#define

Learn to define constants and macros with #define.

#define: defining constants and macros

The #define preprocessor directive gives a name to a value. Before compilation, the preprocessor replaces the name with the value.

#define PI 3.14
#define MAX_USERS 10

printf("%.2f", PI);  /* 3.14 */

Here PI and MAX_USERS are constants; their names are used throughout the code, and they turn into their values before compilation. This is the standard way to name magic numbers. Since the value has no type, the compiler does not check it; use carefully.

Avoiding magic numbers with constants

Using named constants with #define instead of meaningless numbers makes code more readable.

#define BUFFER_SIZE 256
char buf[BUFFER_SIZE];
fgets(buf, BUFFER_SIZE, stdin);

Here the number 256 appears twice; but since it is named BUFFER_SIZE, changing it is done in one place. The meaning of the number is also clear from its name. This approach is very valuable especially for repeated values.

Exercise Define PI and print it

Define a constant with #define PI 3.14. In main print the value of PI with printf using the %.2f format.

Show example solution

#define PI 3.14
printf("%.2f\n", PI);

Test yourself

What does #define do?
- Declares a function
- Declares a variable
- Defines constants and macros
- Defines a type
Show answer

Defines constants and macros — #define defines constants and macros; replaced before compilation.
Fill in the blank: #define constant
```
____ PI 3.14
```
Show answer
```
#define PI 3.14
```
Match directive to task

#define

Defines a constant
#include

Includes a header
#ifdef

Conditional compilation
#endif

Ends a block

Next lesson: #include

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