preprocessor directives

src/Learning-C/Preprocessor_Directives/introduction.md

@@ -0,0 +1,51 @@
+# Preprocessor Directives
+
+Manipulating source code before it undergoes compilation. These directives serve to modify or augment the source code in various ways, facilitating efficient development and customization of programs. 
+
+`.c` -> `preprocessor` -> pure high level `c` -> `compiler` -> `assembly machine code`
+
+## File Inclusion
+
+One of the fundamental tasks of the preprocessor is to include header files into the source code using the #include directive. This directive allows for modularization and reuse of code by incorporating external files containing function prototypes, declarations, and definitions.`
+
+```C
+#include <stdio.h>
+#include "myheader.h"
+```
+
+## Macro
+
+Macros provide a powerful mechanism for defining reusable code snippets, constants, or inline functions using the `#define` directive. These macros undergo substitution by their respective definitions before the actual compilation process begins.
+
+Before compilation macro template will be replaced with the macro expansion in the entire program.
+
+macro template is also known as symbolic constant i.e it can't be changed during program execution.
+
+```C
+#define SQR(x) x*x
+
+void main(){
+    int y = 5, z;
+    z = y / SQR(y); 
+    // z = y / y * y;
+    // z = 5 / 5 * 5;
+    // z = 1 * 5;
+    printf("%d", z); 
+}
+```
+
+```C
+#define MAX(a, b) ((a) > (b) ? (a) : (b))
+
+void main() {
+    int x = 10, y = 20;
+    int max_value = MAX(x, y);
+    printf("Maximum value is %d", max_value);
+}
+```
+
+| `const int x = 10;`                                            | `#define N 10`         |
+| -------------------------------------------------------------- | ---------------------- |
+| memory is needed                                               | no memory is needed    |
+| Scope can be local or global depending on where  it is defined | Scope is always global |
+

src/README.md

@@ -23,6 +23,7 @@ This is to document my progress and to share my learnings with others who may fi
 - [Functions](./Learning-C/Functions/Functions.md)
 - [Recursion](./Learning-C/Recursion/introduction.md)
 - [Storage Classes](./Learning-C/Storage-Classes/introduction.md)
+- [Preprocessor Directives](./Learning-C/Preprocessor-Directives/introduction.md)
 - [Acknowledgements](./Learning-C/Acknowledgements.md)
 
 </details>

src/SUMMARY.md

@@ -33,6 +33,7 @@
 - [Recursion](./Learning-C/Recursion/introduction.md)
     - [Problems](./Learning-C/Recursion/problems.md)
 - [Storage Classes](./Learning-C/Storage-Classes/introduction.md)
+- [Preprocessor Directives](./Learning-C/Preprocessor-Directives/introduction.md)
 - [Acknowledgements](./Learning-C/Acknowledgements.md)
 
 ---