Collection For in Dart

Introduction

Collection for is a Dart feature that lets you generate multiple elements inside a collection literal using a for loop, all in a single expression. It is a concise alternative to building collections with map() or traditional loop-and-add patterns.

Why It Matters

Generating collections from sequences — like turning a list of strings into a list of widgets, or creating a numbered list — is extremely common. Collection for keeps this inline and readable without needing temporary variables or helper functions.

Syntax

var list = [
  for (var item in iterable) transform(item),
];

var list2 = [
  for (int i = start; i < end; i++) expression,
];

Code Example

void main() {
  // Generate squares 1–5
  var squares = [for (int i = 1; i <= 5; i++) i * i];
  print(squares);   // [1, 4, 9, 16, 25]

  // Transform strings to uppercase
  var fruits = ['apple', 'banana', 'cherry'];
  var upper = [for (var f in fruits) f.toUpperCase()];
  print(upper);     // [APPLE, BANANA, CHERRY]

  // Generate a Map from a list
  var names = ['Alice', 'Bob', 'Carol'];
  var nameMap = {
    for (int i = 0; i < names.length; i++) i: names[i],
  };
  print(nameMap);   // {0: Alice, 1: Bob, 2: Carol}

  // Combine collection for with collection if
  var numbers = [1, 2, 3, 4, 5, 6];
  var evenSquares = [
    for (var n in numbers)
      if (n % 2 == 0) n * n,
  ];
  print(evenSquares);   // [4, 16, 36]

  // Generate Set with collection for
  var uniqueDoubles = {for (var n in [1, 2, 2, 3]) n * 2};
  print(uniqueDoubles);   // {2, 4, 6}
}

[1, 4, 9, 16, 25]
[APPLE, BANANA, CHERRY]
{0: Alice, 1: Bob, 2: Carol}
[4, 16, 36]
{2, 4, 6}

Explanation

• for (var item in iterable) iterates over each element and adds the resulting expression to the collection.
• Traditional C-style for (int i = 0; i < n; i++) is also supported inside collection literals.
• Collection for can be nested and combined with collection if for powerful inline data transformations.
• Works in List, Set, and Map literals.

Notes

• Collection for is equivalent to calling map().toList() but is often more readable for simple transformations.
• Use it to generate UI elements (e.g., Flutter widget lists) from data collections.
• Combining collection for with collection if produces concise filter-and-transform pipelines.

Common Mistakes

• Forgetting to use in keyword in for-in form: for (var x in list).
• Trying to add multiple elements per iteration without a spread: use ...[a, b].
• Using collection for for very complex multi-step transformations — a regular loop is more readable in those cases.

Summary

Collection for generates multiple elements inside a collection literal using a loop, working with both for-in and C-style for loops. It supports List, Set, and Map literals and can be combined with collection if for inline filter-and-transform operations. It is a clean alternative to map().toList() for simple data transformations.