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Preprocessor Directives in C

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Introduction

Preprocessor directives are special instructions in C that are processed before compilation.

👉 They start with # and are used to control the compilation process.

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What are Preprocessor Directives?

Preprocessor directives are:
👉 Commands given to the compiler before actual compilation starts

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Why Use Preprocessor Directives?

• ✔ Include files
• ✔ Define constants
• ✔ Control compilation
• ✔ Improve code readability

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Types of Preprocessor Directives

• #include
• #define
• #undef
• Conditional directives (#if, #else, etc.)

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1. #include Directive

👉 Used to include header files

#include <stdio.h>

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2. #define Directive

👉 Used to define constants

#define PI 3.14

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Example

#include <stdio.h>

#define PI 3.14

int main() {
    float r = 2;
    float area = PI * r * r;

    printf("Area = %.2f", area);

    return 0;
}

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Output

Area = 12.56

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3. #undef Directive

👉 Used to remove a defined macro

#undef PI

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4. Conditional Compilation

👉 Used to compile code based on condition

#include <stdio.h>

#define VALUE 10

int main() {
#if VALUE > 5
    printf("Value is greater than 5");
#else
    printf("Value is small");
#endif

    return 0;
}

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Output

Value is greater than 5

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Preprocessor Directives Table

Directive	Purpose
#include	Include header files
#define	Define constants
#undef	Remove macro
#if	Conditional compilation

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Important Notes

• No semicolon needed
• Processed before compilation
• Case-sensitive
• Used globally

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Common Mistakes

• ❌ Using semicolon after #define
• ❌ Wrong macro usage
• ❌ Forgetting header files
• ❌ Misusing conditions

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Pro Tips

• ✔ Use meaningful macros
• ✔ Avoid overuse of macros
• ✔ Use conditional compilation wisely
• ✔ Keep code clean

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Conclusion

Preprocessor directives are powerful tools in C that help control compilation and improve code efficiency.

Master them for better programming practices.

👉 This article is part of Dharani Tech Edu Hub — where learning programming is made simple and practical.