Lists, Maps, and Sets in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flutter List display?

Consider this Flutter code snippet that displays a list of names in a ListView. What will be shown on the screen?

Flutter

List<String> names = ['Anna', 'Bob', 'Cara'];

ListView(
  children: names.map((name) => Text(name)).toList(),
)

A A single Text widget showing 'AnnaBobCara' concatenated

B An empty screen with no text

C A vertical list showing: Anna, Bob, Cara each on its own line

D A runtime error because ListView children must be widgets

💡 Hint

Think about how map and toList create widgets for each name.

The map function creates a Text widget for each name. The ListView shows these widgets vertically.

🧠 Conceptual

intermediate

How does a Dart Map store data?

In Dart, what is the main characteristic of a Map collection?

A It stores only integer values indexed by position

B It stores only unique values without keys

C It stores ordered elements like a list

D It stores key-value pairs where keys are unique and values can be any type

💡 Hint

Think about how you look up a phone number by a person's name.

A Map stores data as pairs: each key is unique and maps to a value.

📝 Syntax

advanced

What error does this Dart Set code cause?

What error will this Dart code produce? Set numbers = {1, 2, 3, 3, 4}; print(numbers.length);

Flutter

Set<int> numbers = {1, 2, 3, 3, 4};
print(numbers.length);

A Prints 4 because duplicates are ignored in sets

B Prints 5 because duplicates are allowed in sets

C SyntaxError because duplicate values are not allowed in set literals

D Runtime error because sets cannot contain integers

💡 Hint

Remember what a set means in math: no duplicates.

Sets automatically remove duplicates, so the length counts unique values only.

❓ lifecycle

advanced

How does Flutter rebuild widgets when a List changes?

If you update a List used in a Flutter widget's build method, what triggers the UI to update?

A Calling setState() to notify Flutter to rebuild the widget

B Changing the List automatically rebuilds the UI without extra code

C You must manually refresh the screen outside Flutter

D Flutter rebuilds only when the app restarts

💡 Hint

Think about how Flutter knows when to redraw the screen.

Flutter rebuilds widgets when setState() is called to notify changes.

🔧 Debug

expert

Why does this Flutter Map access cause an error?

Given this Dart Map: Map ages = {'Alice': 30, 'Bob': 25}; What error occurs when running: print(ages['Cara']!);

Flutter

Map<String, int> ages = {'Alice': 30, 'Bob': 25};
print(ages['Cara']!);

A Prints null because 'Cara' is not in the map

B Throws a runtime error because 'Cara' key does not exist and ! forces non-null

C Syntax error because ! cannot be used with map access

D Prints 0 because missing keys default to zero

💡 Hint

Consider what happens when you force a null value to be non-null.

The ! operator asserts the value is not null, but 'Cara' key is missing, so it throws an error.

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