Variables and Types

In programming, variables are used to store values. These values can be of different types, such as integers, floats, and strings.

Defining Variables

In C++, we must define the the variables before we can use them. To illustrate, try the following example...

int foo1 = 100;
unsigned int foo2 = 60000;
long foo3 = 2000000000;
float foo4 = 123.456;
char foo5 = 'A';
String foo6 = "Hello World";
bool foo7 = true;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
}

void loop() {
  // put your main code here, to run repeatedly:
  Serial.println(foo1);
  Serial.println(foo2);
  Serial.println(foo3);
  Serial.println(foo4);
  Serial.println(foo5);
  Serial.println(foo6);
  Serial.println(foo7);
  delay(5000);
}

int foo1 = 100 : This defines the variable foo1 as type int (integer) and a starting value of 100. foo1 is the name of the variable, and you can use (almost) any name you want, as long as it starts with a alphabet and does not contain spaces or special characters. On most Arduinos, an int is a 16-bits value, and it can store a number that ranges from -32,768 to 32,767. It can only store integers (...whole numbers), and cannot store non-integer numbers such as 1.2 or 45.6.

unsigned int foo2 : This defines the variable foo2 as type unsigned int (unsigned integer). On most Arduinos, an unsigned integer is a 16-bits value, and it can store a number that ranges from 0 to 65,535. It can store numbers twice the size of int, but cannot store negative numbers.

long foo3 : This defines a the variable foo3 as type long (long integer). On most Arduinos, an int is a 16-bits value, and it can store a number that ranges from -2,147,483,648 to 2,147,483,647. There is also an unsigned version of long (unsigned long) that allows you to store numbers twice the size, at the cost of being unable to handle negative numbers.

float foo4 : This defines the variable foo4 as type float (floating point number), a number that has a decimal point. A floating point number has around 6-7 decimal digits of precision. That's the total number of digits, not the number to the right of the decimal point. It can store a number that ranges from -3.4028235E+38 to 3.4028235E+38.

char foo5 = 'A' : This defines the variable foo5 as type char (character). char are used to store a single character. Note that when specifying the value for the character, we use single quotes 'A'; double quotes are used for strings.

String foo6 = "Hello World" : This defines the variable foo6 as type String. Strings are used to store words, sentences, or paragraphs, and when specifying the value for a String, we use double quotes "Hello World".

bool foo7 = true : This defines the variable foo6 as type bool (boolean). Booleans have only two possible values; true or false.

Variable Scope

In the above example, all of the variables are defined outside of setup and loop. Such variables are called global variables, and they can be used in any functions.

Functions that are defined inside of a function are called local variables, and they can only be used within that function itself. Here's an example that illustrates the difference...

int foo1 = 100;

void setup() {
  int foo2 = 200;
  Serial.begin(9600);

  // Both foo1 and foo2 can be used here
  Serial.println(foo1);
  Serial.println(foo2);
}

void loop() {
  Serial.println(foo1);
  Serial.println(foo2); // This line will cause an error as foo2 isn't available outside of setup.
  delay(5000);
}

The above code won't work as foo2 is defined inside of setup, so it cannot be used in loop. If you remove the Serial.println(foo2); in loop, the code will then work.