Blog · C Programming · 03 Jul 2026 · Hindi + English

Difference Between const and #define in C (Comparison Table)

const is a typed, scoped read-only variable while #define is a textual macro replaced before compilation. Full comparison table with 6 key differences.

const vs #define: the core idea

One-line answer: const creates a real, typed, read-only variable checked by the compiler, while #define is a plain text replacement done by the preprocessor before compilation even starts.

const int MAX = 100;    /* compiler: typed read-only variable */
#define MAX 100         /* preprocessor: text replacement     */

Comparison table: 6 key differences

Point	const	#define
Handled by	Compiler	Preprocessor (before compilation)
Data type	Yes — type checking works	No type — just text
Scope	Follows block/function/file scope	Active from #define line till end of file (or #undef)
Memory	Usually occupies memory, has an address	No memory, no address
Debugger visibility	Visible with its name	Invisible — already replaced by value
Usable in case labels / #if	No (in C)	Yes

Program showing the difference

#include <stdio.h>

#define TAX 18          /* text replacement */

int main() {
    const int limit = 500;    /* typed variable */

    printf("Address of limit: %p\n", (void*)&limit);  /* OK   */
    /* printf("%p", &TAX); */  /* ERROR: TAX has no address  */

    switch (18) {
        case TAX:  printf("Tax slab matched\n"); break;  /* OK        */
        /* case limit: */      /* ERROR: not a constant expression in C */
    }
    return 0;
}

Address of limit: 0x7ffd2c... Tax slab matched

The classic #define trap

#define SQUARE(x) x * x

int main() {
    int r = SQUARE(2 + 3);   /* expands to: 2 + 3 * 2 + 3 = 11, not 25! */
    return 0;
}

Because #define is blind text replacement, SQUARE(2 + 3) becomes 2 + 3 * 2 + 3 = 11. const values never suffer from this because they are real evaluated values, not text. (Fix for macros: #define SQUARE(x) ((x)*(x)).)

When to use which?

Use const for typed values: const double RATE = 8.5; — you get type checking, scoping and debugger support.
Use #define when you need a value in case labels, array sizes on very old compilers, or conditional compilation like #if VERSION > 2.
In modern C, the general rule taught in interviews is: "Prefer const over #define for constants; use #define only where const cannot work."

const vs #define: core idea

One-line answer: const एक real, typed, read-only variable बनाता है जिसे compiler check करता है, जबकि #define plain text replacement है जो compilation शुरू होने से पहले preprocessor करता है.

const int MAX = 100;    /* compiler: typed read-only variable */
#define MAX 100         /* preprocessor: text replacement     */

Comparison table: 6 मुख्य अंतर

Point	const	#define
Handle कौन करता है	Compiler	Preprocessor (compilation से पहले)
Data type	हां — type checking काम करती है	कोई type नहीं — सिर्फ text
Scope	Block/function/file scope follow करता है	#define line से file के अंत तक active (या #undef तक)
Memory	आमतौर पर memory लेता है, address होता है	न memory, न address
Debugger में दिखना	Name के साथ दिखता है	नहीं दिखता — पहले ही value से replace हो चुका
case labels / #if में use	नहीं (C में)	हां

अंतर दिखाने वाला program

#include <stdio.h>

#define TAX 18          /* text replacement */

int main() {
    const int limit = 500;    /* typed variable */

    printf("Address of limit: %p\n", (void*)&limit);  /* OK   */
    /* printf("%p", &TAX); */  /* ERROR: TAX का कोई address नहीं */

    switch (18) {
        case TAX:  printf("Tax slab matched\n"); break;  /* OK        */
        /* case limit: */      /* ERROR: C में constant expression नहीं */
    }
    return 0;
}

Address of limit: 0x7ffd2c... Tax slab matched

#define का classic trap

#define SQUARE(x) x * x

int main() {
    int r = SQUARE(2 + 3);   /* बनता है: 2 + 3 * 2 + 3 = 11, 25 नहीं! */
    return 0;
}

#define blind text replacement है, इसलिए SQUARE(2 + 3) बन जाता है 2 + 3 * 2 + 3 = 11. const values के साथ यह problem कभी नहीं आती क्योंकि वे real evaluated values हैं, text नहीं. (Macro का fix: #define SQUARE(x) ((x)*(x)).)

कब कौन-सा use करें?

const use करें typed values के लिए: const double RATE = 8.5; — type checking, scoping और debugger support मिलता है.
#define use करें जब value case labels, बहुत पुराने compilers पर array sizes, या #if VERSION > 2 जैसी conditional compilation में चाहिए.
Modern C में interviews में यही rule सिखाया जाता है: "Constants के लिए const prefer करें; #define सिर्फ वहां use करें जहां const काम नहीं कर सकता."

Frequently Asked Questions

What is the main difference between const and #define in C?

const is a typed read-only variable processed by the compiler with scope and type checking, while #define is untyped text replacement done by the preprocessor before compilation.

Does #define occupy memory?

No. #define does not create a variable, so it takes no memory and has no address; the value is pasted directly into the code.

Can const be used in a switch case label in C?

No. In C a const variable is not a constant expression, so case labels need #define macros, enum constants or literals.