1. lec8

2. Introduction A function is a subprogram that performs a specific task.
Functions you know:
printf("Hi");
scanf("%i",&number);
Y = sqrt(x);

3. Pre-defined and User-defined Functions
Pre-defined Function
Is a function that is already defined in one of the many C libraries.
It is included in the program through the preprocessor directive
statement #include< > and used in the program to perform a
specific task.
Ex: #include<stdio.h>
Defines printf & scanf functions
User-defined Function
Is a function that is created by the user.
It is defined before the main program through a function prototype
and called upon in the main program to perform a specific task.

4. Types of Functions
1. Program sends data to the function and receives data from the
function.
Ex. y=sqrt(x); x y
sqrt
2. Program sends data to the function and doesn’t receive data from
the function.
Ex. Printf("Hi");
"Hi"
printf

5. Types of Functions
3. Program doesn’t sends data to the function and receives data from
the function.
Ex. Number = Get_Number();
Get_Number
Number
4. Program doesn’t sends data to the function and doesn’t receive
data from the function.
Ex. Display_Comment();
Display_Comment

6. Top-Down Design
Often the algorithms needed to solve a problem is more complex
than those we have seen so far.
Often we must break up the problem into subproblems to develop
the program solution.
In attempting to solve a subproblem at one level, we introduce new
subproblems at lower levels.
This process is called top-down design.

7. Top-Down Design

8. Top-Down Design
One way that we can implement top-down design in our programs is
by defining our own functions.
Often, we will write one function subprogram for each subproblem
in the structured chart

9. Advantages of Writing Functions
1. Decreases the main program’s length and reduces the chance to
make errors.
2. Makes it easier to define programming tasks between
programmers. Each programmer can be responsible for a particular
set of functions.
3. Allows procedural abstraction; a programming technique in
which a main function consists of a sequence of function calls and
each function is implemented separately. This helps to focus on one
function at a time instead of writing the complete program all at
once.
4. Allows code reuse of function programs.

10. Code Reuse
Reusing program fragments that have already been written and
tested whenever possible.
It is a way to help in writing error free code.

11. User-defined Functions
Function Prototype
A way to declare a function.
This tells the compiler:
The name of the function
The data type of received data(if there is).
The type & name of sent data (if there is). Also called the
parameters of the function.
defines a function called drawcircle with not sent nor received data
(when the program doesn’t send to the function and doesn’t receive from the
function, the function is called a void function)
Syntax (function without arguments)
ftype fname (void);
Example
void drawcircle(void);

12. User-defined Functions
Function Call
A function call transfers control from the main program to the
function or subprogram.
Syntax (function without arguments)
fname ();
Example
calls function drawcircle and causes it to begin execution
Drawcircle();

13. User-defined Functions
Function Definition
Defines the function operation by writing the code for the function
in a similar way to writing the code for the main program.
Syntax (function without arguments)
Ftype fname(void)
{local declaration
executable statements}
does not contain the return statement
because this function will not send any
data to the program “main function”
(i.e. a function should contain the
return statement only if it is going to
send data to the program)
Example
void drawcircle(void)
{printf(" *** ");
printf("* *");
printf(" *** ");}

14. User-defined Functions
Main Program
void drawcircle (void);
int main(void) {
drawcircle(); }
void drawcircle (void)
Function Prototype
Function Call
Function Definition

15. User-defined Functions
Main Program
void drawcircle (void);
void drawcircle (void) { }
int main(void)
drawcircle();
Function Prototype
Function Definition
Function Call

16. User-defined Functions
Order of Execution
void function1 (void);
void function2 (void);
int main(void) {
function1();
function2(); }
void function1 (void)
{ }
void function2 (void)
transfer of control to function1
transfer of control back to program
transfer of control to function2
transfer of
control back to program

17. Example - Drawing a Stick Figure*
* *
* *
/ \
/ \
/____\
/ \

18. Example - Drawing a Stick Figure
/* draws a stick figure */
#include <stdio.h>
void draw_circle(void); /* Function prototype */
void draw_intersect(void); /* Function prototype */
void draw_base(void); /* Function prototype */
void draw_triangle(void); /* Function prototype */
int
main(void)
{draw_circle(); /* function call */
draw_triangle(); /* function call */
draw_intersect(); /* function call */
return(0); }
void draw_circle(void) /* Function definition */
{printf(" * \n");
printf("* *\n");
printf(" * * \n");}
continue

19. Example - Drawing a Stick Figure - Continue
/* draws a stick figure - continue*/
void draw_intersect(void) /* Function definition */
{printf(" /\\ \n");
printf(" / \\ \n");
printf("/ \\\n");}
void draw_base(void) /* Function definition */
{printf("------\n");}
void draw_triangle(void) /* Function definition */
{draw_intersect();
draw_base();}

20. Comparison of Various Functions
Function Type
Declaration/Prototype
Call
Definition
Input/Output
#include <stdio.h>
printf("Hi");
Pre-Defined
Mathematical
#include<math.h>
y=sqrt(x);
Pre-Defined
Standard Lib
#include<stdlib.h>
y=abs(x);
Pre-Defined
After the main program or prototype
User-Defined
void drawcircle(void)
drawcircle();

21. Using User-defined Functions
1. Declare the function before the main program.
2. Call the function inside the main program.
3. Define the function after the main program.

