Lists, Maps, and Sets in Flutter

Introduction

Lists, Maps, and Sets help you store and organize groups of data in your app. They make it easy to keep track of many items at once.

When you want to keep a list of tasks or names in order.

When you need to connect keys to values, like a phone book with names and numbers.

When you want to store unique items without duplicates, like a set of favorite colors.

Syntax

Flutter

List<String> fruits = ['apple', 'banana', 'orange'];

Map<String, int> scores = {'Alice': 90, 'Bob': 85};

Set<String> uniqueNames = {'Alice', 'Bob', 'Alice'};

Lists keep items in order and allow duplicates.

Maps store pairs of keys and values for quick lookup.

Sets store unique items without order.

Examples

This is an empty list. You can add items later.

Flutter

List<int> numbers = [];
// An empty list with no items

Maps connect keys to values. Here, countries to their capitals.

Flutter

Map<String, String> capitals = {'USA': 'Washington', 'France': 'Paris'};
// Map with country names as keys and capitals as values

Sets remove duplicates automatically.

Flutter

Set<String> letters = {'a', 'b', 'c', 'a'};
// Set will only keep unique letters: 'a', 'b', 'c'

Sample App

This Flutter app shows how to use a List, a Map, and a Set. It displays the fruits list, the scores map, and the unique names set on the screen.

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    List<String> fruits = ['Apple', 'Banana', 'Orange'];
    Map<String, int> scores = {'Alice': 90, 'Bob': 85};
    Set<String> uniqueNames = {'Alice', 'Bob', 'Alice'};

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Lists, Maps, and Sets')),
        body: Padding(
          padding: const EdgeInsets.all(16),
          child: Column(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: [
              Text('List of fruits: ' + fruits.join(', ')),
              const SizedBox(height: 10),
              Text('Scores Map:'),
              for (var entry in scores.entries) Text('${entry.key}: ${entry.value}'),
              const SizedBox(height: 10),
              Text('Unique names in Set: ' + uniqueNames.join(', ')),
            ],
          ),
        ),
      ),
    );
  }
}

Important Notes

Lists keep the order of items and allow duplicates.

Maps let you find values quickly using keys.

Sets automatically remove duplicate items.

Use Lists when order matters, Maps when you want key-value pairs, and Sets when you want unique items.

Summary

Lists store ordered items and allow duplicates.

Maps store key-value pairs for quick lookup.

Sets store unique items without order.

Practice

1. Which of the following correctly describes a Set in Flutter?

easy

A. A fixed-size list that cannot be changed.

B. A collection of key-value pairs for fast lookup.

C. An ordered list that allows duplicate items.

D. A collection of unique items without any specific order.

Solution

1. Step 1: Understand Set characteristics

   A Set stores unique items and does not maintain order.

2. Step 2: Compare with other collections

   Maps store key-value pairs, Lists allow duplicates and are ordered.

3. Final Answer:

   A collection of unique items without any specific order. -> Option D

4. Quick Check:

   Set = unique unordered collection [OK]

Hint: Remember: Sets have unique items, no order [OK]

Common Mistakes:

• Confusing Sets with Lists (which allow duplicates)
• Thinking Sets maintain insertion order
• Mixing up Maps and Sets

2. Which of the following is the correct syntax to create a Map in Flutter?

easy

A. var myMap = {'key1': 'value1', 'key2': 'value2'};

B. var myMap = Set('key1', 'value1');

C. var myMap = ['key1', 'value1', 'key2', 'value2'];

D. var myMap = {1, 2, 3};

Solution

1. Step 1: Identify Map syntax

   Maps use curly braces with key-value pairs separated by colons.

2. Step 2: Check each option

   var myMap = {'key1': 'value1', 'key2': 'value2'}; correctly uses {'key': 'value'} pairs. Others are list, set, or invalid syntax.

3. Final Answer:

   var myMap = {'key1': 'value1', 'key2': 'value2'}; -> Option A

4. Quick Check:

   Map syntax = {'key': 'value'} [OK]

Hint: Maps use curly braces with colon-separated pairs [OK]

Common Mistakes:

• Using commas instead of colons for key-value pairs
• Confusing list syntax with Map syntax
• Trying to create Map with Set syntax

3. What will be the output of this Flutter code?

var list = [1, 2, 3, 2];
var set = list.toSet();
print(set);

medium

A. {1, 2, 3}

B. [1, 2, 3, 2]

C. {1, 2, 3, 2}

D. [1, 2, 3]

Solution

1. Step 1: Convert list to set

   Calling toSet() removes duplicates and creates a Set.

2. Step 2: Understand Set output

   Set prints with curly braces and unique values only: {1, 2, 3}.

3. Final Answer:

   {1, 2, 3} -> Option A

4. Quick Check:

   List to Set removes duplicates [OK]

Hint: toSet() removes duplicates and changes brackets [OK]

Common Mistakes:

• Expecting list brackets [] instead of set braces {}
• Thinking duplicates remain after toSet()
• Confusing output format of Set vs List

4. Identify the error in this Flutter code snippet:

var map = {'a': 1, 'b': 2};
map.add('c', 3);

medium

A. Keys in Map must be integers, not strings.

B. The method add() does not exist for Map.

C. Map cannot have more than two key-value pairs.

D. The syntax for Map declaration is incorrect.

Solution

1. Step 1: Check Map methods

   Maps do not have an add() method; use map[key] = value to add entries.

2. Step 2: Validate other options

   Keys can be strings, Maps can have many pairs, and declaration syntax is correct.

3. Final Answer:

   The method add() does not exist for Map. -> Option B

4. Quick Check:

   Use map[key] = value to add entries [OK]

Hint: Use map[key] = value to add, not add() method [OK]

Common Mistakes:

• Trying to use add() on Map instead of assignment
• Thinking Map keys must be integers
• Assuming Map size is limited

5. Given this Flutter code, what will be the final content of result?

var list = [1, 2, 2, 3];
var map = {for (var i in list) i: i * 2};
var result = map.values.toSet().toList();
print(result);

hard

A. [1, 4, 6]

B. [1, 2, 3]

C. [2, 4, 6]

D. [2, 2, 4, 6]

Solution

1. Step 1: Create Map from list

   Map comprehension uses list items as keys and doubles as values. Duplicate keys overwrite, so map = {1:2, 2:4, 3:6}.

2. Step 2: Extract values and remove duplicates

   map.values = [2, 4, 6]. Converting to Set removes duplicates (none here), then toList() keeps order.

3. Final Answer:

   [2, 4, 6] -> Option C

4. Quick Check:

   Map keys unique, values doubled, duplicates removed [OK]

Hint: Map keys unique; duplicates overwrite values [OK]

Common Mistakes:

• Expecting duplicate keys to keep all entries
• Confusing keys and values in map comprehension
• Not converting Set back to List