State variables in Blockchain / Solidity

Introduction

State variables store information permanently on the blockchain. They keep data safe and shared for everyone using the program.

When you want to save a user's balance in a cryptocurrency app.

When you need to remember who owns a digital item in a game.

When you want to track votes in a blockchain voting system.

When you want to store contract settings that affect how it works.

Syntax

Blockchain / Solidity

type variableName;

// Example:
uint256 myNumber;

State variables are declared inside a contract but outside any function.

They are stored permanently on the blockchain and cost gas to change.

Examples

This declares a state variable named count that stores a number.

Blockchain / Solidity

contract Example {
    uint256 count;
}

This declares a state variable named name that stores text.

Blockchain / Solidity

contract Example {
    string name;
}

This declares a state variable named isActive that stores true or false.

Blockchain / Solidity

contract Example {
    bool isActive;
}

Sample Program

This contract has a state variable storedNumber that saves a number. The setNumber function changes it, and getNumber returns it.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract SimpleStorage {
    uint256 storedNumber;

    function setNumber(uint256 _number) public {
        storedNumber = _number;
    }

    function getNumber() public view returns (uint256) {
        return storedNumber;
    }
}

Important Notes

Changing state variables costs gas because it updates the blockchain.

Reading state variables (view functions) does not cost gas if called externally.

State variables keep their value between function calls and transactions.

Summary

State variables store data permanently on the blockchain.

They are declared inside contracts but outside functions.

Use them to remember important information for your blockchain app.

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