Trouble accessing the object's array inside the array and their size

Question

I have a quick question and it's an easy one. Please somebody help.

I am an absolute beginner in flutter. I am having trouble accessing the object inside the list and their size. Here is a quick example of what I am trying to achieve.

final questions = [
    {
      'questionText': 'What's your favorite color?',
      'answers': [
        {'text': 'Black', 'score': 3},
        {'text': 'Blue', 'score': 1},
        {'text': 'White', 'score': 1}
      ]
    },
    {
      'questionText': 'What's your favorite animal?',
      'answers': [
        {'text': 'Lion', 'score': 3},
        {'text': 'Snake', 'score': 5},
        {'text': 'Cat', 'score': 1}
      ]
    },
    {
      'questionText': 'What's your favorite teacher?',
      'answers': [
        {'text': 'Ali', 'score': 3},
        {'text': 'Aslam', 'score': 3},
        {'text': 'Akram', 'score': 3}
      ]
    }
  ];


  print(questions.length);    // 3
  print(questions[0].length);    // 2
print(questions[0]['answers']);   //[{text: Black, score: 3}, {text: Blue, score: 1}, {text: White, score: 1}]

But when I tried to get the length of "answers" within the questions by following below method. I get this error (given inside comments).

print(questions[0]['answers'].length);  //Error: The getter 'length' isn't defined for the class 'Object'.

When I tried to print the first element of 'answers' array within the questions (which is "{'text': 'Black', 'score': 3}"), I get the below error

print(questions[0]['answers'][0]); // The method '[]' isn't defined for the class 'Object'

Also tried these

print(questions[0]['answers']['text']); // Error: The method '[]' isn't defined for the class 'Object'.
print(questions[0]['answers'[0]]);  // null
print(questions[0]['answers'['text']]); // Error: The argument type 'String' can't be assigned //to the parameter type 'int'.

How can I do this? Also, what should I do if I only want to print all three options('text') of 'answers'. I know it's a kinda dumb question but I am completely stuck here. Any help would highly be appreciated.

Felix · Accepted Answer · 2019-11-24 19:24:11Z

By the notation you're using Dart infers the types to a list of maps with string keys but mixed value types (questionText is string, answers is a list of maps again). That forces Dart to use Object as value type, what makes accessing nested fields impossible without casting.

questions[0].runtimeType // Type (_InternalLinkedHashMap<String, Object>)

You can also see that easily in the debugger by inspecting the questions object.

I suggest defining classes with properly typed properties.

class Question {
  Question(this.text, this.answers);

  final String text;

  final List<Answer> answers;

  String toString() => 'Question { text: $text, answers: $answers }';
}

class Answer {
  Answer(this.text, this.score);

  final String text;

  final int score;

  String toString() => 'Answer { text: $text, score: $score }';
}

main(List<String> args) {
  final questions = [
    Question('What\'s your favorite color?', [
      Answer('Black', 3),
      Answer('Blue', 1),
      Answer('White', 1),
    ]),
    Question('What\'s your favorite animal?', [
      Answer('Lion', 3),
      Answer('Snake', 5),
      Answer('Cat', 1),
    ]),
    Question('What\'s your favorite teacher?', [
      Answer('Ali', 3),
      Answer('Aslam', 3),
      Answer('Akram', 3),
    ]),
  ];

  print(questions.length); // 3
  print(questions[0].text); // What's your favorite color?
  print(questions[0].answers); // [Answer { text: Black, score: 3 }, Answer { text: Blue, score: 1 }, Answer { text: White, score: 1 }]
  print(questions[0].answers.length); // 3
  questions[0].answers.forEach((answer) => print(answer.text)); // print all three options('text') of 'answers'
  print(questions[0].answers[0]); // Answer { text: Black, score: 3 }
  print(questions[0].answers[0].text); // Black
  print(questions[0].answers[0].score); // 3
}

SUX · Accepted Answer · 2019-11-24 19:48:25Z

2

You can use this trick.

var answers = questions[0]['answers'] as List<Map<String, Object>>;
print('${answers.length}'); //3
print('${answers[0]}'); //{text: Black, score: 3}

Or one line.

print('${(questions[0]['answers'] as List<Map<String, Object>>).length}'); //3

answered Nov 24, 2019 at 19:48

SUX

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1 Comment

Faizan Kamal Over a year ago

That's helpful indeed. Thanks @suhaib

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