> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Dart Collection For Dart's collection `for` is a syntactic feature that allows the embedding of loop constructs directly within collection literals (`List`, `Set`, and `Map`). It evaluates the loop during the collection's instantiation, dynamically generating and appending elements to the enclosing collection based on the loop's iterations. ## Syntax The collection `for` supports standard C-style `for` loops, `for-in` loops, and asynchronous `await for` loops. **List and Set Literals:** ```dart theme={"dark"} // Standard for loop [for (; ; ) ] // For-in loop {for ( in ) } ``` **Map Literals:** ```dart theme={"dark"} // Maps require key-value pair expressions {for ( in ) : } ``` ## Mechanics and Behavior * **Evaluation:** The loop executes at runtime at the exact point the collection literal is evaluated. The `` is evaluated once per iteration, and the resulting value is immediately appended to the collection in sequential order. * **Scoping:** Variables declared within the `for` loop header are lexically scoped to the loop body, which in this context is the single expression (or key-value pair) following the loop declaration. * **Type Inference:** The static type of the resulting collection is inferred from the type of the evaluated expression(s). If explicit type arguments are provided to the collection literal (e.g., `[]`), the evaluated expressions must be statically assignable to that type. * **Expression vs. Statement:** Within a collection literal, `for` acts as an element generator rather than a control flow statement. It does not support block bodies (`{}`) or statements like `return` or `break`. ## Code Examples **Standard `for` Loop in a List:** ```dart theme={"dark"} final integers = [1, 2, for (var i = 3; i <= 5; i++) i]; // Result: [1, 2, 3, 4, 5] ``` **`for-in` Loop in a Set:** ```dart theme={"dark"} final source = [1, 2, 3]; final squaredSet = {for (final num in source) num * num}; // Result: {1, 4, 9} ``` **`for` Loop in a Map:** ```dart theme={"dark"} final keys = ['a', 'b', 'c']; final charMap = {for (var i = 0; i < keys.length; i++) keys[i]: i}; // Result: {'a': 0, 'b': 1, 'c': 2} ``` ## Advanced Composition Because the collection `for` yields elements, it can be deeply nested and combined with other collection operators, such as the collection `if` and the spread operator (`...`). **Nesting and Collection `if`:** ```dart theme={"dark"} final matrix = [ for (var i = 0; i < 3; i++) for (var j = 0; j < 3; j++) if (i == j) 1 else 0 ]; // Result: [1, 0, 0, 0, 1, 0, 0, 0, 1] ``` **Asynchronous Iteration (`await for`):** When constructing a collection within an `async` function, `await for` can be used to consume a `Stream` directly inside the collection literal. The collection instantiation will suspend until the stream is fully consumed. ```dart theme={"dark"} Future> consumeStream(Stream stream) async { return [ 0, await for (final value in stream) value, 99 ]; } ```

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