Mappings in Blockchain / Solidity - Mini Project: Build & Apply

Understanding Mappings in Solidity

📖 Scenario: You are creating a simple blockchain contract to store and retrieve user balances. This contract uses a mapping to link each user's address to their balance.

🎯 Goal: Build a Solidity contract that uses a mapping to store balances for addresses, set a balance for one user, and then retrieve and display that balance.

📋 What You'll Learn

Create a mapping named balances that links address to uint

Create a uint variable named initialBalance and set it to 100

Write a function setBalance that sets the initialBalance for the sender's address in balances

Write a function getBalance that returns the balance of the sender's address

Print the balance of the sender after setting it

💡 Why This Matters

🌍 Real World

Mappings are used in blockchain contracts to store data linked to user addresses, like balances, permissions, or votes.

💼 Career

Understanding mappings is essential for blockchain developers to manage user data efficiently and securely in smart contracts.

1

Create the mapping to store balances

Create a mapping named balances that links address to uint inside a contract named BalanceTracker.

Blockchain / Solidity

contract BalanceTracker {
    // Your code here
}

Hint

Think of mapping as a dictionary where each address has a number (uint) balance.

2

Add an initial balance variable

Inside the BalanceTracker contract, create a uint variable named initialBalance and set it to 100.

Blockchain / Solidity

contract BalanceTracker {
    mapping(address => uint) public balances;
    // Your code here
}

Hint

This variable will hold the starting balance for users.

3

Write a function to set the sender's balance

Add a public function named setBalance inside BalanceTracker that sets the sender's balance in balances to initialBalance.

Blockchain / Solidity

contract BalanceTracker {
    mapping(address => uint) public balances;
    uint initialBalance = 100;

    // Your code here
}

Hint

Use msg.sender to get the address calling the function.

4

Write a function to get the sender's balance and print it

Add a public view function named getBalance inside BalanceTracker that returns the sender's balance from balances. Then, write a comment showing how to call setBalance and getBalance to print the balance.

Blockchain / Solidity

contract BalanceTracker {
    mapping(address => uint) public balances;
    uint initialBalance = 100;

    function setBalance() public {
        balances[msg.sender] = initialBalance;
    }

    // Your code here
}

Hint

The view keyword means the function does not change the contract state.

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