Changing the execution flow of a program

Dart has the usual control structures with no surprises here (refer to control.dart).

An if...else statement (with an optional else) is as follows:

var n = 25;
if (n < 10) {
  print('1 digit number: $n');
} else if (n >=  10 && n < 100){
  print('2+ digit number: $n'); // 2+ digit number: 25
} else {
  print('3 or more digit number: $n');
}

Single-line statements without {} are allowed, but don't mix the two. A simple and short if…else statement can be replaced by a ternary operator, as shown in the following example code:

num rabbitCount = 16758;
(rabbitCount > 20000) ? print('enough for this year!') : print('breed on!');   // breed on!

If the expression before ? is true, the first statement is executed, else the statement after : is executed. To test if a variable v refers to a real object, use: if (v != null) { … }.

Testing if an object v is of type T is done with an if statement: if (v is T).

In that case we can safely cast v to type T and access all members of T:

if (v is T) {
  (v as T).methodOfT()
}

For example, if we don't know for sure that ba2 is a BankAccount, the code in line (1) in the following code will generate an error; we can avoid this with an if test in line (2):

var ba1, ba2;
ba1 = new BankAccount("Jeff", "5768-346-89", 758.0);
if (ba1 is BankAccount) ba1.deposit(42.0);
print('${ba1.balance}'); // 800.0
(ba2 as BankAccount).deposit(100.0); <-- NoSuchMethodError  (1)
if (ba2 is BankAccount) {                                   (2)
  (ba2 as BankAccount).deposit(100.0);
  print('deposited 100 on ba2'); // statement not reached
} else {
  print('ba2 is not a BankAccount'); // ba2 is not a BankAccount
}

We can replace multiple if...else if with a switch case statement; switch tests the value of an integer or string variable in () against different constant values in case clauses:

switch(ba1.owner) {
  case 'Jeff':
    print('Jeff is the bank account owner'); // this is printed
    break;
  case 'Mary':
    print('Mary is the bank account owner');
    break;
  default:
    print('The bank account owner is not Jeff, nor Mary');
}

Each case must end with a break or a continue with a label; use default when no other case matches; multiple cases can be combined.

Repetition can be coded with a for loop if the number of repetitions is known or with a while or do...while loop if the looping depends on a condition:

var langs = ["Java","Python","Ruby", "Dart"];
for (int i = 0; i < langs.length; i++) {
  print('${langs[i]}');
}

Notice that the condition i value should be less than the length of the List.

If you don't need the index i, the for...in loop provides a simpler alternative:

var s = '';
var numbers = [0, 1, 2, 3, 4, 5, 6, 7];
for (var n in numbers) {
  s = '$s$n ';
}
print(s);  // 0 1 2 3 4 5 6 7

In each loop the variable n takes the value of the next collection element.

Conditions without counters are best tested in a while loop:

while (rabbitCount <= 20000) {
  print('keep breeding');
  rabbitCount += 4;
}

Don't get involved in an infinite loop by forgetting a statement that changes the condition! You can always break out from a loop with a break:

while (true) {
  if (rabbitCount > 20000) break;
  rabbitCount += 4;
}

Likewise, skip the execution of the body of the loop with a continue:

s = '';
for (var n in numbers) {
  if (n % 2 == 0) continue; // skip even numbers
  s = '$s$n ';
}
  print('$s');  // 1 3 5 7