const

The const keyword stands for “constant”. It is a variable qualifier that modifies the behavior of the variable, making a variable “read-only”. This means that the variable can be used just as any other variable of its type, but its value cannot be changed. You will get a compiler error if you try to assign a value to a const variable.

Constants defined with the const keyword obey the same rules of variable scoping that govern other variables. This, and the pitfalls of using #define, often makes using the const keyword a superior method for defining constants than #define.

Example

// this defines a variable called "pi", which cannot be changed:
const float pi = 3.14;
float x;

// ....

x = pi * 2;    // it's fine to find the value of a const variable

pi = 7;        // illegal - you can't write to (modify) a constant

#define or const

You can use either const or #define for creating numeric or string constants. For arrays, you will need to use const. In general, const is preferred over #define for defining constants.

See Also

• #define
• volatile

License and Attribution

Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.