It is very useful for you

1. Preprocessors

2. Definition
Preprocessors are some special
commands that start with hash (#)
symbol.
Preprocessing is done before
compilation and it generates expanded
source code.
Following diagram show the concept of
preprocessing:

3. Source Code .C/.CPP File
Preprocessing
Expanded Source
Code
Compiling
Object Code .OBJ File
Linking
Executable Code .EXE File

4. Important Preprocessors
1) #define
2) #include
3) #ifdef
4) #ifndef
5) #undef

5. #define
It can be used in 3 ways
1) It can be used to define constants.
2) It can be used to create shortcuts.
3) It can be used like a small function
(macro).

6. Example 1 – Defining Constants
#include<stdio.h>
# define N 10
void main()
{
int x;
x=N*5;
printf(“%d”, x);
}

7. Output
50

8. Example 2 – Creating Shortcut
#include<stdio.h>
# define P printf(“Enter a number :”)
void main()
{
int x;
P;
scanf(“%d”, &x);
printf(“Value of x = %d”, x);
}

9. Output
Enter a number : 25
Value of x = 25

10. Example 3 - Macro
#include<stdio.h>
# define ADD(a,b) a+b
void main()
{
int x;
x=ADD(10,20);
printf(“%d”, x);
}

11. Output
30

12. #include
This preprocessor is used to include header
files in our programs.
Header files contain functions and macros.
The code of header file is attached with our
program after preprocessing. So we can use
all the things defined in the header file in our
program.
E.g. printf() function is defined in a header
file called <stdio.h> . So we need to include
this header file to use printf() function in our
program.

13. #ifdef
This preprocessor is used for conditional
compilation.
Syntax
#ifdef MacroName
Statements
#else
Statements
#endif

14. Example
#include<stdio.h>
void main()
{
#ifdef N
printf(“Hello”);
#else
printf(“Welcome”);
#endif
}

15. Output
Welcome

16. #ifndef
This preprocessor is also used for
conditional compilation.
Syntax
#ifndef MacroName
Statements
#else
Statements
#endif

17. Example
#include<stdio.h>
void main()
{
#ifndef N
printf(“Hello”);
#else
printf(“Welcome”);
#endif
}

18. Output
Hello

19. #undef
This preprocessor is used to undefine a
macro.
We can also redefine the macro after
undefining it.

20. Example
#include<stdio.h>
#define N 10
void main()
{
int x;
x = N*5;
printf(“%d n”, x);
#undef N
#define N 20
x = N*5;
printf(“%d n”, x);
}

21. Output
50
100