Functions in C

1. FUNCTIONS IN C
Lecture 1
CSC-103 Programming Fundamentals
CSC-141 Introduction to Computer Programming
Slides prepared by: Dr. Shaista Jabeen

2. Lecture Outline
• What is a function?
• Types of functions
• Why do we need functions?
• Syntax of a function
• Main function
• Function declaration with an example
• Function definition with an example
• Function call (Hierarchical function calls)
• Functions with and without arguments
• Returning value from functions
• An Example Code
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3. A Brief Review:
Structure of a C Program:
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https://data-flair.training/blogs/basic-structure-of-c-program/

4. What is a function?
• Suppose you are building an application in C language and in one of
your program, you need to perform a same task more than once.
• In such case you have two options:
a) Use the same set of statements every time you want to perform the task
b) Create a function to perform that task, and just call it every time you need
to perform that task.
A function is a block of statements that performs a specific task.
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5. Types of Functions
Predefined
Library functions
Declaration in header files
User-Defined
User customized functions,
to reduce complexity of
big programs
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6. Types of Functions
1) Predefined - standard library functions such as puts(), gets(), printf(),
scanf() etc.
- These are the functions which already have a definition in header files (.h
files like stdio.h), so we just call them whenever there is a need to use them.
-we don’t define these functions
2) User Defined functions
- The functions that we create in a program are known as user defined
functions.
- There can be any number of user defined functions in a program.
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7. Why do we need Functions?
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• Reusability
• Function is defined only once -it can be called multiple times within the
program
• Organization:
• Dividing a big/complicated task into smaller code chunks
• Improved readability of code
• Easier Debugging
• Ease in testing – redundancy is reduced
• Extensibility
• When we need to extend our program to handle a case it didn’t handle
before; functions allow us to make the change in one place and have that
change take effect every time the function is called.

8. Example: A function to add two numbers
int addition (int num1, int num2)
{ int sum;
sum = num1+num2;
/* Function return type is integer, so we are returning an integer value, the sum of
the passed numbers. */
return sum;
}
Syntax of a function:
return_type function_name (argument list)
{
Set of statements – Block of code…
}
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9. main function:
• A C program always starts execution with the main () function
• There is always one main function in a C program
• All other functions are called inside the main function with the
sequence specified
• When a function completes its task, the control returns to the main
function main ()
{ Statement1;
………
…….
Statement 100;
}
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10. A simple Function: Example
#include<stdio.h>
void message(); //function prototype declaration
int main()
{
message(); // function call
printf(“this is the end of main functionn”);
return 0;
}
void message() // function definition
{
printf(“time is preciousn”);
}
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11. Multiple functions Calls inside main ();
int main()
{
Function_1(); // function call
printf(“this is the end of first function”);
Function_2(); // function call
printf(“this is the end of second function”);
Function_3(); // function call
printf(“this is the end of third function”);
Function_4(); // function call
printf(“this is the end of fourth function”);
return 0;
}
Run Example in Code Blocks
(function definitions are given
there)
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12. Elements Of User Defined Functions:
1. Function Prototype/Declaration
2. Function Definition
3. Function Call
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13. Function Declaration or Prototype
• A function prototype is simply the declaration of a function that
specifies function's name, parameters and return type.
• It gives information to the compiler that the function may later be
used in the program.
Syntax:
returnType functionName (type1 argument1, type2 argument2, ...);
Example:
void display();
int addition (int , int );
Parameters List
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14. Function Definition
It has two parts;
1. Function Header
• Function Name
• Function Type (return type)
• List of Parameters
• No semi-colon at the end
2. Function Body
• Local Variable Declarations
• Function Statements
• A return Statement
• All enclosed in curly braces
int function_name (void)
{ int x, y;
statements….
…..
return 0;
}
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15. Function Definition: Example
• It contains the function body.
• Function body is the block of code to perform a specific task.
Example: (A function to add two numbers)
int sum (int num1, int num2)
{
int result;
result = num1+num2;
/* Function return type is integer, so we
are returning an integer value, the result
of the passed numbers. */
return result;
}
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Function Header
Function Body

16. Function Call:
• A user-defined function is called from the main function.
• When a program calls a function, the program control is transferred
to the called function.
main()
{
………..
…………
funct_1();
…………
…………
}
return_type funct_1()
{
…………….
…………….
……………..
…………………
}
Calling Function
(Boss Function)
Called Function
(Worker Function) 16

17. Hierarchical Boss/Worker Function Relationship
Any called function can call another function and control passes between those two functions
in hierarchical manner.
main ()
Called function in first level
Called function in second level
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18. Hierarchical Function Calling Example
int addition(int , int );
void display(int);
int main()
{
sum(4,5); // main is calling a user-defined function
return 0;
}
//__________________________________________________
int sum(int a, int b) // Called Function in First Level by main function
{ int y;
y=a+b;
display(y); // sum is calling another function display()
return 0;
} //__________________________________________________
void display(int z) // Called function in second level by sum function
{
printf("The sum of two numbers is %dn", z);
}
• A function called
inside main function
can call another
function.
• The control passes
between the calling
and called function
at each level of
hierarchy.
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Funct_2();

19. Passing Arguments to/between Functions
• Arguments are used to communicate between the calling and the
called function
Functions
With
Arguments
//Function Declaration
int sum (int , int );
// Function Call
sum(10,20);
Without
Arguments
//Function Declaration
int display();
// Call
display();
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20. Returning a value from function
A function may or may not return a
result. But if it does, we must use the
return statement to output the result.
return statement also ends the
function execution.
return statement can occur anywhere
in the function depending on the
program flow.
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21. Function Example in Code Blocks
Example:
Take two points from user that define a point location in 2D cartesian co-
ordinate. Write a C program using functions to convert this point location to
polar coordinates.
Solution:
Domain Knowledge: Mathematics
• To convert from Cartesian Coordinates (𝑥, 𝑦)to Polar Coordinates 𝑟, 𝜃
𝑟 = (𝑥2+ 𝑦2
)
𝜃 = 𝑡𝑎𝑛−1
𝑦
𝑥
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22. int main()
{
float x,y, theta, r;
printf("Enter a number for x coordinate: n");
scanf("%f", &x);
printf("Enter a number for y coordinate: n");
scanf("%f", &y);
r = distance(x,y); //calling function distance
theta = angle(x,y); //calling function angle
printf("The polar coordinates of %f and %f are; n r = %f n
theta = %f n", x,y,r, theta);
return 0;
}
float distance(float a, float b)
{ float d;
/* Calculating r */
d = sqrt(a*a + b*b);
return d;
}
C Code for Given Example:
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23. Cartesian to Polar
Coordinates
Example:
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float angle(float a, float b)
{ float ang,m;
m=b/a;
ang= atan(m);
/* Converting theta from radian to degrees */
printf("The angle in radians is %fn", ang);
ang= ang * (180/ PI);
/* Conditional Check for Polar angle */
if((a<0) && (b>0))
{ ang= 180 + ang;
return ang;
}
else if ((a>0) && (b<0))
{ ang = -(abs(ang));
return ang; }
else if ((a<0) && (b<0))
{ ang = 180 + ang;
return ang; }
else
return ang;
}
angle function in C:

24. Summarizing about Functions:
1) main() in C program is also a function.
2) A C program can have any number of functions, There is no limit on number
of functions. One of those functions must be a main function
3) Functions can be defined in any order
4) Function names in a program must be unique
5) A function gets called when the function name is followed by a semi-colon
6) A function cannot be defined inside a function.
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