Briefly explain #define deirective.

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Q. Briefly explain #define deirective.

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Concept understanding — The C Preprocessor and Directives

The C preprocessor is a program that runs before the compiler. It scans the source file for lines that begin with a hash symbol #, called preprocessor directives, and carries out the instructions they contain, producing an expanded source file that is then compiled.

The two most important directives are:

#include - inserts the contents of another file into the program. It has two forms: #include <file name> searches the standard system directories (used for library headers such as stdio.h), while #include "file name" also searches the current directory (used for the programmer's own files). One included file may include others, but a file cannot include itself.
#define - creates a macro or symbolic constant. Its general form is #define identifier string; wherever the identifier later appears, the preprocessor substitutes the string. For example #define PI 3.1415926 lets you write PI in place of the number throughout the program, so a value used in many places can be changed in a single line.

Directives are not ordinary statements: they are not terminated by a semicolon and are handled entirely by the preprocessor, not the compiler. Using them makes programs shorter, easier to read, and easier to modify - which is why the link and definition sections at the top of a C program are built almost entirely from #include and #define.

[!TLDR]

#define creates a symbolic constant or macro that the preprocessor substitutes throughout the program.

The #define directive is used to give a name to a constant value or a small piece of code (a macro). Before compilation, the preprocessor replaces every use of that name with the defined text. This makes programs easier to read and to modify, because the value need be changed in only one place.

#define PI 3.14
#define MAX 100

Here, wherever PI appears in the program, it is replaced by 3.14.

[!ANSWER]

The #define directive defines a symbolic constant or macro, and the preprocessor substitutes the defined value wherever the name is used (e.g. #define PI 3.14).

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