselves
• Can only solve a base case
• Divide a problem up into
–What it can do
–What it cannot do
– What it cannot do resembles original
problem
– The function launches a new copy of
itself (recursion step) to solve what
it cannot do
23

24. Recursion
b Example: factorials
• 5! = 5 * 4 * 3 * 2 * 1
• Notice that
–5! = 5 * 4!
–4! = 4 * 3! ...
• Can compute factorials
recursively
• Solve base case (1! = 0! = 1)
then plug in
–2! = 2 * 1! = 2 * 1 = 2;
–3! = 3 * 2! = 3 * 2 = 6; 24

25. Example Using Recursion: The
Fibonacci Series
b Fibonacci series: 0, 1, 1, 2,
3, 5, 8...
• Each number is the sum of the
previous two
• Can be solved recursively:
–fib( n ) = fib( n - 1 ) + fib( n –
2 )
• Code for the fibaonacci function
long fibonacci( long n )
{ 25

26. Function Prototypes
b A function prototype tells the
compiler:
• The number and type of arguments that
are to be passed to the function.
• The type of the value that is to be
returned by the function.
b General Form of a Function
Prototype
type function_name( parameter type list);
26

27. Examples of Function Prototypes
double sqrt(double);
b The parameter list is typically a
comma-separated list of types.
Identifiers are optional.
void f(char c, int i);
is equivalent to
void f(char, int);
27

28. The Keyword void
b void is used if:
• A function takes no arguments.
• If no value is returned by the
function.
28

29. Function Invocation
b As we have seen, a function is
invoked (or called) by writing
its name and an appropriate
list of arguments within
parentheses.
• The arguments must match in
number and type the parameters
in the parameter list of the
function definition.
29

30. Call-by-Value
b In C, all arguments are
passed call-by-value.
• This means that each argument is
evaluated, and its value is used
in place of the corresponding
formal parameter in the called
function.
30

31. Demonstration Program for Call-by-Value
#include <stdio.h>
int compute_sum(int n);
int main(void)
{
int n = 3, sum;
printf(“%dn”, n); /* 3 is printed */
sum = compute_sum(n);
printf(“%dn”, n); /* 3 is printed */
printf(“%dn”, sum);
return 0;
}
int compute_sum(int n)
{
int sum = 0;
for (; n > 0; --n) /* in main(), n is unchanged */
sum += n;
printf(“%dn”, n); /* 0 is printed */
return sum;
}
31