Starting with Dart

1 Dart Basics

1.1 Comments

Types of Dart Comments:

• Dart Single line Comment.

• Dart Multiline Comment.

• Dart Documentation Comment.

    void main()
    {
        // This is a single comment
        /*
        This is a multiline comment
        */
        /// This is a documentation comment.
    }

1.2 Variables

To declare a variable: type variable_name;

To declare multiple variables of the same type: type variable1_name, variable2_name, variable3_name, ....variableN_name;

Type of the variable can be among:

• Static Variable

• Dynamic Variable

• Final Variables and Constants.

1.2.1 Example of Dart Variable

    void main()
    {
        
        int variable1 = 10;
        
        
        double variable2 = 0.2; 
    
        bool variable3 = false; 
        String variable4 = "0", variable5 = "Geeks for Geeks";
        
        print(variable1);
        print(variable2);
        print(variable3);
        print(variable4);
        print(variable5);
    }   

    void main()
    {
        // Declaring and initialising a variable
        int variable1 = 10;
        
        // Declaring another variable
        double variable2 = 0.2; // must declare double a value or it
        // will throw error
        bool variable3 = false; // must declare boolean a value or it
        // will throw error
        
        // Declaring multiple variable
        String variable4 = "0", variable5 = "Some text";
        
        // Printing values of all the variables
        print(variable1); // Print 10
        print(variable2); // Print 0.2
        print(variable3); // Print default string value
        print(variable4); // Print default bool value
        print(variable5); // Print Geeks for Geeks
    }   

1.2.2 Dynamic Type Variable in Dart

This is a special variable initialised with keyword dynamic. The variable declared with this data type can store implicitly any value during running the program. Syntax: dynamic variable_name;

    void main()
    {
        // Assigning value to variable
        dynamic variable = "Some text";
        
        // Printing variable 
        print(variable);
        
        // Reassigning the data to variable and printing it
        variable = 3.14157;
        print(variable);
    }       

If we use var instead of dynamic in the above code, it will show an error because var gets the type of the first initialized value.

1.2.3 Final and Const

These keywords are used to define constant variable in Dart i.e. once a variable is defined using these keyword then its value can’t be changed in the entire code. These keyword can be used with or without data type name. A final variable can only be set once and it is initialized when accessed. Syntax for Final: final variable_name (without datatype) or final data_type variable_name (with datatype).

    void main() 
    {
        // Assigning value to variable without datatype
        final variable1 = "Some text";
        // Printing variable1
        print(variable1);
        
        // Assigning value to variable2 with datatype
        final String variable2 = "Some other text";
        // Printing variable2
        print(variable2);
        
        // Now, if we try to reassign the variable1 we get an error
    }       

A constant variable is a compile-time constant and its value must be known before the program runs. Syntax for Const: const variable_name; (without datatype) or const data_type variable_name; (with datatype).

    void main() 
    {
        // Assigning value to variable1 without datatype
        const variable1 = "Some text";
        // Printing variable1
        print(variable1);
        
        // Assigning value to variable2 with datatype
        const variable2 = "Some text again!!";
        // Printing variable variable2
        print(variable2);
    }   

1.2.4 Null Safety in Dart

In Dart, by default a variable can’t be assigned Null value till it is defined that the variable can store Null value in it. This to avoid cases where user assign null value in Dart. To declare a variable as nullable, you append a ‘?’ to the type of the variable. The declared variable will by default store null as value.

1.3 Operators in Dart

The following are the most useful types of operators in Dart.

• Arithmetic Operators.

• Relational Operators.

• Assignment Operators.

• Logical Operators.

