Variable

A variable is a way of naming and storing a value for later use by the program, such as data from a sensor or an intermediate value used in a calculation.

Declaring Variables

Before they are used, all variables have to be declared. Declaring a variable means defining its type, and optionally, setting an initial value (initializing the variable). Variables do not have to be initialized (assigned a value) when they are declared, but it is often useful.

int inputVariable1; int inputVariable2 = 0; // both are correct

Programmers should consider the size of the numbers they wish to store in choosing variable types. Variables will roll over (see the next part) when the value stored exceeds the space assigned to store it. See below for an example.

Variable Scope

Another important choice that programmers face is where to declare variables. The specific place that variables are declared influences how various functions in a program will see the variable. This is called variable scope.

There are two types of variables:

  • Global variable
  • Local variable

Global Variables

A global variable is the variable declared outside of all functions (e.g. setup(), loop(), etc. ). The global variable can be accessed by every functions in a program.

Local Variables

A local variable is the variable declared inside a function or a block of code (inside a curly brackets). The local variable only visible to the function or block of code in which they are declared.

Example Code

int gPWMval; // a global variable: all functions can access this variable void setup() { // ... } void loop() { int i; // a local variable: i is only visible inside of loop() function float f; // a local variable: f is only visible inside of loop() function // ... for (int j = 0; j < 100; j++) { // a local variable: j can only be accessed inside the for-loop brackets } }

Initializing Variables

  • Variables may be initialized (assigned a starting value) when they are declared or not. It is always good programming practice however to double check that a variable has valid data in it, before it is accessed for some other purpose. For example:
int calibrationVal = 17; // declare calibrationVal and set initial value
  • If a global variable is not explicitly initialized, it will be initialized to 0. For example:
int calibrationVal; // the it's value is 0 void setup() {} void loop() {}
  • If a local variable is not explicitly initialized, it's value is unpredictable. For example:
void setup() { int calibrationVal; // the it's value is unpredictable } void loop() {}

Variable Rollover

When variables are made to exceed their maximum capacity they "roll over" back to their minimum capacity, note that this happens in both directions.

byte x; x = 0; x = x - 1; // x now contains 255 - rolls over in neg. direction
byte x; x = 255; x = x + 1; // x now contains 0 - rolls over

Using Variables

Once variables have been declared, they can be defined by setting the variable equal to the value one wishes to store with the assignment operator (single equal sign). The assignment operator tells the program to put whatever is on the right side of the equal sign into the variable on the left side.

inputVariable1 = 7; // sets the variable named inputVariable1 to 7 inputVariable2 = analogRead(2); // sets the variable named inputVariable2 to the // (digitized) input voltage read from analog pin #2

Examples

int lightSensVal = 1234; char currentLetter = 'a'; unsigned long speedOfLight = 186000UL; char errorMessage[] = "choose another option"; // see string

Once a variable has been set (assigned a value), you can test its value to see if it meets certain conditions, or you can use its value directly. For instance, the following code tests whether the inputVariable2 is less than 100, then sets a delay based on inputVariable2 which is a minimum of 100:

if (inputVariable2 < 100) { inputVariable2 = 100; } delay(inputVariable2);

This example shows all three useful operations with variables. It tests the variable ( if (inputVariable2 < 100) ), it sets the variable if it passes the test ( inputVariable2 = 100 ), and it uses the value of the variable as an input parameter to the delay() function ( delay(inputVariable2) )

※ NOTES AND WARNINGS:

You should give your variables descriptive names, so as to make your code more readable. Variable names like tiltSensor or pushButton help you (and anyone else reading your code) understand what the variable represents. Variable names like var or value, on the other hand, do little to make your code readable.

You can name a variable any word that is not already one of the keywords in Arduino. Avoid beginning variable names with numeral characters.

Variable Scope

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