avoid_accessing_collections_by_constant_index

v0.3.0 Warning Collection & Type

Accessing a collection with a constant index (like list[0]) inside a loop is suspicious because the index never changes with the loop iteration. This usually means the access should be moved outside the loop, or the index should depend on the loop variable.

Why use this rule

A constant index inside a loop always reads the same element on every iteration. This is either redundant work that belongs before the loop, or a bug where the developer forgot to use the loop variable as the index.

See also: Dart collections

Don’t

const array = [1, 2, 3, 4, 5];

void example() {
  // Constant index inside for-in loop
  for (final element in array) {
    array[0];
  }

  // Constant index inside for loop
  for (var i = 0; i < array.length; i++) {
    array[0];
  }

  // Const variable index inside loop
  const idx = 2;
  for (final element in array) {
    array[idx];
  }

  // Constant index inside while loop
  var j = 0;
  while (j < array.length) {
    array[0];
    j++;
  }
}

Do

const array = [1, 2, 3, 4, 5];

void example() {
  // Access outside of a loop
  final first = array[0];

  // Loop variable used as index
  for (var i = 0; i < array.length; i++) {
    array[i];
  }

  // Mutable variable used as index
  var idx = 0;
  for (final element in array) {
    array[idx];
    idx++;
  }

  // Expression-based index
  for (var i = 0; i < array.length; i++) {
    array[i + 1];
  }
}

Configuration

This rule is in the recommended preset, so it is on with preset: recommended or preset: opinionated. Add it to preset: core with avoid_accessing_collections_by_constant_index: true.

To turn it off:

many_lints.yaml

rules:
  avoid_accessing_collections_by_constant_index: false

To keep the rule on but skip certain paths, use per-rule exclude.

Related rules

• avoid_unsafe_collection_methods — Check for emptiness before using first, last, single or reduce.
• prefer_safe_collection_access — list.first throws where list.head returns None.
• avoid_missing_enum_constant_in_map — Cover every enum constant in a map keyed by that enum.
• avoid_collection_equality_checks — Avoid comparing collections with == or != as it checks reference equality, not contents.