1.3.1 Arithmetic Operators

    void main()
    {
        int a = 2;
        int b = 3;
        
        // Adding a and b
        var c = a + b;
        print("Sum  ($a + $b) = $c");
        
        // Subtracting a and b
        var d = a - b;
        print("Difference ($a - $b) = $d");
        
        // Using unary minus
        var e = -d;
        print("Negation -($a - $b) = $e");
        
        // Multiplication of a and b
        var f = a * b;
        print("Product ($a * $b) = $f");
        
        // Division of a and b
        var g = b / a;
        print("Division ($b / $a) = $g");
        
        // Using ~/ to divide a and b
        var h = b ~/ a;
        print("Quotient ($b ~/ $a) = $h");
        
        // Remainder of a and b
        var i = b % a;
        print("Remainder ($b % $a) = $i");
    }   

1.3.2 Relational Operators

    void main()
    {
        int a = 2;
        int b = 3;
        
        // Greater between a and b
        var c = a > b;
        print("a is greater than b ($a > $b) : $c");
        
        // Smaller between a and b
        var d = a < b;
        print("a is smaller than b ($a < $b) : $d");
        
        // Greater than or equal to between a and b
        var e = a >= b;
        print("a is greater than b ($a >= $b) : $e");
        
        // Less than or equal to between a and b
        var f = a <= b;
        print("a is smaller than b ($a <= $b) : $f");
        
        // Equality between a and b
        var g = b == a;
        print("a and b are equal ($b == $a) : $g");
        
        // Unequality between a and b
        var h = b != a;
        print("a and b are not equal ($b != $a) : $h");
    }

Note that the == operator can’t be used to check if the object is same. So, to check if the object are same we use identical() function.

1.3.3 Assignment Operators

    void main()
    {
        int a = 5;
        int b = 7;
        
        // Assigning value to variable c
        var c = a * b;
        
        print("assignment operator used c = $a*$b so now c = $c\n");
        
        // Assigning value to variable d
        var d;
        
        // Value is assign as it is null
        d ??= a + b;
        
        print("Assigning value only if d is null");
        print("d ??= $a+$b so d = $d \n");
        
        // Again trying to assign value to d
        d ??= a - b;
        // Value is not assign as it is not null
        
        print("Assigning value only if d is null");
        print("d ??= a-b so d = $d");
        print("As d was not null value was not updated");
    }

There also compound assignment operators such as +=, -=, *=, /=,  /=, %=, =̂, &=, and |=.

1.3.4 Logical Operators

    void main()
    {
        int a = 5;
        int b = 7;
        
        // Using And Operator
        bool c = a > 10 && b < 10;
        print(c);
        
        // Using Or Operator
        bool d = a > 10 || b < 10;
        print(d);
        
        // Using Not Operator
        bool e = !(a > 10);
        print(e);
    }

Logical operator can only be application to boolean expression and in dart.

1.4 Standard Input/Output

1.4.1 Standard Input in Dart

In Dart programming language, you can take standard input from the user through the console via the use of .readLineSync() function. To take input from the console you need to import a library, named dart:io from libraries of Dart.

    void main()
    {
        print("Enter your name?");
        // Reading name 
        String? name = stdin.readLineSync(); // null safety in name string
        
        // Printing the name
        print("Hello, $name! \nWelcome!!");
    }

Taking integer value as input:

    // Importing dart:io file
    import 'dart:io';
    void main()
    {
        print("Enter your name?");
        // Reading name 
        String? name = stdin.readLineSync(); // null safety in name string
        
        // Printing the name
        print("Hello, $name! \nWelcome!!");
    }

    // Importing dart:io file
    import 'dart:io';
    void main()
    {
        // Asking for favourite number
        print("Enter your favourite number:");
        
        // Scanning number
        int n = int.parse(stdin.readLineSync()!);
        // Here ! is for null safety
        
        // Printing that number
        print("Your favourite number is $n");
    }

1.4.2 Standard Output in Dart

In dart, there are two ways to display output in the console:

• Using print() statement.

• Using stdout.write() statement.

    import 'dart:io';
        
    void main()
    {
        // Printing in first way
        print("Welcome!"); // printing from print statement
        
        // Printing in second way
        stdout.write("Welcome!\n");  // printing from stdout.write()
    }

The print() statement brings the cursor to next line while stdout.write() does not bring the cursor to the next line, it remains in the same line.