Define macro - Step-by-Step Execution

Concept Flow - Define macro

Start

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Preprocessor reads #define

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Store macro name and replacement

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Replace macro name with replacement in code

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Compile replaced code

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Run program

The C preprocessor reads the #define directive, stores the macro, replaces occurrences in code, then compilation and execution happen.

Execution Sample

C

#define PI 3.14
float area = PI * r * r;

Defines a macro PI as 3.14 and uses it to calculate area.

Execution Table

Step	Code Line	Action	Resulting Code
1	#define PI 3.14	Store macro PI with value 3.14	No code output, macro stored
2	float area = PI * r * r;	Replace PI with 3.14	float area = 3.14 * r * r;
3	Compile replaced code	Compile float area = 3.14 * r * r;	Ready to run
4	Run program	Calculate area using 3.14	Area computed using 3.14

💡 All macros replaced, code compiled and executed

Variable Tracker

Variable	Start	After Macro Replacement	Final
PI	undefined	3.14	3.14
area	undefined	undefined	calculated value at runtime

Key Moments - 2 Insights

Why does the macro replacement happen before compilation?

Because the preprocessor runs first and replaces all macro names with their values before the compiler sees the code, as shown in execution_table step 2.

Is PI a variable that takes memory?

No, PI is just text replaced by 3.14 before compilation, so it does not use memory like a variable. See variable_tracker where PI is replaced.

Visual Quiz - 3 Questions

Look at the execution_table step 2, what does the code line become after macro replacement?

A float area = PI * r * r;

B float area = 3 * r * r;

C float area = 3.14 * r * r;

D float area = r * r;

💡 Hint

Check the 'Resulting Code' column in step 2 of execution_table

At which step does the macro get stored by the preprocessor?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' column in execution_table step 1

If we change #define PI 3.14 to #define PI 3.14159, what changes in the execution_table?

A Step 1 macro storage changes to PI = 3.14

B Step 2 code becomes float area = 3.14159 * r * r;

C Step 3 compilation fails

D No change at all

💡 Hint

Macro replacement in step 2 depends on the macro value stored in step 1

Concept Snapshot

#define MACRO_NAME replacement
- Defines a macro for text replacement
- Preprocessor replaces MACRO_NAME with replacement before compilation
- Macros do not use memory
- Useful for constants or code snippets

Full Transcript

In C, #define creates a macro that replaces text before compilation. The preprocessor reads the #define line, stores the macro name and its replacement text. Then, wherever the macro name appears in code, it is replaced by the replacement text. This happens before the compiler runs. For example, #define PI 3.14 replaces all PI with 3.14. Macros are not variables and do not take memory. The execution table shows storing the macro, replacing it in code, compiling, and running. This helps write readable and maintainable code.