Constants in C const vs define

Learn how to use constants in C programming on Debian 12. This tutorial covers both const keyword and #define preprocessor with practical examples.

Understanding Constants in C Programming

Constants in C are fixed values that cannot be altered during program execution. Using constants makes your code more readable and maintainable. In C, there are two main ways to define constants: using the const keyword and the #define preprocessor directive.

The const Keyword

The const keyword creates a read-only variable. Once initialized, its value cannot be changed. The syntax is:

const type name = value;

For example:

const float PI = 3.14159;
const int MAX_USERS = 100;

Key characteristics:

• Has data type and scope rules like normal variables
• Allocated storage in memory
• Can be used for array sizes in C99 and later

The #define Preprocessor

#define creates macro constants that are replaced by the preprocessor before compilation:

#define NAME value

For example:

#define PI 3.14159
#define MAX_USERS 100

Key characteristics:

• No memory allocation - simple text replacement
• No type checking
• Global scope from point of definition

Example Program

Let's write a program demonstrating both constant types:

#include 

#define TAX_RATE 0.07

int main() 
    const float DISCOUNT = 0.15;
    float price = 100.0;
    
    printf(\\"Original price: $%.2f\
\\", price);
    printf(\\"After %.0f%% discount: $%.2f\
\\", 
           DISCOUNT*100, price*(1-DISCOUNT));
    printf(\\"Plus %.0f%% tax: $%.2f\
\\",
           TAX_RATE*100, price*(1-DISCOUNT)*(1+TAX_RATE));
    
    return 0;

When to Use Each

Use const when:

• You need type safety
• You want scope control
• Working with pointers to constants

Use #define when:

• You need compile-time constants for array sizes
• Creating header file guards
• Defining values used in multiple source files

Best Practices

• Use uppercase names for constants (CONSTANT_NAME)
• Prefer const for type safety in most cases
• Place #define directives at the top of files
• Document the purpose of each constant

Conclusion

Both const and #define have important roles in C programming. Understanding their differences helps you write better, more maintainable code. Practice using both in different scenarios to become comfortable with each approach.

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