In a previous tutorial, the term "preprocessor commands" was used. Preprocessor commands are... well... commands that come before the program is processed. Preprocessor commands can be recognized by the # sign. In Dev-cpp, they are higlighted as green. In most other C/C++ compilers, they will have some form of highlight. Some preprocessor commands that are useful are...
#include
This preprocessor command copies a file from one location to another, like a .h [header] to a
.c file. Used to include prototypes for functions such as printf() and
system(). You can find at least one of these in most programs, the most common being
#include <stdlib.h>. The syntax is #include then either a < for
C standard library functions or " for custom/homemade header functions [prototypes]. There is
no semicolon (;) after this command.
#define
This preprocessor command declares a constant. Opposed to a variable, a constant is [usually] a
number that, once defined can be used again and again without changing. An example that I use is
PI. I go into calc and go 'pi' and it gives me a number; I copy it and paste it after a #define PI
like this:
#define PI 3.1415926535897932384626433832795
You can use this for other things, too, such as including header files [more later, explained with
section on header files]. No semicolon after this one.
#ifndef
If not defined. Much like an if statement, does what's after it
[pre-processor stuff] if what's supplied is not defined. Confusing? Here's an example:
#ifndef PI
#define PI 3.1415926535897932384626433832795
Which brings us up to the next one...
#endif
Used to end an if statement. Used after preprocessor if statements, like #ifndef. No semicolon.
