Efficient data structures in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity mapping example?

Consider the following Solidity code snippet. What will be the value of balances[0x123] after execution?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    mapping(address => uint) public balances;

    function deposit() public {
        balances[msg.sender] += 100;
    }

    function test() public {
        deposit();
    }
}

A 100

B 0

C Compilation error

D Undefined

💡 Hint

Think about what happens when deposit() is called by the sender.

The deposit function adds 100 to the sender's balance. Calling deposit() inside test() increases balances[msg.sender] by 100. So balances[0x123] will be 100 if 0x123 calls test().

🧠 Conceptual

intermediate

Which data structure is best for constant time membership checks in smart contracts?

In blockchain smart contracts, which data structure allows you to check if an element exists in constant time (O(1))?

A Mapping

B Array

C Linked List

D Binary Tree

💡 Hint

Think about how mappings work in Solidity.

Mappings in Solidity provide O(1) access to values by keys, making membership checks very efficient compared to arrays or linked lists.

🔧 Debug

advanced

Why does this Solidity code cause a runtime error?

Examine the following Solidity code. Why does calling getValue(5) cause a runtime error?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint[] public values;

    constructor() {
        values.push(10);
        values.push(20);
    }

    function getValue(uint index) public view returns (uint) {
        return values[index];
    }
}

A Type error because values is not an array

B Compilation error due to missing return statement

C Index out of bounds error because index 5 does not exist

D No error, returns 0

💡 Hint

Check the size of the array and the index used.

The array 'values' has only 2 elements at indices 0 and 1. Accessing index 5 causes an out-of-bounds runtime error.

📝 Syntax

advanced

Which option correctly declares a fixed-size byte array in Solidity?

Select the correct syntax to declare a fixed-size byte array of length 4 in Solidity.

A byte[4] data;

B fixed bytes data = 4;

C bytes data[4];

D bytes4 data;

💡 Hint

Remember Solidity has a special type for fixed-size byte arrays.

In Solidity, 'bytes4' declares a fixed-size byte array of length 4.

🚀 Application

expert

How many items are stored after this Solidity struct array operation?

Given the following Solidity code, how many Person structs are stored in the people array after execution?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract PeopleList {
    struct Person {
        string name;
        uint age;
    }

    Person[] public people;

    function addPeople() public {
        people.push(Person("Alice", 30));
        people.push(Person("Bob", 25));
        people.pop();
        people.push(Person("Charlie", 20));
    }
}

A 1

B 2

C 3

D 0

💡 Hint

Remember what the pop() function does to the array.

The code pushes Alice and Bob, then removes the last element (Bob), then adds Charlie. So the array has Alice and Charlie, total 2.

Practice

1. Which data structure is best for quickly finding a user's balance by their blockchain address?

easy

A. Array

B. Mapping (key-value pairs)

C. Struct

D. Linked list

Solution

1. Step 1: Understand the need for quick lookup

   We want to find a balance by address fast, so we need a structure that supports direct access by key.

2. Step 2: Identify the best data structure

   Mappings provide key-value pairs allowing O(1) access by address, unlike arrays or structs which require searching.

3. Final Answer:

   Mapping (key-value pairs) -> Option B

4. Quick Check:

   Fast key-based lookup = Mapping [OK]

Hint: Use mappings for direct key lookups in blockchain [OK]

Common Mistakes:

• Choosing arrays which require looping to find an address
• Using structs alone without a key for lookup
• Thinking linked lists are efficient for random access

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) balances;

C. mapping[address] uint balances;

D. mapping{address: uint} balances;

Solution

1. Step 1: Recall Solidity mapping syntax

   Mappings use the syntax mapping(keyType => valueType) variableName;

2. Step 2: Match the correct syntax

   mapping(address => uint) balances; matches this exactly: mapping(address => uint) balances;

3. Final Answer:

   mapping(address => uint) balances; -> Option A

4. Quick Check:

   Correct mapping syntax uses '=>' [OK]

Hint: Remember mapping uses '=>' between key and value types [OK]

Common Mistakes:

• Using commas instead of '=>' in mapping
• Using square brackets or curly braces incorrectly
• Omitting the semicolon at the end

3. What will be the output of this Solidity code snippet?

struct User { uint id; string name; }
User[] users;
users.push(User(1, "Alice"));
users.push(User(2, "Bob"));
string memory name = users[1].name;

medium

A. "Alice"

B. Empty string

C. Compilation error

D. "Bob"

Solution

1. Step 1: Understand array indexing

   Arrays start at index 0, so users[0] is Alice, users[1] is Bob.

2. Step 2: Identify the accessed element

   The code accesses users[1].name, which is "Bob".

3. Final Answer:

   "Bob" -> Option D

4. Quick Check:

   Index 1 in array = "Bob" [OK]

Hint: Remember arrays start at zero index [OK]

Common Mistakes:

• Confusing index 1 with index 0
• Assuming structs print as variable names
• Expecting compilation error due to string usage

4. Identify the error in this Solidity code snippet:

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

medium

A. Cannot use '+=' on mapping values

B. Function lacks visibility modifier

C. No initialization needed for mapping values

D. Mapping keys must be uint, not address

Solution

1. Step 1: Check mapping usage

   Mappings default to zero for uint values if key not set, so no initialization needed.

2. Step 2: Verify function and operation

   Using '+=' on balances[user] is valid; function has public visibility.

3. Final Answer:

   No initialization needed for mapping values -> Option C

4. Quick Check:

   Mapping uint defaults to 0, so '+=' works [OK]

Hint: Mapping values default to zero, no init needed [OK]

Common Mistakes:

• Thinking mapping values must be initialized before use
• Confusing visibility modifiers
• Assuming keys must be uint instead of address

5. You want to store user profiles with id, name, and balance, and quickly find a profile by id. Which data structure combination is most efficient in Solidity?

hard

A. Mapping from id to struct

B. Array of structs only

C. Struct with embedded array

D. Linked list of structs

Solution

1. Step 1: Analyze the need for quick lookup by id

   Quick lookup by id requires direct access, which arrays or linked lists cannot provide efficiently.

2. Step 2: Choose the best data structure

   Mapping from id to struct allows O(1) access to user profiles by id, combining grouping (struct) and fast lookup (mapping).

3. Final Answer:

   Mapping from id to struct -> Option A

4. Quick Check:

   Fast key access + grouped data = mapping to struct [OK]

Hint: Use mapping from id to struct for fast profile lookup [OK]

Common Mistakes:

• Using arrays which require looping to find id
• Using linked lists which are slow for random access
• Embedding arrays inside structs without mapping