22. Types of Functions
1. Program sends data to the function and receives data from the
function.
Prototype: return_type function_name (formal parameter list);
Call: variable_name = function_name (actual argument list);
Definition: return_type function_name (formal parameter list) {...}
2. Program sends data to the function and doesn’t receive data from
the function.
Prototype: void function_name (formal parameter list);
Call: function_name (actual argument list);
Definition: void function_name (formal parameter list) {...}

23. Types of Functions
3. Program doesn’t sends data to the function and receives data from
the function.
Prototype: return_type function_name (void);
Call: variable_name = function_name ();
Definition: return_type function_name (void) {...}
4. Program doesn’t sends data to the function and doesn’t receive
data from the function.
Prototype: void function_name (void);
Call: function_name ();
Definition: void function_name (void) {...}

24. Argument List Correspondence
1. The number of actual arguments used in a function call must be
the same as the number of the formal parameters listed in the
function prototype and definition.
2. The Order of the arguments in the lists determines correspondence
and must be the same in arguments and parameters.
The first actual argument corresponds to the first formal parameter,
the second actual argument corresponds to the second formal
parameter and so on.
3. The data types of the actual argument and the formal parameter
must match.

25. Example Prototype& definition: float squared (int x, float y)
What is returned from
the function (i.e. result)
What is sent to the function
Call:
y = squared (x,y)
What is returned from
the function (i.e. result)
What is sent to the function

26. Examples of Function Prototype & definition
float squared (int number)
Returns back a float type
to the main program
An integer type called number is sent
to the function squared
void print_report (int report_number)
Returns nothing
to the main program
An integer type called report_number
is sent to the function print_report

27. Examples of Function Prototype & definition
char get_menu_choice (void)
Returns back a character type to the main program
Nothing is sent to the function
get_menu_choice
void print_comment (void)
Returns nothing
to the main program
Nothing is sent to the function
print_comment

28. Program Example 1 /* computes half of the value of x */ ..........
float half_of(float k); /* function prototype */
int main(void) {
y=half_of(x); /* calling function */ }
float half_of(float k) /* function definition */
k = k / 2;
return(k);

29. Program Example 2 /* computes the cube of num */ ..........
int cubed(int x); /* function prototype */
int main(void) {
y=cubed(num); /* calling function */ }
int cubed(int x) /* function definition */
return(x*x*x);

30. Program Example 3 /* displays the largest of two numbers */ ..........
int larger(int x,int y); /* function prototype */
int main(void) {
y=larger(num1,num2); /* calling function */ }
int larger(int x,int y) /* function definition */
return(larger_num);

31. Program Example 4
/* calculates the area and circumferencce of a circle */
float find_area(float r); /* function prototype */
float find_circum(float r); /* function prototype */
int main(void) {
area=find_area(r); /* calling function */
circum = find_circum(r); /* calling function */ }
float find_area(float r) /* function definition */
{ return(PI*POW(r,2);}
float find_circum(float r)
{return(2.0*PI*r)}

32. Example continue #include<stdio.h> int menu (void);
int main(void)
{int n1, n2, choice;
do{choice=menu();
if (choice==1 || choice==2)
{printf("\nEnter two numbers: ");
scanf("%i%i", &n1, &n2);}
if (choice==1) printf("\n%i + %i = %i\n", n1, n2, n1+n2);
else if (choice==2) printf("\n%i - %i = %i\n", n1, n2, n1-n2);
else if (choice!=3) printf("\nInvalid choice... Try again...\n");}
while (choice!=3);
return(0);}
continue

33. Example int menu (void) {int choice;
printf("\n \n");
printf("1. Add two numbers\n");
printf("2. Subtract two numbers\n");
printf("3. Quit\n");
printf(" \n\n");
printf("Please Enter your choice: ");
scanf("%i", &choice);
return(choice);}

34. Global and Local Declarations
When the declaration is placed at the bigenning of the main function or sub-functions.
Global Declaration
When the declaration is placed immediately after the pre-processor
directives.
IMPORTANT: in the C programming language local variables
have precedence over global variables.
Functions 2

35. Global and Local Declarations
#include <stdio.h>
/* global declarations */
void function1(void);
int main(void) {
/* local declarations */ }
void function1(void)
Functions 3