Preprocessing happens before the compilation of a program, hence the name. Available preprocessor commands are explained below.
Macro names are always in uppercase by convention.
In a macro processor, a conditional is a command that allows a part of the program to be ignored during compilation, on some conditions. In the C preprocessor, a conditional can test either an arithmetic expression or whether a name is defined as a macro. A conditional in the C preprocessor resembles in some ways an if statement in C, but it is important to understand the difference between them. The condition in an if statement is tested during the execution of your program. Its purpose is to allow your program to behave differently from run to run, depending on the data it is operating on.