Preprocessor is a special subsystem of the MQL5 compiler that is intended for preparation of the program source code immediately before the program is compiled.
Preprocessor allows enhancement of the source code readability. The code can be structured by including of specific files containing source codes of mql5-programs. The possibility to assign mnemonic names to specific constants contributes to enhancement of the code readability.
Preprocessor also allows determining specific parameters of mql5-programs:
If the # symbol is used as the first character in a line of the program, this line is considered as a preprocessor directive. A preprocessor directive ends with a line feed character.