Introduction to Programming in C

📘 Preprocessor Directives and Macros in C

Before a C program is compiled, it is passed through a preprocessor. This preprocessor handles directives that begin with the # symbol, such as #include, #define, and #ifdef. These directives allow you to manage files, define constants, create macros, and write conditional compilation logic.

🔹 Common Preprocessor Directives

• #include: Includes a header file
• #define: Defines a constant or macro
• #undef: Undefines a macro
• #ifdef: Checks if a macro is defined
• #ifndef: Checks if a macro is not defined
• #endif: Ends conditional directive
• #if, #else, #elif: Conditional compilation

🔧 Example: #define and Constants

#define PI 3.14159
#define SITE "https://udaanpath.com"

int main() {
    printf("Welcome to %s\n", SITE);
    printf("Area of circle: %f\n", PI * 2 * 2);
    return 0;
}

🌀 Macros with Arguments

#define SQUARE(x) ((x) * (x))

int main() {
    printf("Square of 5: %d\n", SQUARE(5));  // Output: 25
}

Note: Always use parentheses in macro definitions to avoid unexpected behaviors.

📂 Conditional Compilation

#define DEBUG

int main() {
#ifdef DEBUG
    printf("Debug mode ON\n");
#endif
    printf("Running program...\n");
}

📚 UdaanPath Use Case: Toggle Features in Development

Let’s say you want to print test logs only during development at UdaanPath:

#define DEV_MODE

int main() {
#ifdef DEV_MODE
    printf("🧪 Running test logs...\n");
#endif
    printf("Welcome to UdaanPath!\n");
}

⚙️ #undef Example

#define SITE "UdaanPath"
#undef SITE
#define SITE "NewWebsite"

int main() {
    printf("Visit %s\n", SITE);
}

🧠 Benefits of Preprocessor Directives

• Improves maintainability of large codebases
• Helps in debugging and logging
• Supports platform-specific compilation
• Used for configuration flags in embedded systems

📝 Practice Exercises

1. Define a macro to calculate the cube of a number
2. Use #ifdef to conditionally print debug output
3. Write a program that includes custom header files using #include
4. Create macros for printing error messages consistently

✅ Summary

• #define is used to define constants and macros
• Preprocessor directives execute before compilation
• Use conditional compilation for feature toggles
• Always use parentheses in macro expressions

🚀 Coming Next

In the next chapter, we’ll dive into Inline Functions, Static & Extern Keywords — crucial concepts for memory control and modular programming.