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Arduino Programming: Variables, constants, and more

SCHOOL OF MAKING

sketch. Constants defined within any function are

local constants and can only be accessed within

the function.

As with variables, to use a constant in a sketch,

simply refer to the constant any place where

you want to access the constant’s value. For

example, to use the pi constant in a circumference

calculation, you would use the following:

const float pi = 3.14;

float circumference;

float radius;

radius = 2.52;

circumference = 2 * pi * radius;

Constants are used primarily to increase code

readability and maintainability. If your sketch

measures the radius of

a circle, then calculates

the circle’s area and

circumference, you

could insert the value

of 3.14 into each of the

formulas. What happens

then if we want to

use a different value

for pi (perhaps including more decimal places)?

You’d have to change the value for pi in each of the

expressions that references it. By storing the value

in a constant, you’d only have to change its value in

one place to effect all of the expressions that use

it. Additionally, from a readability standpoint, seeing

pi in your code is much clearer in meaning to the

reader than 3.14.

When you try to change the value assigned to

a constant in a sketch, the Arduino compiler will

complain, displaying the error message shown

in Figure 2. You will not be able to deploy this

sketch to the Arduino device until you’ve resolved

this error.

MEMORY MANAGEMENT

Arduino devices are inexpensive programmable

microcontrollers, and in order to meet its imposed

cost limitation, the devices have limited memory

in separate functions; however, doing this is likely

to lead to an intense headache if you have to debug

this code. Figure 1 shows the compiler error you’ll

see when you violate scope.

CONSTANTS

In C, constants define storage locations for data

that cannot be modified; the data is referenced by

a sketch, but never changed. They’re typically used

to define values used in calculations or string values

used repeatedly by a sketch. Constant values are

stored in memory and can be easily accessed by

any part of your sketch.

Constants are defined

the same way you

define variables, except

the constant definition

always starts with the

const keyword and you

must assign a value to

the constant. To define a

constant in a sketch, use the following expression:

const DATA_TYPE CONSTANT_NAME = VALUE;

Constants are referenced by name, so

CONSTANT_NAME defines the name associated with

the constant. VALUE defines the data value stored

in the constant. Here are some example constant

definitions, creating (in order) an integer constant,

floating-point constant (decimal value), and a string

constant (a character array) :

const int myConstValue = 3;

const float pi = 3.14;

const char myString = “My string array”;

Constants have the same scope options as

variables. Constants defined at the beginning of

a sketch, before the sketch’s setup() and loop()

functions, have a global scope which means

they can be accessed from anywhere within the

If you learn to program on

devices with limited RAM,

it will set up up with good

habits for the rest of your

coding career

QUICK TIP

Constant values are stored

in memory and can be

easily accessed by any part

of your sketch

”

”

Good variable names are essential for making your code

easy to read. Think about them at the start as they’re

awkward to change later

QUICK TIP

Figure 3

Keep an eye on the

size of the sketch as

you’re developing

it to make sure you

don’t suddenly run

out of space on your

chosen hardware