I have some data in this form as below located at the start of the file

const _questions = [
  {
    'question': 'How long is New Zealand’s Ninety Mile Beach?',
    'answers': [
      '88km, so 55 miles long.',
      '55km, so 34 miles long.',
      '90km, so 56 miles long.'
    ],
    'answer': 1,
  },
  {
    'question':
        'In which month does the German festival of Oktoberfest mostly take place?',
    'answers': ['January', 'October', 'September'],
    'answer': 2,
  },
  {
    'question': 'Who composed the music for Sonic the Hedgehog 3?',
    'answers': [
      'Britney Spears',
      'Timbaland',
      'Michael Jackson',
    ],
    'answer': 1,
  },
]

I have a class like this

class QuestionNumber with ChangeNotifier {
List<int> answerlist= [1, 2, 1];
}

I want to initialise the list to have all of the 'answer' numbers so that when that the list is initialised to List answerlist= [];

Thanks for any help!

CodePudding user response：

You need a model class to handle this(easy approach), and in some cases you don't have answer filed, for that I'm using default value 0,

Model Class

class Question {
  final String question;
  final List<String> answers;
  final int answer;
  Question({
    required this.question,
    required this.answers,
    required this.answer,
  });

  Map<String, dynamic> toMap() {
    return {
      'question': question,
      'answers': answers,
      'answer': answer,
    };
  }

  factory Question.fromMap(Map<String, dynamic> map) {
    return Question(
      question: map['question'] ?? '',
      answers: List<String>.from(map['answers']),
      answer: map['answer']?.toInt() ?? 0,
    );
  }

  String toJson() => json.encode(toMap());

  factory Question.fromJson(String source) =>
      Question.fromMap(json.decode(source));
}

It is more like parsing json

Then do like

  List<Question> questionlist =
      _questions.map((q) => Question.fromMap(q)).toList();
  List<int> answerlist = [];
  for (final q in questionlist) {
    answerlist.add(q.answer);
  }
  

Check on dartPad

CodePudding user response：

check this below it may help you,

DataModel,

class QuestionsModel {
  List<Lists> lists;

  QuestionsModel({this.lists});

  QuestionsModel.fromJson(Map<String, dynamic> json) {
    if (json['lists'] != null) {
      lists = new List<Lists>();
      json['lists'].forEach((v) {
        lists.add(new Lists.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    if (this.lists != null) {
      data['lists'] = this.lists.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Lists {
  String question;
  List<String> answers;
  int answer;

  Lists({this.question, this.answers, this.answer});

  Lists.fromJson(Map<String, dynamic> json) {
    question = json['question'];
    answers = json['answers'].cast<String>();
    answer = json['answer'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['question'] = this.question;
    data['answers'] = this.answers;
    data['answer'] = this.answer;
    return data;
  }
}

QuestionsModel questionsModel = new QuestionsModel.fromJson({
    "lists": [
      {
        "question": "How long is New Zealand’s Ninety Mile Beach?",
        "answers": [
          "88km, so 55 miles long.",
          "55km, so 34 miles long.",
          "90km, so 56 miles long."
        ],
        "answer": 1
      },
      {
        "question":
            "In which month does the German festival of Oktoberfest mostly take place?",
        "answers": ["January", "October", "September"],
        "answer": 2
      },
      {
        "question": "Who composed the music for Sonic the Hedgehog 3?",
        "answers": ["Britney Spears", "Timbaland", "Michael Jackson"],
        "answer": 1
      }
    ]
  });

To get List of answer number from ClassModel,

var answers = questionsModel.lists.map((e) => e.answer);
print("Answers Number : ${answers}"); // 1, 2, 1

To get object value from Class using list builder,

ListView.builder(
  itemCount: questionsModel.lists.length,
  itemBuilder: (BuildContext context, int index) {
    return ListTile(
      title: Text(questionsModel.lists[index].question), // access value like this 
    );
  },
)