> ## 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.

# Constants

> Declaring and using constants in C with const and #define

# Constants in C

A **constant** is a fixed value that **cannot be changed** during the execution of a program. It is also called a **read-only** value. Constants must be initialized at the time of declaration; otherwise, they may hold garbage values.

***

## Ways to Define Constants

### 1. Using the `const` keyword

```c theme={null}
const data_type var_name = value;
const float PI = 3.14;
```

### 2. Using `#define` (Preprocessor Macro)

```c theme={null}
#define CONSTANT_NAME value
#define PI 3.14
```

***

## Examples

**Using `const`:**

```c theme={null}
#include <stdio.h>
int main() {
    const float PI = 3.14;
    printf("Value of PI: %.2f\n", PI);
    return 0;
}
```

**Using `#define`:**

```c theme={null}
#include <stdio.h>
#define PI 3.14
int main() {
    printf("Value of PI: %.2f\n", PI);
    return 0;
}
```

Both produce the same output: `Value of PI: 3.14`

***

## Attempting to Change a Constant

```c theme={null}
const int a = 10;
a = 20;   // ❌ Error: assignment of read-only variable 'a'
```

***

## `const` vs `#define`

| Feature          | `const`             | `#define`                          |
| ---------------- | ------------------- | ---------------------------------- |
| Handled by       | Compiler            | Preprocessor                       |
| Memory allocated | Yes                 | No                                 |
| Type safety      | Enforced            | Not enforced                       |
| Scope            | Follows block rules | File-wide only                     |
| Error checking   | Compile-time        | None                               |
| Debugging        | Easier — has a name | Harder — replaced before compiling |

**Best practice:** Prefer `const` over `#define` for typed constants in modern C. Use `#define` for values that are truly compile-time substitutions (like include guards).