ZetCode

Dart basics

last modified December 7, 2020

Dart basics tutorial covers the basic programming concepts of Dart language.

Dart is a client-optimized language for fast applications on any platform. It is used to build mobile, desktop, server, and web applications.

Dart simple example

The following is a simple example in Dart.

simple.dart
main() {
  print('First program in Dart');
}

The program prints a message to the console. Dart programs have the .dart extension. The main function is the entry point to the program. The body of the function is enclosed in a pair of curly brackets. The print function displays a message in console. The statements are terminated with a semicolon.

$ dart simple.dart 
First program in Dart

We run the program.

Dart comments

Comments are used by humans to clarify source code. There are three types of comments in Dart: single line comments (//), multi-line comments (/* */), and documentation comments (///).

Documentation comments are used to produce documentation by dartdoc. They are used also by IDEs.

comments.dart
/*
  This is comments.dart 
  Author: Jan Bodnar
  ZetCode 2020
*/

// Program starts here
main() {
  print("This is a Dart program");
}

In the example, we have a multi-line and a single line comment. Comments are ignored by the compiler.

Dart variables

Variables store references to values. Every value is an object -- an instance of a class.

variables.dart
main() {
  String name = 'John Doe';
  int age = 34;
  double height = 172.5;

  print('$name is $age years old; his height is $height cm');
}

In the example, we have three variables: a string, an integer, and a double variable. The data type of the variables are explicitly specified with String, int, and double.

String name = 'John Doe';

We can create string literals both with single and double quotes.

print('$name is $age years old; his height is $height cm');

Dart supports variable interpolation in strings. The variables preceded with the $ character are evaluated to their values inside strings.

$ dart variables.dart 
John Doe is 34 years old; his height is 172.5

Dart var keyword

When we use the var keyword, the compiler infers the data type of a variable from the right side of the assignment.

varkeyw.dart
main() {
  var name = 'John Doe';
  var age = 34;
  var height = 172.5;

  print('$name is $age years old; his height is $height cm');

  print(name.runtimeType);
  print(age.runtimeType);
  print(height.runtimeType);
}

The data types of the three variables are inferred by the compiler.

print(name.runtimeType);
print(age.runtimeType);
print(height.runtimeType);

We can use the runtimeType attribute to get the data type of a variable.

$ dart varkeyw.dart 
John Doe is 34 years old; his height is 172.5 cm
String
int
double

Dart dynamic keyword

With the dynamic keyword, we can declare a dynamically typed variable. We can assign values of different data types to the same variable.

dynamickeyw.dart
main() {
  dynamic n = 3;
  print(3);

  n = 'three';
  print(n);
}

In the example, we first assign value 3 to the n variable; later, we assign the string value 'three'.

$ dart dynamickeyw.dart 
3
three

Dart user input

The dart:io library provides file, socket, HTTP, and other I/O support for non-web applications.

user_input.dart
import 'dart:io';

void main() {
  stdout.write("Enter your name: ");
  var name = stdin.readLineSync();
  print('Hello $name\n');
}

The example prompts the user for his name and prints a message.

stdout.write("Enter your name: ");

We can use the stdout.write function to write to the console without a newline character.

var name = stdin.readLineSync();

We read the user input with stdin.readLineSync.

$ dart read_input.dart 
Enter your name: Peter
Hello Peter

Dart conditionals

Conditionals are created with the if, else if, and else keywords.

conditionals.dart
import 'dart:io';

main() {
  stdout.write("Enter a number: ");
  var input = stdin.readLineSync();

  var n = int.parse(input);

  if (n > 0) {
    print('$n is a positive value');
  } else if (n == 0) {
    print('$n is zero');
  } else {
    print('$n is a negative value');
  }
}

In the example, we ask for a number from the user. Depending on the received value, we print a message to the console.

var n = int.parse(input);

Since the readLineSync returns a string, we transform the string to a number with int.parse.

if (n > 0) {
  print('$n is a positive value');
} else if (n == 0) {
  print('$n is zero');
} else {
  print('$n is a negative value');
}

After the if keyword, we place the condition between two round brackets. If the condition is true, the body of the if statement is executed. Other branches are skipped. If the condition is false, the compiler tests the condition after the else if statement. If it is true, the following body is executed. If both conditions fail, the body after the else statement is executed.

