Introduce C preprocessor and header files and use them

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The C preprocessor is a part of the C compiler that processes source code before compilation. It's responsible for performing text replacements, file inclusion, and conditional compilation.

One of the most common uses of the C preprocessor is to include header files. Header files contain function and variable declarations that are needed by your C program. They typically have a ".h" file extension and are included in your C code using the "#include" directive.

For example, if you have a header file called "stdio.h" that contains declarations for standard input/output functions like "printf" and "scanf", you can include it in your C code like this:

    #include <stdio.h>

This tells the C preprocessor to copy the contents of "stdio.h" into your C code before compilation. This way, your code can call the functions declared in "stdio.h" without having to define them itself.

Header files can also define macros, which are pieces of code that are pieces of code that are replaced by other code during compilation. Macros are defined using the "#define" directive. For example, you could define a macro that calculates the square of a number like this:

#define SQUARE(x) ((x) * (x))

This defines a macro called "SQUARE" that takes one arguments "x" and returns the square of that argument. You can then use the macro in your C code like this:

int x = 5; int square = SQUARE(x); printf("The square of %d is %d\n", x, square);

This would output "The square of 5 is 25".

Overall, the C preprocessor and header files are powerful tools that allow you to reuse code, define macros, and perform other text substitutions before compilation. They're an important part of the C programming language and are used in many C programs, both big and small.