Dart Types

Basic Types

The Dart language has special support for the following basic types:

• numbers
• strings
• booleans

Numbers

Dart has two kind of numbers: discrete: integer and continuous: double-precision. With these two it keeps things simple and covers a wide range of computations. Both numbers occupy 64-bit.

Examples:

//simple Dart literals
var x = 10;         //integer
var h = 0xDEADBEEF; //integer
var y = 1.1;        //double
var e = 1.25e3;     //double

Implicit coercion:

Numbers are converted implicit when possible in this direction: int->double. In next example we use an integer literal but a double literal is required. However the compiler will not complain.

void main() {
  //implicit coercion
  double x = 10;
  print(x); // 10.0
}

Explicit coercion:

For conversion in this direction: double->int we can loose some data. In this case only explicitly coercion is possible: There are functions available to do the job.

void main() {
  //explicit coercion
  double y = 2.25;
  int a = y.floor();
  int b = y.ceil();

  print(a); // 2
  print(b); // 3
}

Homework: Let's try this example on-line: declare-vars

Constants

In Dart, a constant is an immutable value that can be assigned only once.

To create a constant, use the "const" keyword followed by the value you want to assign.

For example:

const int x = 5;
const String name = 'John';

The "const" keyword can be used with various data types in Dart, including numbers, strings, lists, and maps.

Constants are useful when you need to ensure that a value doesn't change during runtime, such as when you're working with configuration values or mathematical constants.

Note that constants are different from variables, which can be reassigned with a new value during runtime.

Strings

A string is a sequence of code units. You can use either single or double quotes to create a string. Dart strings are encoded using Unicode UTF-16. You can use single or double quotes to create string literals.

Examples:

//simple Dart strings
var message = 'Sage-Code teaches Computer Science for free!';
var why = "It's a hobby of the author.";

Conversion:

There is a technique often used to convert numbers into strings so that you can print the number withing a text. It is also possible to convert a string into a number but this require special attention since the string may contain something else and conversion may fail.

Example:

Number to string conversion:

// int -> String
String literalString = 101.toString(); // '101'

// double -> String
String piAsString = 3.1415.toStringAsFixed(2); // 3.14

Example:

String to number conversion:

// String -> int
var number = int.parse('101'); // 101

// String -> double
var piNumber = double.parse('3.14'); // 3.14

Interpolation:

You can use a special notation to insert the result of an expression inside to a string. The expression can be as simple as a variable or complex like the result of a function.

Example 1:

You can use "$var_name" notation to include a variable value into a string:

// using variable name
var name = "Elucian";
var twitter = "@elucian_moise";
print("May name is: $name");
print("My twitter is: $twitter");

Output:

May name is: Elucian
My twitter is: @elucian_moise

Example 2:

You can use "${expression}" notation to include result of expression into a string. This is very convenient way to create output strings.

// using toUpperCase() function
var language = "html";
print("Is: ${language.toUpperCase()} a programming language?");

Output:

Is: HTML a programming language?

Concatenation:

Strange but you can use "+" to concatenate two strings. This is the usual method, but in Dart there are other methods to concatenate strings, as we show in next examples.

Example 1:

You can not concatenate a string to a number. For this you can use interpolation or you can convert the number to string before be able to concatenate.

// using concatenation
void main() {
   var test = '10' + 10.toString();
   print(test); //1010
}

Output:

1010

Example 2:

Dart offer you surprises. Adjacent strings are automatically concatenated without "+" this can be useful for long strings that do not feet on a single line just in case you want to break it. The end of line is not included in string:

// using concatenation
void main() {
   var test = 'this is' " a "
              'test';
   print(test); //this is a test
}

Raw strings:

A regular string support escape sequences, for example "\n" represents new line. However there is a way to create "raw" strings that are immune to escape substitution. For this you add a small "r" before the string literal.

// using raw string
var test = r'escape sequence \n '
            'do not work inside '
            'raw strings.';
print(test); //

Output:

escape sequence \n do not work inside raw strings.

Triple quotes:

Python has introduced first time triple quoted strings. This notation is now popular among new computer languages, also Java was upgraded and so it is supported in Dart.

//using triple quoted strings
void main() {
    //incorrect indentation
    var s1 = '''
            You can create
            multi-line strings like this one.
            ''';
    print(s1);
    //correct indentation
    var s2 = """This is also a
multi-line string.""";
    print(s2);
}

Booleans

Boolean type is also known as logic type. It supports only two values: true and false. You can declare a Boolean using "var" or "bool" keywords. I mean, hold on, I lied. Boolean null is partially supported. A "bool" variable is null when not assigned. But this values are not valid in Boolean expressions.

Example:

//using Boolean
void main() {
    var b1 = true;
    bool b2 = false;
    bool b3;    //incorrect declaration
    print(b1);  //true
    print(b2);  //false
    print(b3);  //null (surprise)
}

The table of truth

Next table will help you understand better all possible combinations of values and the result yield by logical operators: "!" = NOT, "||" = OR, "&& "=  AND.

Comparison

Comparison operators like "==" or "!=" will create Boolean results based on non Boolean values. This operator can be used for: numbers, string as well as Boolean values. A less know trick is to assign result of expressions to a Boolean variable. Check this out:

//using comparison operators
void main() {
    //compare numbers
    bool b1 = 1 == 1;
    assert (b1); // true

    //compare booleans
    bool b2 = true == b1;
    assert (b2); // true

    //compare strings
    bool b3 = "this"!="that";
    assert (b3); // true

    //compare strings
    bool b4 = "yes is winter" =="yes " "is " "winter";
    assert (b4); // true

    print("done.");
}

Homework: Open on-line then modify this program to demonstrate that (1 < 2) and ( 2 > 0 ): dart-comparison