In this article, we will discuss the difference between #define and #undef pre-processor in C language.
Pre-Processor:
- Pre-processor is a program that performs before the compilation.
- It only notices the # started statement.
- # is called preprocessor directive.
- Each preprocessing directive must be on its own line.
- The word after # is called the preprocessor command.
#define:
The #define directive defines an identifier and a character sequence (a set of characters) that will be substituted for the identifier each time it is encountered in the source file.
Syntax:
#define macro-name char-sequence.
The identifier is referred to as a macro name and replacement process as a macro replacement.
Example:
#define PI 3.14
Here. PI is the macro-name and 3.14 is the char-sequence.
Program 1:
Below is the C program illustrating the use of #define:
C
// C program illustrating the use of // #define #include <stdio.h> #define PI 3.14 // Driver Code int main() { int r = 4; float a; a = PI * r * r; printf("area of circle is %f", a); return 0; } |
area of circle is 50.240002
Explanation:
- In this example, PI is the macro-name and the char-sequence is 3.14.
- When the program runs the compiler will check the #define command first and assign the PI as 3.14.
- Now in the entire program wherever the compiler sees the PI word it will replace it with 3.14.
Program 2:
Below is the C program printing product of two numbers using #define:
C
// C program to find the product of // two numbers using #define #include <stdio.h> #define PRODUCT(a, b) a* b // Driver Code int main() { printf("product of a and b is " "%d", PRODUCT(3, 4)); return 0; } |
product of a and b is 12
Explanation:
- In this example, a macro-name as the product is defined and passes two arguments as a and b and gives the char-sequence as the product of these two arguments.
- When the compiler sees the macro-name in the print statement, it replaces the macro-name with the product of a and b and gives the answer as their product.
#undef:
The #undef preprocessor directive is used to undefined macros.
Syntax:
#undef macro-name
Program 3:
Below is the C program to illustrate the use of #undef in a program:
C
// C program to illustrate the use // of #undef in a program #include <stdio.h> #define PI 3.14 #undef PI // Driver Code int main() { int r = 6; float a; a = PI * r * r; printf("area of circle is %f", a); return 0; } |
Output:
Explanation: In this example, when #undef is used, then it will delete the #define command and the macro will get undefined and the compiler will show the error.
