How do I cast a multidimensional json array to a multidimensional dart list?

Question

I got a JSON file with a long list of data like this.

[
  [
    [6.25575, 51.83439],
    [6.26408, 51.83342]
  ],
  [
    [6.25575, 51.83439],
    [6.26408, 51.83342]
  ]
]

When I write:

final List<List<List<double>>> data = await getJson('assets/data.json') as List<List<List<double>>>;

It gives the error: _CastError (type 'List<dynamic>' is not a subtype of type 'List<List<List<double>>>' in type cast).

getJson function:

Future<dynamic> getJson(String file) async {
  final String jsonData = await rootBundle.loadString(file);
  final dynamic data = await jsonDecode(jsonData);

  return data;
}

How can a cast this list of lists to a List<List<List<double>>>?

Victor Eronmosele · Accepted Answer · 2021-01-10 00:12:22Z

2

You need to cast the different Lists individually like below:

final List<List<List<double>>> data = (await getJson('assets/data.json') as List).map((e)=> (e as List).map((e) => (e as List).map((e) => e as double).toList()).toList()).toList();

answered Jan 10, 2021 at 0:12

Victor Eronmosele

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1 Comment

lrn Over a year ago

Alternative syntax for the same thing: var data = [for (var l1 in getJson('assets/data.json)) [for (var l2 in l1) [ for (var v3 in l2) v3 as double]]];.

julemand101 · Accepted Answer · 2021-01-10 00:13:42Z

I don't know if this solution is more readable than the one Victor Eronmosele did come up with but since I ended up making it I will just post it. But the principle is the same. Just another syntax for creating this new list object.

import 'dart:convert';

void main() {
  const input = '''
[
  [
    [6.25575, 51.83439],
    [6.26408, 51.83342]
  ],
  [
    [6.25575, 51.83439],
    [6.26408, 51.83342]
  ]
]
''';

  final typedList = fromJson(jsonDecode(input) as List<dynamic>);
  print(typedList); // [[[6.25575, 51.83439], [6.26408, 51.83342]], [[6.25575, 51.83439], [6.26408, 51.83342]]]
  print(typedList.runtimeType); // List<List<List<double>>>
}

List<List<List<double>>> fromJson(List<dynamic> list1) => [
      for (final list2 in list1)
        [
          for (final list3 in list2)
            [for (final number in list3) number as double]
        ]
    ];

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