Constants and immutables in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constant and immutable variables in Solidity

What is the output of the following Solidity contract when calling getValues()?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    uint256 public constant CONST_VAL = 100;
    uint256 public immutable IMMUTABLE_VAL;

    constructor(uint256 _val) {
        IMMUTABLE_VAL = _val;
    }

    function getValues() public view returns (uint256, uint256) {
        return (CONST_VAL, IMMUTABLE_VAL);
    }
}

A (0, 200)

B (100, 200)

C (100, 0)

D (200, 100)

💡 Hint

Remember that constant variables are fixed at compile time, and immutable variables are set once during construction.

The constant variable CONST_VAL is set to 100 at compile time. The immutable variable IMMUTABLE_VAL is set once in the constructor to the passed value (200). So the function returns (100, 200).

🧠 Conceptual

intermediate

Understanding immutables in smart contracts

Which statement about immutable variables in Solidity is correct?

A Immutable variables can be changed multiple times after contract deployment.

B Immutable variables must be assigned a value at declaration and cannot be changed later.

C Immutable variables can be assigned once during contract construction and then cannot be changed.

D Immutable variables behave exactly like constant variables and are replaced at compile time.

💡 Hint

Think about when the value of an immutable variable is set.

Immutable variables are assigned once during the contract's constructor execution and cannot be changed afterward. This allows flexibility compared to constants, which must be assigned at compile time.

🔧 Debug

advanced

Identify the error with constant variable assignment

What error will the Solidity compiler produce for this code snippet?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Demo {
    uint256 public constant MY_CONST;

    constructor() {
        MY_CONST = 10;
    }
}

A TypeError: Constant variables must be assigned at declaration.

B SyntaxError: Missing semicolon after constant declaration.

C RuntimeError: Cannot assign to constant variable in constructor.

D No error, code compiles and runs fine.

💡 Hint

Check where constant variables must be assigned.

Constant variables must be assigned a value where they are declared. Assigning them in the constructor causes a compile-time error.

❓ Predict Output

advanced

Gas cost difference between constant and immutable

Given the following Solidity contract, which statement about gas cost when accessing CONST_VAL and IMMUTABLE_VAL is true?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract GasTest {
    uint256 public constant CONST_VAL = 123;
    uint256 public immutable IMMUTABLE_VAL;

    constructor() {
        IMMUTABLE_VAL = 456;
    }

    function readConst() public pure returns (uint256) {
        return CONST_VAL;
    }

    function readImmutable() public view returns (uint256) {
        return IMMUTABLE_VAL;
    }
}

A Reading CONST_VAL costs less gas than reading IMMUTABLE_VAL.

B Reading IMMUTABLE_VAL costs less gas than reading CONST_VAL.

C Reading CONST_VAL costs gas, but reading IMMUTABLE_VAL is free.

D Both CONST_VAL and IMMUTABLE_VAL cost the same gas to read.

💡 Hint

Consider where the values are stored and how the EVM accesses them.

Constant variables are replaced by their value at compile time and do not require storage access, so reading them costs less gas. Immutable variables are stored in storage slots and require a storage read, which costs more gas.

🧠 Conceptual

expert

Why use immutables instead of constants in Solidity?

Which is the best reason to use immutable variables instead of constant variables in Solidity?

A Immutable variables are stored in memory, constants are stored in storage.

B Immutable variables use less gas than constants when accessed.

C Immutable variables can be changed after deployment, constants cannot.

D Immutable variables allow setting values at runtime during contract deployment, while constants require compile-time values.

💡 Hint

Think about when you know the value you want to assign.

Immutable variables let you assign values during contract deployment (constructor), which is useful when the value depends on deployment parameters. Constants must have fixed values known at compile time.

Practice

1. What is the main difference between constant and immutable variables in a blockchain smart contract?

easy

A. Both constant and immutable variables can be changed after deployment.

B. constant variables can be changed anytime, immutable variables cannot be changed.

C. immutable variables are fixed at compile time, constant variables are set during contract creation.

D. constant variables are fixed at compile time, immutable variables are set once during contract creation.

Solution

1. Step 1: Understand constant variables

   constant variables are assigned a value at compile time and cannot be changed later.

2. Step 2: Understand immutable variables

   immutable variables are assigned once during contract creation (deployment) and then remain fixed.

