Mappings in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mapping in blockchain smart contracts?

A mapping is like a digital dictionary that links one value (a key) to another value. It helps store and find data quickly in smart contracts.

beginner

How do you declare a mapping in Solidity?

You write: mapping(KeyType => ValueType) public name; where KeyType is the type of the key and ValueType is the type of the value stored.

intermediate

Can you iterate over all keys in a mapping directly?

No, mappings do not store keys or allow iteration. You must keep a separate list of keys if you want to loop through them.

beginner

What happens if you access a mapping with a key that does not exist?

You get the default value for the value type (like 0 for numbers or false for booleans), because mappings always return a value even if the key was never set.

beginner

Why are mappings useful in blockchain smart contracts?

They let you store and quickly find data linked by keys, like user balances or permissions, without using lots of storage or complex searching.

How do you declare a mapping from addresses to unsigned integers in Solidity?

A mapping(uint, address) public balances;

B mapping(uint => address) public balances;

C mapping(address => uint) public balances;

D mapping(address, uint) public balances;

The correct syntax uses '=>' to link key type to value type: mapping(address => uint).

What value do you get when accessing a mapping with a key that was never set?

A An error is thrown

B The default value of the value type

C Null

D Random data

Mappings return the default value for the value type if the key does not exist.

Can you loop through all keys in a mapping directly?

A No, mappings do not store keys for iteration

B Yes, mappings support iteration

C Only if keys are numbers

D Only in test environments

Mappings do not keep track of keys, so you cannot loop through them directly.

What is a common use of mappings in smart contracts?

A Running loops

B Creating user interfaces

C Compiling code

D Storing user balances

Mappings are often used to store balances or permissions linked to user addresses.

Which of these is NOT a valid key type for a mapping in Solidity?

A mapping

B uint

C string

D address

Mappings cannot have another mapping as a key type.

Explain what a mapping is and how it works in blockchain smart contracts.

Think of it like a digital dictionary that stores data linked by keys.

Describe how you would store and retrieve user balances using a mapping in Solidity.

Focus on how keys and values relate and how to get a balance.

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