> ## Documentation Index
> Fetch the complete documentation index at: https://learn.acadlink.app/llms.txt
> Use this file to discover all available pages before exploring further.

# #define Statement

> Using #define to create macros and symbolic constants in C

# `#define` Statement in C

The `#define` directive is a **preprocessor command** that tells the C preprocessor to replace a name with a value (or code) everywhere it appears in the program — **before** compilation begins.

It is the most basic form of a macro in C.

***

## Syntax

```c theme={null}
#define MACRO_NAME replacement_text
```

* `MACRO_NAME` — by convention written in **UPPERCASE**
* `replacement_text` — the value or code that replaces the name
* No semicolon (`;`) at the end

***

## Symbolic Constants

The most common use: replacing magic numbers with readable names.

```c theme={null}
#include <stdio.h>
#define PI       3.14159
#define MAX_SIZE 100
#define GREETING "Hello, World!"

int main() {
    float r = 5.0;
    float area = PI * r * r;

    printf("Area = %.2f\n", area);
    printf("%s\n", GREETING);

    return 0;
}
```

**Output:**

```
Area = 78.54
Hello, World!
```

The preprocessor replaces every occurrence of `PI` with `3.14159` before the compiler sees the code.

***

## Object-like Macros

```c theme={null}
#define TRUE  1
#define FALSE 0
#define NULL  0
#define NEWLINE '\n'
```

***

## Undefining a Macro

You can remove a macro definition mid-file using `#undef`:

```c theme={null}
#define LIMIT 50
// ... use LIMIT ...
#undef LIMIT
// LIMIT no longer exists below this line
```

***

## `#define` vs `const`

|               | `#define`      | `const`             |
| ------------- | -------------- | ------------------- |
| Handled by    | Preprocessor   | Compiler            |
| Type checking | None           | Yes                 |
| Memory        | None allocated | Allocated           |
| Scope         | File-wide only | Follows block rules |
| Debugging     | Harder         | Easier              |

**Prefer `const` for typed constants. Use `#define` for compile-time substitutions like include guards.**