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.