Transpose a 2D rectangle Dart list

transpose_list.dart

List<List<R>> transposeList<R>(List<List<R>> input) {
  List<List<R>> output = [];

  // Check for rectangle matrix
  if (input.any((element) => element.length != input[0].length)) {
    throw FormatException('Not a rectangle Matrix');
  }

  for (int i = 0; i < input[0].length; i++) {
    output.add(List<R>.generate(input.length, (idx) => null));
  }

  for (int i = 0; i < input.length; i++) {
    List<R> column = input[i];
    for (int j = 0; j < input[0].length; j++) {
      R rowItem = column[j];
      output.elementAt(j).removeAt(i);
      output.elementAt(j).insert(i, rowItem);
    }
  }

  return output;
}