How to join the values in-between two values in a list

Question

I have this list [null, 3, 5, null] and I want to join the values in-between nulls and put it into the same list For example:

[null, 3, 5, null] into [null, 35, null]

I made this one extension that groups all the values between two nulls

extension GroupByNull<T> on List<T> {
  List<List<T>> groupByNull() {
    final groupedList = <List<T>>[];
    var list = this;

    for (var i = 0; i < list.length; i++) {
      if (list[i] == null) {
        if (i > 0) {
          groupedList.add(list.sublist(0, i));
        }
        list = list.sublist(i + 1);
        i = 0;
      }
    }

    if (list.isNotEmpty) {
      groupedList.add(list);
    }

    return groupedList;
  }
}

Which returns [[3, 5]] for [null, 3, 5, null]... But I want it to be joined together and added to the original list in the same index

How can I solve this... thank you.

Answer 1

Note that you can't operate on an arbitrary List<T> because there's no general way to combine two elements of some arbitrary type T. What you want could make sense for List<int?> or maybe List<String?>. (Or maybe it could work on an arbitrary List<T> input if you want a List<String?> output.)

Assuming that you want to operate on List<int?>, then basically as you iterate over your input list, keep track of your current accumulated value. If you encounter null, add the current value (if any) to your output List along with the null. Don't forget to add the current accumulated value (if any) when you're done iterating in case there isn't a final null element.

extension GroupByNull on List<int?> {
  List<int?> groupByNull() {
    var result = <int?>[];
    int? currentValue;
    for (var element in this) {
      if (element != null) {
        currentValue = (currentValue ?? 0) * 10 + element;
      } else {
        if (currentValue != null) {
          result.add(currentValue);
          currentValue = null;
        }
        result.add(currentValue);
      }
    }

    if (currentValue != null) {
      result.add(currentValue);
    }
    return result;
  }
}

void main() {
  print([null, 3, 5, null].groupByNull()); // Prints: [null, 35, null]

  print([3, 5, null].groupByNull()); // Prints: [35, null]
  print([3, 5, null, null].groupByNull()); // Prints: [35, null, null]
  print([null, 3, 5].groupByNull()); // Prints: [null, 35]
  print([null, null, 3, 5].groupByNull()); // Prints: [null, null, 35]
  print([null, 0, 0, 0, null].groupByNull()); // Prints: [null, 0, null]
  print([null, 1, 2, null, 3, 4, null]
      .groupByNull()); // Prints: [null, 12, null, 34, null]
  print([null].groupByNull()); // Prints: [null]
  print([null, null].groupByNull()); // Prints: [null, null]}
}