Dart For Loop - SyntBlaze

A for loop in Dart is a control flow statement used to execute a block of code repeatedly based on a boolean condition, or to iterate over the elements of an Iterable or Stream. Dart provides three primary syntactical constructs for iteration: the traditional C-style for loop, the for-in loop for synchronous collections, and the asynchronous await for loop for streams.

Standard `for` Loop

The standard for loop relies on an explicit loop counter and consists of three optional expressions: initialization, condition, and update.

for (var i = 0; i < 5; i++) {
  print(i);
}

Initialization: Executed exactly once before the loop begins. Variables declared here are lexically scoped to the loop block.
Condition: Evaluated before each iteration. If it resolves to true, the execution block runs. If false, the loop terminates.
Update: Executed at the end of each iteration, typically used to mutate the loop counter.

Closure Capture Mechanics: Dart captures the state of the loop variable inside closures correctly. Each iteration of the loop creates a new lexical scope for the loop variable, meaning asynchronous callbacks or closures defined within the loop will capture the exact value of the variable for that specific iteration, rather than a reference to the final mutated variable.

var callbacks = <Function>[];
for (var i = 0; i < 3; i++) {
  callbacks.add(() => print(i)); // Captures 0, 1, 2 respectively
}

`for-in` Loop

The for-in loop is a declarative construct used to traverse objects that implement the Iterable interface (such as List or Set). It abstracts away the underlying Iterator mechanics (moveNext() and current).

var numbers = [1, 2, 3];
for (var number in numbers) {
  print(number);
}

Pattern Matching (Dart 3.0+): The for-in loop supports Dart’s pattern matching and destructuring. If the Iterable contains structured data like Records or instances of a class, they can be destructured directly in the loop declaration.

var coordinates = [(1, 2), (3, 4), (5, 6)];

for (var (x, y) in coordinates) {
  print('X: $x, Y: $y');
}

Asynchronous `await for` Loop

The await for loop is an asynchronous control flow construct used to consume elements from a Stream. It suspends execution of the surrounding async function until the stream emits a new event, executing the loop body for each data event and terminating when the stream is closed.

Future<void> processStream(Stream<int> numberStream) async {
  await for (var number in numberStream) {
    print(number);
  }
}

Control Transfer Statements

Dart provides keywords to alter the standard execution flow within any for loop:

break: Immediately terminates the innermost enclosing loop.
continue: Bypasses the remaining statements in the current execution block. In a standard for loop, it jumps directly to the update expression. In a for-in or await for loop, it advances to the next element.

Labeled Statements: For nested loops, Dart supports labels to direct break or continue statements to a specific loop level.

void processMatrix() {
  outerLoop:
  for (var i = 0; i < 5; i++) {
    for (var j = 0; j < 5; j++) {
      if (i == 2 && j == 2) {
        break outerLoop; // Terminates both loops
      }
    }
  }
}

The `forEach` Method

While technically a higher-order method rather than a loop statement, forEach is a functional alternative available on all Iterable objects. It accepts a callback function and applies it to every element.

var items = ['A', 'B', 'C'];
items.forEach((element) {
  print(element);
});

Note: Control transfer statements (break and continue) cannot be used inside the forEach callback, as the block is a separate function scope, not a loop body. To terminate iteration early, a standard for or for-in loop must be used.

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