Control Flow Statements

You can control the flow of your Dart code using any of the following:

If and else

Dart supports if statements with optional else statements, as the next sample shows.

// if Statement
if(condition){
	// Statements
}

// if-else Statement
if(condition){
	// Statements
}else{
	// Statements
}

// if-else Ladder
if(condition1){
	// Statements
}else if(condition2){
	// Statements
}else if(condition3){
	// Statements ...
}
...
...
...
else{
	// Statements
}

Here is the following example:

if (isRaining()) {
  you.bringRainCoat();
} else if (isSnowing()) {
  you.wearJacket();
} else {
  car.putTopDown();
}

Unlike JavaScript, conditions must use boolean values, nothing else.

For loops

You can iterate with the standard for loop. For example:

var message = StringBuffer('Dart is fun');
for (var i = 0; i < 5; i++) {
  message.write('!');
}

forEach method

If the object that you are iterating over is an Iterable, you can use the forEach() method. Using forEach() is a good option if you don’t need to know the current iteration counter:

candidates.forEach((candidate) => candidate.interview());

// or,

candidates.forEach(function(candidate){
	return candidate.interview();
});

for-in Statement

Iterable classes such as List and Set also support the for-in form of iteration:

var collection = [0, 1, 2];
for (var x in collection) {
  print(x); // 0 1 2
}

while loop

A while loop evaluates the condition before the loop:

while(condition){
	// Statements
}

For example,

while (!isDone()) {
  doSomething();
}

The condition is tested first. If the condition is evaluated as true, then the statement of while body executed otherwise the control goes out of the while loop.

do-while loop

A do-while loop evaluates the condition after the loop:

do{
	// Statements
}while(condition);

The statements in the do body is executed first, then the condition is checked. If the condition becomes true, the statements get executed for the second time. That means even if in the first attempt, the condition becomes false, there's a gaurentee that the statements will be executed for at least once.

do {
  printLine();
} while (!atEndOfPage());

break statement

Use break statement to terminate the loop and let the control goes out of it.

while (true) {
  if (shutDownRequested()) break;
  processIncomingRequests();
}

When the control goes to break statement, the loop breaks. Generally the break statement appears inside a if statement.

continue statement

Use continue to skip to the next loop iteration:

for (int i = 0; i < candidates.length; i++) {
  var candidate = candidates[i];
  if (candidate.yearsExperience < 5) {
    continue;
  }
  candidate.interview();
}

Whenever the control meets the continue statement, the following statements after it gets skipped and starts a new iteration. Generally continue statement is appeared within the if statement.

switch and case

switch(value){
	case value1:
		statements1
		break;
	case value2:
		statements2
		break;
	...

	default:
		statements
}

The value should be integer, string, or compile time constants.

When the control gets inside the switch statement, the value of first case statement is checked against the value in the switch statement, if it matches, then the corresponding statements under that case statement gets executed. If the value doesn't match, then the next case statement gets checked. This way it goes to the last case statement until a match is found. If no match s found against any case statement, then the default statements gets executed if there is any.

To terminate the execution of the case statement, use break statement. As a rule, if there is any statement present under the case statement, the break statement must be present to terminate the switch statement. Otherwise, there will be an error.

The following example omits the break statement in a case clause, thus generating an error:

var command = 'OPEN';
switch (command) {
  case 'OPEN':
    executeOpen();
    // ERROR: Missing break

  case 'CLOSED':
    executeClosed();
    break;
}

However, Dart does support empty case clauses, allowing a form of fall-through:

var command = 'CLOSED';
switch (command) {
  case 'CLOSED': // Empty case falls through.
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

Here is an example of switch-case statement

var command = 'OPEN';
switch (command) {
  case 'CLOSED':
    executeClosed();
    break;
  case 'PENDING':
    executePending();
    break;
  case 'APPROVED':
    executeApproved();
    break;
  case 'DENIED':
    executeDenied();
    break;
  case 'OPEN':
    executeOpen();
    break;
  default:
    executeUnknown();
}

If you want both, statements as well as fall-through mechanism, then you can use continue statement to achieve this goal:

var command = 'CLOSED';
switch (command) {
  case 'CLOSED':
    executeClosed();
    continue nowClosed;
  // Continues executing at the nowClosed label.

  nowClosed:
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeNowClosed();
    break;
}

assert statement

During development, use an assert statement to disrupt normal execution if a boolean condition is false. Here are some more:

assert(condition, optionalMessage);

// Make sure the variable has a non-null value.
assert(text != null);

// Make sure the value is less than 100.
assert(number < 100);

// Make sure this is an https URL.
assert(urlString.startsWith('https'));

To attach a message to an assertion, add a string as the second argument to assert.

assert(urlString.startsWith('https'),
    'URL ($urlString) should start with "https".');

The first argument of assert is a condition. But you can use a expression or a function call that returns a boolean value.