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Dart Variables Tutorial with Examples

  1. What is variable?
  2. Declare variable with var keyword
  3. Declare variable with dynamic keyword
  4. Declare variable with specific data type
  5. Declare variable with final keyword
  6. Declare variable with const keyword

1. What is variable?

In the Dart programming language, a variable is a name (an identifier) that points to an address where data is stored in memory.
Example:
var myVariable1 = 100;
  • var is a keyword to declare a variable.
  • myVariable1 is the variable name.
  • 100 is the value of this variable.
Here are the rules for naming variables:
  • The keyword cannot be used to name a variable. For example "var" is a keyword, which cannot be used to name variables.
  • The variable's name consists of letters of the Latin alphabet and numbers.
  • The variable name cannot contain spaces or special characters except for the underscore ( _ ) or the dollar sign ( $ ).
  • Variable names cannot begin with a digit.

2. Declare variable with var keyword

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Variable declaration is mandatory before use. For each name you only need to declare it once.
The simplest way to declare a variable is to use the var keyword:
var myVariable1 = "My Text"; // Declare a variable and assign a value to it.

var myVariable2; // Declare a variable but have not assigned a value.
You can assign a new value to the variable, which means that the variable has pointed to a new data storage address in memory.
myVariable1 = "New My Text";
Example:
Variable_var_ex1.dart
void main() {
  var myVariable1 = "My Text";
  print(myVariable1);

  myVariable1 = "New My Text";  
  print(myVariable1);
}
Output:
My Text
New My Text
Using the "var" keyword to declare a variable means you're telling Dart: "Detect the variable's data type yourself". The data type of the variable will be determined the first time it is assigned a value. If you declare a variable but never assign a value to it, its data type is Null.
Variable_var_ex2.dart
void main() {
  var myVariable1 = "My Text"; // Data type: String
  print("data type of myVariable1: " + myVariable1.runtimeType.toString());

  var myVariable2 = 1000; // Data type: int
  print("data type of myVariable2: " + myVariable2.runtimeType.toString());

  var myVariable3; // Data type: Null
  print("data type of myVariable3: " + myVariable3.runtimeType.toString());

  myVariable3 = true; // Data type: bool
  print("data type of myVariable3: " + myVariable3.runtimeType.toString());
}
Output:
data type of myVariable1: String
data type of myVariable2: int
data type of myVariable3: Null
data type of myVariable3: bool
  • Dart Data Types
A variable is declared with the var keyword and the data type is detected as non-Null. You cannot assign it a value with a different data type.
Example: A variable has been determined by Dart that its data type is String. The compiler will report an error if you assign a numeric value to it.
void main() {
  var myVariable1 = "My Text"; // Data type: String
  print("data type of myVariable1: " + myVariable1.runtimeType.toString());
 
  // Can not assign to it an integer value.
  myVariable1 = 1000; // Error at compile time.
}
Compiler error message:
Error compiling to JavaScript:
Warning: Interpreting this as package URI, 'package:dartpad_sample/main.dart'.
lib/main.dart:6:17:
Error: A value of type 'int' can't be assigned to a variable of type 'String'.
  myVariable1 = 1000; // Error at compile time.
                ^
Error: Compilation failed.

3. Declare variable with dynamic keyword

The dynamic keyword is also used to declare a variable, which is quite similar to the var keyword. The difference is that the data type of this variable can be changed.
dynamic v1 = 123;   // v1 is of type int.
v1 = 456;           // changing value of v1 from 123 to 456.
v1 = 'abc';         // changing type of v1 from int to String.

var v2 = 123;       // v2 is of type int.
v2 = 456;           // changing value of v2 from 123 to 456.
v2 = 'abc';         // ERROR: can't change type of v2 from int to String.

4. Declare variable with specific data type

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You can declare a variable with a specific data type instead of asking Dart to automatically detect its data type.
String myVariable1 = "My Text";

int myVariable2 = 1000;
Specifying a specific data type for the variable at the time of declaration will help reduce errors in the coding process.
For example: You declare a variable of String type but assign it a numeric value, the compiler will give you an error.
String myVariable1  = 1000; // Error at compile time.
  • Dart Data Types

5. Declare variable with final keyword

The final keyword is also used to declare a variable, and you can only assign a value to this variable once. The data type of this variable will be determined by Dart in the first assignment.
void main() {
  final myVariable1; // Declare a variable.

  myVariable1 = 2; // Assign a value to it.

  myVariable1 = 3; // ERROR at compile time.

  final myVariable2 = "Some Text"; // Declare and assign value to variable.

  myVariable2 = "New Text"; // ERROR at compile time.
}
Using the final keyword helps you prevent accidentally assigning a new value to a variable when it already has a value. This ensures that the variable only points to a single address in memory during its lifetime.
A variable is declared with the final keyword. The data in the memory area to which this variable points can still change internally.
Example: The value of a variable is an array, the elements of the array can still change.
Variable_final_ex2.dart
void main() {
  final myArray = ["A", "B", "C"];
  print(myArray);
 
  myArray[2] = "E";
  print(myArray);
}
Output:
[A, B, C]
[A, B, E]
  • Dart Switch

6. Declare variable with const keyword

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The const keyword is used to declare a special variable, also known as a constant. When declaring a variable with the const keyword, you should assign a value to it, otherwise its value will be null. Once declared you cannot assign a value to this variable. The data area that the variable points to will become immutable, meaning it cannot change the data internally.
Variable_const_ex1.dart
void main() {
  const myVariable1 = 100; // Declare and assign value to variable.

  myVariable1 = 2; // ERROR: Can not assign new value.

  const myVariable2; // Declare a variable (Its value is considered null).

  myVariable2 = "Some Text"; // ERROR: Can not assign new value.
}
A variable is declared with the const keyword. The memory area it points to becomes immutable.
Example: The value of a variable is an array, this array will become immutable.
Variable_const_ex2.dart
void main() {
  const myArray = ["A", "B", "C"];
  print(myArray);

  myArray[2] = "E"; // ERROR: myArray is immutable.
  print(myArray);
}
Output:
[A, B, C]
: Unsupported operation: indexed setError: Unsupported operation: indexed set
  • Dart Switch
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