3. Final Answer:

   constant fixed at compile time, immutable set once at creation -> Option D

4. Quick Check:

   constant = compile time, immutable = creation time [OK]

Hint: Constant fixed at compile, immutable set once at deployment [OK]

Common Mistakes:

• Thinking immutable can be changed after deployment
• Confusing when constants are assigned
• Assuming both can be modified anytime

2. Which of the following is the correct way to declare a constant integer named MAX_SUPPLY with value 1000 in Solidity?

easy

A. constant uint MAX_SUPPLY = 1000;

B. uint constant MAX_SUPPLY = 1000;

C. uint MAX_SUPPLY constant = 1000;

D. constant MAX_SUPPLY uint = 1000;

Solution

1. Step 1: Recall Solidity constant syntax

   In Solidity, the type comes first, then the keyword constant, then the variable name.

2. Step 2: Check each option

   uint constant MAX_SUPPLY = 1000; matches the correct syntax: uint constant MAX_SUPPLY = 1000;. Others have wrong order.

3. Final Answer:

   uint constant MAX_SUPPLY = 1000; -> Option B

4. Quick Check:

   Type + constant + name = correct order [OK]

Hint: Type first, then constant keyword, then variable name [OK]

Common Mistakes:

• Placing constant before type
• Putting constant after variable name
• Missing semicolon at end

3. Consider the Solidity contract snippet:

contract Example {
    uint public immutable creationTime;
    constructor() {
        creationTime = block.timestamp;
    }
}

What will creationTime hold after deployment?

medium

A. The timestamp when the contract was deployed

B. The timestamp when the contract was compiled

C. Zero, because it is not initialized

D. It will cause a compilation error

Solution

1. Step 1: Understand immutable assignment

   immutable variables can be assigned once during contract creation, here in the constructor.

2. Step 2: Analyze the assignment

   creationTime = block.timestamp; sets it to the current block timestamp at deployment.

3. Final Answer:

   The timestamp when the contract was deployed -> Option A

4. Quick Check:

   Immutable set at deployment time [OK]

Hint: Immutable set once in constructor at deployment [OK]

Common Mistakes:

• Thinking it holds compile time value
• Assuming default zero value
• Confusing immutable with constant

4. What is wrong with this Solidity code snippet?

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

medium

A. You cannot assign to an immutable variable outside the constructor.

B. The variable value should be declared as constant.

C. The function setValue should be marked as view.

D. There is no error; the code is correct.

Solution

1. Step 1: Recall rules for immutable variables

   immutable variables can only be assigned once during contract creation, typically in the constructor.

2. Step 2: Check assignment location

   Here, value is assigned inside a public function, which is not allowed.

3. Final Answer:

   Cannot assign immutable outside constructor -> Option A

4. Quick Check:

   Immutable assigned only once in constructor [OK]

Hint: Immutable only set in constructor, not in other functions [OK]

Common Mistakes:

• Trying to assign immutable in regular functions
• Confusing constant and immutable rules
• Thinking public functions can set immutable

5. You want to store a fixed token symbol in your smart contract that never changes and a deployment timestamp that is set once at deployment. Which declarations are correct?

contract Token {
    // Declare symbol
    // Declare deploymentTime
    constructor() {
        deploymentTime = block.timestamp;
    }
}

hard

A. string public constant symbol;
uint public immutable deploymentTime = block.timestamp;

B. string public immutable symbol = "TKN";
uint public constant deploymentTime = block.timestamp;

C. string public constant symbol = "TKN";
uint public immutable deploymentTime;

D. string public immutable symbol = "TKN";
uint public immutable deploymentTime = block.timestamp;

Solution

1. Step 1: Declare fixed symbol as constant

   Since symbol never changes and is known at compile time, declare as constant with value assigned inline.

2. Step 2: Declare deploymentTime as immutable

   Deployment timestamp is set once at deployment, so declare as immutable and assign in constructor.

3. Final Answer:

   symbol constant with value, deploymentTime immutable assigned in constructor -> Option C

4. Quick Check:

   Constant fixed value, immutable set once at deployment [OK]

Hint: Constant for fixed value, immutable for deployment-time set [OK]

Common Mistakes:

• Assigning immutable variables inline without constructor
• Declaring constant without value
• Confusing when to use constant vs immutable