Mappings in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a mapping from address to uint.

Blockchain / Solidity

mapping(address => uint) public [1];

Drag options to blanks, or click blank then click option'

Aaccounts

Bbalance

Cbalances

Dusers

Attempts:

3 left

💡 Hint

Common Mistakes

Using singular names like 'balance' which is less common.

Using unrelated names like 'users' which is vague.

The mapping name is usually plural to represent multiple entries. balances is the common name for mapping addresses to uint values.

2fill in blank

medium

Complete the code to assign 100 to the sender's balance in the mapping.

Blockchain / Solidity

[1][msg.sender] = 100;

Drag options to blanks, or click blank then click option'

Abalances

Baddress

Cmapping

Dmsg

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'address' or 'msg' which are not mapping variables.

Trying to assign directly to 'mapping' keyword.

You assign a value to a key in the mapping by using the mapping name, here balances, with the key msg.sender.

3fill in blank

hard

Fix the error in the code to correctly check if an address has a balance greater than zero.

Blockchain / Solidity

if ([1][user] > 0) {
    // do something
}

Drag options to blanks, or click blank then click option'

Abalance

Baddress

Cmapping

Dbalances

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'balance' which is undefined.

Using 'mapping' or 'address' which are keywords or types, not variables.

The mapping variable name is balances. Using balances[user] accesses the stored value.

4fill in blank

hard

Fill both blanks to create a mapping from address to bool and set the sender's value to true.

Blockchain / Solidity

mapping([1] => [2]) public approved;
approved[msg.sender] = true;

Drag options to blanks, or click blank then click option'

Aaddress

Bbool

Cuint

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using uint or string as value type instead of bool.

Swapping key and value types.

The mapping key type is address and the value type is bool to store true/false values.

5fill in blank

hard

Fill all three blanks to create a mapping from uint to string, assign a value, and retrieve it.

Blockchain / Solidity

mapping([1] => [2]) public names;
names[[3]] = "Alice";
string memory name = names[1];

Drag options to blanks, or click blank then click option'

Auint

Bstring

C1

Daddress

Attempts:

3 left

💡 Hint

Common Mistakes

Using address as key type here instead of uint.

Using wrong key value like address or 0.

The mapping key is uint, the value is string, and the key used to assign is 1.

Practice

1. What is the primary purpose of mapping in blockchain smart contracts?

easy

A. To create user interfaces

B. To store a list of all keys used

C. To perform complex mathematical calculations

D. To connect keys to values for fast data lookup

Solution

1. Step 1: Understand the role of mapping

   Mappings are used to link a key to a value, allowing quick access to data based on the key.

2. Step 2: Compare options

   Only To connect keys to values for fast data lookup correctly describes this purpose; others describe unrelated functions.

3. Final Answer:

   To connect keys to values for fast data lookup -> Option D

4. Quick Check:

   Mapping = key-value fast lookup [OK]

Hint: Mappings link keys to values quickly [OK]

Common Mistakes:

• Thinking mappings store all keys explicitly
• Confusing mappings with arrays or lists
• Assuming mappings perform calculations

2. Which of the following is the correct syntax to declare a mapping from address to uint in Solidity?

easy

A. mapping(address, uint) balances;

B. mapping(address => uint) public balances;

C. mapping[address => uint] balances;

D. mapping(address => uint) balances()

Solution

1. Step 1: Recall Solidity mapping syntax

   The correct syntax uses parentheses with key type, arrow, and value type: mapping(keyType => valueType) visibility name;

2. Step 2: Check each option

   mapping(address => uint) public balances; matches the correct syntax. Options A, C, and D have syntax errors like using commas instead of =>, wrong brackets, or function-like declaration.

3. Final Answer:

   mapping(address => uint) public balances; -> Option B

4. Quick Check:

   Correct mapping syntax uses (key => value) [OK]

Hint: Use parentheses and arrow for mapping syntax [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Missing visibility keyword like public
• Using commas instead of =>

3. Consider the Solidity code:

mapping(address => uint) public balances;

function setBalance(address user, uint amount) public {
  balances[user] = amount;
}

function getBalance(address user) public view returns (uint) {
  return balances[user];
}

// After calling setBalance(0x1234..., 50), what does getBalance(0x1234...) return?

medium

A. 50

B. Error

C. Undefined

D. 0

Solution

1. Step 1: Understand setBalance effect

   The function sets the balance of the given address to the specified amount, here 50.

2. Step 2: Understand getBalance output

   Calling getBalance with the same address returns the stored value, which is 50.

3. Final Answer:

   50 -> Option A

4. Quick Check:

   Set then get returns stored value [OK]

Hint: Set value then get returns that value [OK]

Common Mistakes:

• Assuming default zero is returned after setting
• Thinking mapping keys are enumerable
• Expecting an error for unset keys

4. What is wrong with this Solidity code snippet?

mapping(address => uint) balances;

function addBalance(address user, uint amount) public {
  balances[user] += amount;
}

function getBalance(address user) public view returns (uint) {
  return balances[user];
}

medium

A. The mapping is missing initialization before use

B. The mapping keys are not stored, so balances[user] always returns zero

C. There is no error; the code works as expected

D. The mapping is not declared public, so balances cannot be read

Solution

1. Step 1: Check mapping declaration

   Mapping does not need explicit initialization; default values are zero.

2. Step 2: Check function logic

   addBalance adds amount to the current balance; getBalance returns it. Not declaring public means no automatic getter, but manual getter exists.

3. Final Answer:

   There is no error; the code works as expected -> Option C

4. Quick Check:

   Mapping defaults and manual getter work fine [OK]

Hint: Manual getter works even if mapping not public [OK]

Common Mistakes:

• Assuming mapping must be public to read values
• Thinking mappings need explicit initialization
• Confusing mapping keys storage with values

5. You want to track ownership of unique items by users using mappings in Solidity. Which approach correctly allows you to check if a user owns a specific item ID with a direct lookup starting from the user address?

hard

A. mapping(address => mapping(uint => bool)) ownsItem; // nested mapping user to item ID to ownership

B. mapping(uint => address) itemToOwner; // maps item ID to owner address

C. mapping(address => uint) ownerToItem; // stores one item per user

D. mapping(address => uint[]) userItems; // mapping to array of item IDs

Solution

1. Step 1: Understand ownership tracking needs

   We need to check if a user owns a specific item ID, so a direct boolean check is best.

2. Step 2: Analyze options

   Only mapping(address => mapping(uint => bool)) ownsItem; // nested mapping user to item ID to ownership provides a nested mapping(address => mapping(uint => bool)) ownsItem, allowing direct ownsItem[user][itemID] lookup from the user address. A limits to one item per user; B requires starting from item ID; D requires searching an array.

3. Final Answer:

   mapping(address => mapping(uint => bool)) ownsItem; // nested mapping user to item ID to ownership -> Option A

4. Quick Check:

   Nested mapping allows direct ownership check [OK]

Hint: Use nested mapping for user-item ownership check [OK]

Common Mistakes:

• Using single mapping limiting one item per user
• Using arrays which are not mappings
• Confusing item-to-owner with user-to-item ownership check