Constants and immutables in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a constant value for the token name.

Blockchain / Solidity

contract Token {
    string public constant [1] = "MyToken";
}

Drag options to blanks, or click blank then click option'

ATOKEN_NAME

BtokenName

Ctoken_name

DTokenName

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or camelCase names for constants.

Constants in blockchain smart contracts are usually named in uppercase letters with underscores.

2fill in blank

medium

Complete the code to declare an immutable variable for the contract owner address.

Blockchain / Solidity

contract Ownership {
    address public immutable [1];

    constructor(address _owner) {
        [1] = _owner;
    }
}

Drag options to blanks, or click blank then click option'

AOWNER_ADDRESS

BownerAddress

Cowner

DOwner

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase names for immutable variables.

Trying to assign immutable variables outside the constructor.

Immutable variables are usually named in camelCase and set once during contract deployment.

3fill in blank

hard

Fix the error in the code by choosing the correct keyword to declare a constant.

Blockchain / Solidity

contract Example {
    uint256 public [1] VALUE = 1000;
}

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Alet

Bimmutable

Cconstant

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using immutable instead of constant for fixed values.

Using JavaScript keywords like var or let.

The keyword constant is used to declare compile-time constants in smart contracts.

4fill in blank

hard

Fill both blanks to declare an immutable uint variable and assign it in the constructor.

Blockchain / Solidity

contract Config {
    uint256 public [1] [2];

    constructor(uint256 _value) {
        [2] = _value;
    }
}

Drag options to blanks, or click blank then click option'

Aimmutable

Bconstant

CconfigValue

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names in declaration and assignment.

Forgetting to add the immutable keyword.

The variable name must be the same in declaration and assignment. The keyword immutable is missing and should be added before the variable name.

5fill in blank

hard

Fill all three blanks to declare a constant string, an immutable address, and assign the immutable in the constructor.

Blockchain / Solidity

contract Demo {
    string public constant [1] = "DemoToken";
    address public immutable [2];

    constructor(address _admin) {
        [3] = _admin;
    }
}

Drag options to blanks, or click blank then click option'

ACONSTANT_NAME

Badmin

CADMIN_ADDRESS

DTOKEN_NAME

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up constant and immutable naming conventions.

Assigning to a different variable name in the constructor.

The constant string is named TOKEN_NAME. The immutable address variable is admin, which is assigned in the constructor.

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