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_TypeError (type 'List<dynamic>' is not a subtype of type 'List<int?>') Flutter

I’m using the json decoder on a List<List<int?>> json, and I’m having trouble converting the List<dynamic> to the List<List<int?>> I’d like to end up with. Is there a simple way to do this?

final jsonString = await rootBundle.loadString('lib/Permutations/$filename.json');
final jsonData = await json.decode(jsonString);
final data = List<List<int?>>.from(jsonData);

(TypeError (type ‘List<dynamic>’ is not a subtype of type
‘List<int?>’)

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2

Answers


  1. 0

    seems that jsonData is List<dynamic>, while you are using that list of dynamic to build a two dimensional list of integers.

    so, you have two options either

    • casting that List<dynamic> to List<int?>

    final jsonData = await json.decode(jsonString) as List<int?>;

    Note : it’s not guaranteed to work.

    • build that list as it’s

    final data = List<List>.from(jsonData);

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  2. 0

    The example below only works for the specified json format shown in jsonString., but the code can easily be modified for other formats.

    import 'dart:convert';

void main() {
  // sample data, substitute for your code
  // final jsonString = await rootBundle.loadString('lib/Permutations/$filename.json');
  String jsonString = '[[1,2,3,4,5],[6,7,8,9,10],[11,12,13,14,15]]';
  
  // decode same as before (no need for await)
  final jsonData = json.decode(
    jsonString,
  );

  // map data
  final data = (jsonData as List).map(
    // function runs for each array in the outer array
    // return the correct data type in this case List<int>
    (e) {
      return List<int>.from(
        e,
      );
    },

    // convert to list from iterable
  ).toList();

  print(
    data.runtimeType,
  );
  print(
    data,
  );

}
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