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#define
is a useful C component that allows you to give a name to a constant value before the program is compiled.
Defined constants in arduino don't take up any program memory space on the chip. The compiler will replace references to these constants with the defined value at compile time.
Arduino defines have the same syntax as C defines:
#define constantName value
Note that the # is necessary.
#define ledPin 3 // The compiler will replace any mention of ledPin with the value 3 at compile time.
There is no semicolon after the #define statement. If you include one, the compiler will throw cryptic errors further down the page.
#define ledPin 3; // this is an error
Similarly, including an equal sign after the #define statement will also generate a cryptic compiler error further down the page.
#define ledPin = 3 // this is also an error
Corrections, suggestions, and new documentation should be posted to the Forum.
The text of the Arduino reference is licensed under a Creative Commons Attribution-ShareAlike 3.0 License. Code samples in the reference are released into the public domain.