State variables in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity contract's state variable?

Consider the following Solidity contract. What will be the value of count after calling increment() twice?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Counter {
    uint public count = 0;

    function increment() public {
        count += 1;
    }
}

A 0

B 2

C 1

D Compilation error

💡 Hint

State variables keep their value between function calls.

The count variable starts at 0 and increases by 1 each time increment() is called. After two calls, it becomes 2.

🧠 Conceptual

intermediate

Which statement about state variables in Solidity is true?

Choose the correct statement about state variables in Solidity.

A State variables are stored in memory and lost after function execution.

B State variables are only accessible inside the constructor.

C State variables are stored on the blockchain and persist between transactions.

D State variables cannot be public.

💡 Hint

Think about where data is saved permanently in a smart contract.

State variables are stored on the blockchain and keep their values between transactions.

🔧 Debug

advanced

Why does this contract fail to update the state variable?

Examine the contract below. Why does calling setValue(10) not change the value state variable?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint value;

    function setValue(uint value) public {
        value = value;
    }

    function getValue() public view returns (uint) {
        return value;
    }
}

A The function should be marked as view to modify state variables.

B The state variable is private and cannot be changed.

C The contract is missing a constructor to initialize the variable.

D The function parameter shadows the state variable, so assignment does nothing.

💡 Hint

Look carefully at the variable names inside the function.

The function parameter value has the same name as the state variable, so value = value; assigns the parameter to itself, leaving the state variable unchanged.

📝 Syntax

advanced

Which option correctly declares a public state variable with an initial value?

Choose the correct Solidity code snippet that declares a public uint state variable named totalSupply initialized to 1000.

A uint public totalSupply = 1000;

B public uint totalSupply = 1000;

C uint totalSupply public = 1000;

D uint totalSupply = 1000 public;

💡 Hint

Remember the order of visibility and type in Solidity variable declarations.

The correct syntax is uint public totalSupply = 1000;. Visibility comes after the type.

🚀 Application

expert

What is the value of the state variable after multiple transactions?

Given the contract below, what will be the value of balance after the following sequence of calls?

1. Call deposit(50)
2. Call withdraw(20)
3. Call deposit(30)

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Wallet {
    uint public balance;

    function deposit(uint amount) public {
        balance += amount;
    }

    function withdraw(uint amount) public {
        if (amount <= balance) {
            balance -= amount;
        }
    }
}

A 60

B 100

C 80

D 50

💡 Hint

Add deposits and subtract withdrawals carefully.

Starting from 0, deposit 50 → 50, withdraw 20 → 30, deposit 30 → 60.