Smart contract concept in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity function call?

Consider this Solidity smart contract snippet:

pragma solidity ^0.8.0;

contract SimpleStorage {
    uint storedData;

    function set(uint x) public {
        storedData = x;
    }

    function get() public view returns (uint) {
        return storedData;
    }
}

If we call set(42) and then get(), what will get() return?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleStorage {
    uint storedData;

    function set(uint x) public {
        storedData = x;
    }

    function get() public view returns (uint) {
        return storedData;
    }
}

A Transaction revert error

B 42

C Compilation error

D 0

💡 Hint

Think about what the set function does before calling get.

The set function stores the value 42 in storedData. Then get returns that stored value, so the output is 42.

🧠 Conceptual

intermediate

Which statement best describes a smart contract?

Choose the best description of a smart contract in blockchain technology.

A A centralized database storing user information securely.

B A legal contract signed digitally between two parties.

C A self-executing program that runs on a blockchain and enforces rules automatically.

D A cryptocurrency wallet used to store tokens.

💡 Hint

Think about what makes smart contracts different from traditional contracts.

Smart contracts are programs that run on blockchains and automatically enforce rules without intermediaries.

🔧 Debug

advanced

What error does this Solidity code produce?

Examine this Solidity function:

function divide(uint a, uint b) public pure returns (uint) {
    return a / b;
}

What happens if b is zero when calling divide(10, 0)?

Blockchain / Solidity

function divide(uint a, uint b) public pure returns (uint) {
    return a / b;
}

A Throws a division by zero runtime error and reverts the transaction

B Compiles with a warning but runs normally

C Returns 0

D Returns a large number due to overflow

💡 Hint

What happens in programming when you divide by zero?

Dividing by zero causes a runtime error in Solidity, which reverts the transaction.

📝 Syntax

advanced

Which option correctly declares a payable function in Solidity?

In Solidity, which function declaration allows the function to receive Ether?

A function deposit() public payable {}

B function deposit() public pay {}

C function deposit() public payable returns (uint) {}

D function deposit() public payable() {}

💡 Hint

Look for the correct keyword and syntax for payable functions.

The keyword payable must be used without parentheses to allow receiving Ether.

🚀 Application

expert

How many items are in the mapping after this transaction?

Given this Solidity contract snippet:

contract Registry {
    mapping(address => bool) public registered;

    function register() public {
        registered[msg.sender] = true;
    }
}

If three different users call register() once each, how many entries will registered contain?

A 1

B Cannot be determined

C 0

D 3

💡 Hint

Think about how mappings store data for unique keys.

Each unique address key set to true counts as one entry. Three users mean three entries.

Practice

1. What is a smart contract in blockchain technology?

easy

A. A type of cryptocurrency wallet

B. A program that runs on a blockchain to automate agreements

C. A physical contract signed by two parties

D. A blockchain mining algorithm

Solution

1. Step 1: Understand the definition of smart contracts

   Smart contracts are computer programs that run on blockchains to automate and enforce agreements without middlemen.

2. Step 2: Compare options with the definition

   Only A program that runs on a blockchain to automate agreements describes a program on blockchain automating agreements. Other options describe unrelated concepts.

3. Final Answer:

   A program that runs on a blockchain to automate agreements -> Option B

4. Quick Check:

   Smart contract = automated blockchain program [OK]

Hint: Smart contracts are code on blockchain, not physical papers [OK]

Common Mistakes:

• Confusing smart contracts with physical contracts
• Thinking smart contracts are wallets or mining tools
• Assuming smart contracts require middlemen

2. Which of the following is the correct way to start a simple smart contract in Solidity?

easy

A. contract MyContract { }

B. class MyContract { }

C. function MyContract() { }

D. module MyContract { }

Solution

1. Step 1: Recall Solidity syntax for defining contracts

   In Solidity, smart contracts start with the keyword contract followed by the contract name and braces.

2. Step 2: Check each option's syntax

   contract MyContract { } uses contract keyword correctly. Options A, B, and C use keywords from other languages or incorrect syntax.

3. Final Answer:

   contract MyContract { } -> Option A

4. Quick Check:

   Solidity contracts start with 'contract' keyword [OK]

Hint: Solidity contracts start with 'contract' keyword, not class or function [OK]

Common Mistakes:

• Using 'class' instead of 'contract' keyword
• Confusing functions with contract declarations
• Using module or other unrelated keywords

3. What will be the output of this Solidity function when called?

function getValue() public pure returns (uint) { return 5 + 3; }

medium

A. Error

B. 53

C. 8

D. 0

Solution

1. Step 1: Analyze the function logic

   The function returns the sum of 5 and 3, which is 8.

2. Step 2: Understand Solidity return behavior

   The function is marked pure and returns a uint, so it will output the calculated value 8 without errors.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add numbers inside function to get output quickly [OK]

Common Mistakes:

• Thinking '5 + 3' concatenates as '53'
• Expecting runtime error due to missing state
• Assuming default return is zero

4. Identify the error in this Solidity smart contract snippet:

contract Test {
  uint value;
  function setValue(uint _value) public {
    value = _value
  }
}

medium

A. Missing semicolon after assignment

B. Incorrect function visibility

C. Variable 'value' should be public

D. Function name should be 'set_value'

Solution

1. Step 1: Check syntax line by line

   The line value = _value is missing a semicolon at the end, which is required in Solidity.

2. Step 2: Verify other parts

   Function visibility 'public' is correct, variable can be private, and function naming is valid. So only missing semicolon is error.

3. Final Answer:

   Missing semicolon after assignment -> Option A

4. Quick Check:

   Semicolons end statements in Solidity [OK]

Hint: Check for missing semicolons after statements [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking function visibility is wrong
• Assuming variable must be public

5. You want to create a smart contract that stores user balances and allows deposits only if the amount is positive. Which Solidity code snippet correctly enforces this?

hard

A. function deposit(uint amount) public {
if(amount >= 0) {
balances[msg.sender] = amount;
}
}

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

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

D. function deposit(uint amount) public {
require(amount > 0, "Amount must be positive");
balances[msg.sender] += amount;
}

Solution

1. Step 1: Check input validation for positive amount

   function deposit(uint amount) public { require(amount > 0, "Amount must be positive"); balances[msg.sender] += amount; } uses require(amount > 0) to ensure only positive deposits are accepted.

2. Step 2: Compare other options for correctness

   function deposit(uint amount) public { if(amount >= 0) { balances[msg.sender] = amount; } } allows zero and overwrites balance instead of adding. function deposit(int amount) public { balances[msg.sender] += amount; } uses signed int allowing negative deposits. function deposit(uint amount) public { balances[msg.sender] += amount; } lacks any check.

3. Final Answer:

   function deposit(uint amount) public { require(amount > 0, "Amount must be positive"); balances[msg.sender] += amount; } -> Option D

4. Quick Check:

   Use require() to enforce positive deposits [OK]

Hint: Use require() to check conditions before updating state [OK]

Common Mistakes:

• Not validating input amounts
• Using signed integers for balances
• Overwriting balances instead of adding