$ dart conditionals.dart 
Enter a number: 4
4 is a positive value
$ dart conditionals.dart 
Enter a number: -4
-4 is a negative value
$ dart conditionals.dart 
Enter a number: 0
0 is zero

Dart Exception

Exceptions are errors indicating that something unexpected happened. Exceptions are processed with try, on, and catch keywords.

format_exception.dart
import 'dart:io';

main() {
  stdout.write("Enter a number: ");
  var input = stdin.readLineSync();

  int n;

  try {
    n = int.parse(input);
  } on FormatException {
    print('wrong value');
    return 1;
  }

  if (n > 0) {
    print('$n is a positive value');
  } else if (n == 0) {
    print('$n is zero');
  } else {
    print('$n is a negative value');
  }
}

In the example, we handle the case when the user does not enter a valid number value.

try {
  n = int.parse(input);
} on FormatException {
  print('wrong value');
return 1;
}

The error prone code is placed in the body of the try statement. The FormatException is thrown when a string or some other data does not have an expected format and cannot be parsed or processed.

$ dart format_exception.dart 
Enter a number: 4
4 is a positive value
$ dart format_exception.dart 
Enter a number: f
wrong value

Dart while loop

The while statement is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. The while keyword executes the statements inside the block enclosed by the curly brackets. The statements are executed each time the expression is evaluated to true.

while_loop.dart
main() {
  int i = 0;
  int sum = 0;

  while (i <= 10) {
    sum += i;
    i++;
  }

  print(sum);
}
In the code example, we calculate the sum of values from a range of numbers.

The while loop has three parts. Initialization, testing and updating. Each execution of the statement is called a cycle.

int i = 0;

We initiate the i variable. It is used as a counter.

while (i <= 10) {
   ...
}

The expression inside the round brackets following the while keyword is the second phase, the testing. The statements in the body are executed until the expression is evaluated to false.

sum += i;

We add the current value of i to the sum variable.

i++;

This is the last, third phase of the while loop, the updating. We increment the counter. Note that improper handling of the while loops may lead to endless cycles.

$ dart while_loop.dart 
55

Dart for loop

We can do loops with the for statement. There are two for loops in Dart: the classic for loop and the for range loop.

for_loop.dart
main() {
  var sum = 0;
  for (var i = 0; i < 10; i++) {
    sum += i;
  }

  print(sum);

  var vals = [1, 2, 3, 4, 5];
  for (var e in vals) {
    print(e * e);
  }
}

In the example, we use both for loops.

var sum = 0;
for (var i = 0; i < 10; i++) {
     sum += i;
}

We calculate the sum of values 1..10 with the classic for loop. There are three phases. In the first phase, we initiate the counter i to zero. This phase is done only once. Next comes the condition. If the condition is met, the statement inside the for block is executed. In the third phase the counter is increased. Now we repeat the 2, 3 phases until the condition is not met and the for loop is left. In our case, when the counter i is equal to 10, the for loop stops executing.

var vals = [1, 2, 3, 4, 5];
for (var e in vals) {
     print(e * e);
}

In the second form, we go through the elements of a list one by one. We square all the values of the list.

$ dart for_loop.dart 
45
1
4
9
16
25

Dart command line arguments

Dart programs can receive command line arguments. They follow the name of the program when we run it.

cmd_args.dart
main(List<String> args) {
  for (var val in args) {
    print(val);
  }

  print('---------');

  for (int i = 0; i < args.length; i++) {
    print(args[i]);
  }
}

We receive the arguments in the args list. We go through the arguments with the classic and the range for loops.

$ dart cmd_args.dart 1 2 3 4
1
2
3
4
---------
1
2
3
4

This was an introduction to the Dart programming language.

List all Dart tutorials.