State variables in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a state variable in blockchain smart contracts?

A state variable is a variable whose value is permanently stored on the blockchain. It keeps data between function calls and transactions.

beginner

Where are state variables stored in a blockchain smart contract?

State variables are stored in the contract's storage on the blockchain, making their values persistent and shared across all users.

beginner

How do state variables differ from local variables in smart contracts?

State variables are stored permanently on the blockchain, while local variables exist only temporarily during function execution and are not saved.

intermediate

Why should you be careful when updating state variables in smart contracts?

Updating state variables costs gas (transaction fees) because it changes blockchain storage. Minimizing updates saves cost and improves efficiency.

beginner

Give an example of declaring a state variable in Solidity.

Example:

uint public count;

This declares a state variable named count of type unsigned integer, stored on the blockchain.

What happens to a state variable's value after a smart contract function finishes execution?

A It is saved permanently on the blockchain.

B It is deleted immediately.

C It resets to zero.

D It becomes a local variable.

State variables keep their values permanently on the blockchain after function execution.

Which of these variables is stored temporarily during function execution?

A State variable

B Local variable

C Global variable

D Constant variable

Local variables exist only during function execution and are not saved on the blockchain.

Why does updating a state variable cost gas in Ethereum?

A Because it changes the blockchain storage.

B Because it sends tokens.

C Because it calls another contract.

D Because it reads data.

Changing state variables modifies blockchain storage, which requires gas fees.

Which keyword is used to declare a state variable in Solidity?

A uint

B storage

C memory

D public

The visibility keyword like public is used with the type to declare a state variable accessible outside the contract.

Where are state variables stored?

A In temporary memory during execution

B In the user's local machine

C In the contract's storage on the blockchain

D In the transaction logs only

State variables are stored in the contract's persistent storage on the blockchain.

Explain what state variables are and why they are important in blockchain smart contracts.

Think about how data stays saved between transactions.

Describe the difference between state variables and local variables in smart contracts.

Consider where each variable lives and how long it lasts.

Practice

1. What is a state variable in a blockchain smart contract?

easy

A. A variable that stores data permanently on the blockchain

B. A temporary variable used only inside functions

C. A variable that stores user input off-chain

D. A variable that resets every time a function runs

Solution

1. Step 1: Understand what state variables do

   State variables store data permanently on the blockchain, unlike local variables which are temporary.

2. Step 2: Compare options with definition

   Only A variable that stores data permanently on the blockchain describes permanent storage on the blockchain, which matches the definition of state variables.

3. Final Answer:

   A variable that stores data permanently on the blockchain -> Option A

4. Quick Check:

   State variable = permanent blockchain storage [OK]

Hint: State variables save data permanently on blockchain [OK]

Common Mistakes:

• Confusing state variables with local variables
• Thinking state variables reset after function ends
• Assuming state variables store data off-chain

2. Which of the following is the correct way to declare a state variable in Solidity?

easy

A. let count = uint256;

B. function uint256 count;

C. uint256 public count;

D. var count = 0;

Solution

1. Step 1: Recall Solidity syntax for state variables

   State variables are declared outside functions with a type and visibility, e.g., uint256 public count;.

2. Step 2: Check each option

   uint256 public count; matches correct syntax. Options A, B, and C use incorrect keywords or syntax not valid for state variables.

3. Final Answer:

   uint256 public count; -> Option C

4. Quick Check:

   State variable declaration = type + visibility + name [OK]

Hint: State variables need type and visibility outside functions [OK]

Common Mistakes:

• Using function keyword to declare variables
• Using var or let which are not Solidity keywords
• Declaring variables inside functions as state variables

3. Consider this Solidity contract snippet:

contract Counter {
  uint256 public count = 0;
  function increment() public {
    count += 1;
  }
}

What will be the value of count after calling increment() three times?

medium

A. 1

B. 3

C. 0

D. Error: count is not accessible

Solution

1. Step 1: Understand initial value and increment

   The state variable count starts at 0 and increases by 1 each time increment() is called.

2. Step 2: Calculate after three calls

   After three calls, count = 0 + 1 + 1 + 1 = 3.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   0 + 3 increments = 3 [OK]

Hint: Add 1 to count for each increment call [OK]

Common Mistakes:

• Forgetting count starts at 0
• Assuming count resets after each call
• Thinking count is private and inaccessible

4. Identify the error in this Solidity contract:

contract Example {
  uint256 count;
  function setCount(uint256 _count) public {
    uint256 count = _count;
  }
}

medium

A. Local variable shadows state variable inside function

B. State variable count is not declared

C. Function setCount has wrong return type

D. Missing visibility for state variable count

Solution

1. Step 1: Analyze variable declarations

   The state variable count is declared correctly. Inside setCount, a new local variable count is declared, which hides the state variable.

2. Step 2: Understand shadowing effect

   Assigning to local count does not change the state variable. This is a common mistake called variable shadowing.

3. Final Answer:

   Local variable shadows state variable inside function -> Option A

4. Quick Check:

   Local variable shadows state variable = bug [OK]

Hint: Avoid declaring local variable with same name as state variable [OK]

Common Mistakes:

• Thinking state variable is missing
• Ignoring variable shadowing
• Assuming function needs return type

5. You want to create a smart contract that stores a user's name permanently and allows updating it. Which of these is the best way to declare the state variable and update it?

hard

A. Declare string memory name; as state variable

B. Declare string name; inside the update function and assign new value there

C. Use a local variable string name; in constructor only

D. Declare string public name; as state variable and update it in a public function

Solution

1. Step 1: Understand state variable declaration for permanent storage

   State variables must be declared outside functions with proper type and visibility, e.g., string public name;.

2. Step 2: Update state variable in a public function

   To change the stored name, write a public function that assigns a new value to the state variable.

3. Final Answer:

   Declare string public name; as state variable and update it in a public function -> Option D

4. Quick Check:

   State variable + public update function = correct pattern [OK]

Hint: Declare state variable outside, update inside public function [OK]

Common Mistakes:

• Declaring variables inside functions only
• Using memory keyword for state variables
• Not providing visibility for state variables