Function modifiers in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple modifier in Solidity

What is the output of this Solidity contract when calling test() function?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ModifierExample {
    string public message = "";

    modifier onlyOnce() {
        require(bytes(message).length == 0, "Already set");
        _;
    }

    function test() public onlyOnce {
        message = "Hello, blockchain!";
    }
}

A The contract throws a runtime error on the first call to test().

B "Hello, blockchain!" is stored in message after calling test() once.

C "" (empty string) remains in message after calling test() once.

D The contract throws a compile-time error due to modifier syntax.

💡 Hint

Think about what the modifier checks before allowing the function to run.

The modifier onlyOnce allows the function to run only if message is empty. On first call, message is empty, so test() sets it to "Hello, blockchain!".

❓ Predict Output

intermediate

Effect of modifier order on function execution

Consider these two modifiers and a function using both. What is the output of calling execute()?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ModifierOrder {
    string public log = "";

    modifier first() {
        log = string(abi.encodePacked(log, "1"));
        _;
    }

    modifier second() {
        log = string(abi.encodePacked(log, "2"));
        _;
    }

    function execute() public first second {
        log = string(abi.encodePacked(log, "E"));
    }
}

A "12E"

B "21E"

C "1E2"

D "2E1"

💡 Hint

Modifiers run in the order they are listed before the function body.

Modifiers run in order: first adds "1", then second adds "2", then function adds "E". So log becomes "12E".

🔧 Debug

advanced

Identify the error caused by modifier usage

What error occurs when calling withdraw() in this contract?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract WithdrawExample {
    address public owner;
    uint public balance = 100;

    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");
        _;
    }

    constructor() {
        owner = msg.sender;
    }

    function withdraw(uint amount) public onlyOwner {
        require(amount <= balance, "Insufficient balance");
        balance -= amount;
        payable(msg.sender).transfer(amount);
    }
}

A No error; function executes successfully.

B Compile-time error: 'payable' keyword used incorrectly.

C Runtime error: 'require' fails because msg.sender is not owner.

D Runtime error: 'transfer' fails because contract has no Ether balance.

💡 Hint

Check if the contract has Ether to send before calling transfer.

The contract has a balance variable but no actual Ether. Calling transfer fails because contract's Ether balance is zero.

📝 Syntax

advanced

Identify the syntax error in modifier definition

Which option correctly fixes the syntax error in this modifier?

Blockchain / Solidity

modifier checkValue(uint x) {
    require(x > 0);
    _;
}

A Add a semicolon after require: require(x > 0); _;

B Add a comma after require: require(x > 0), _;

C Add curly braces around require: { require(x > 0) } _;

D Add a colon after require: require(x > 0): _;

💡 Hint

Statements in Solidity must end with a semicolon.

The require statement is missing a semicolon at the end, causing a syntax error.

🚀 Application

expert

Using modifiers to restrict function access and update state

Given this contract, what is the value of counter after calling increment() twice from the owner address?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Counter {
    address public owner;
    uint public counter = 0;

    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");
        _;
    }

    modifier incrementCounter() {
        _;
        counter += 1;
    }

    constructor() {
        owner = msg.sender;
    }

    function increment() public onlyOwner incrementCounter {
        // function body empty
    }
}

A 0

B 1

C 2

D 3

💡 Hint

Consider the order modifiers run and how counter is updated.

Modifiers run in order: onlyOwner checks sender, then incrementCounter runs function body (empty), then increments counter. Each call increments counter by 1, so after two calls counter is 2.

Practice

1. What is the main purpose of a function modifier in blockchain smart contracts?

easy

A. To change the return type of a function

B. To add extra rules or checks around a function

C. To rename a function

D. To delete a function from the contract

Solution

1. Step 1: Understand function modifiers

   Function modifiers are used to add extra rules or steps before or after a function runs.

2. Step 2: Compare options

   Only To add extra rules or checks around a function correctly describes adding extra rules or checks around a function.

3. Final Answer:

   To add extra rules or checks around a function -> Option B

4. Quick Check:

   Function modifiers add rules [OK]

Hint: Modifiers add rules or checks around functions [OK]

Common Mistakes:

• Thinking modifiers rename or delete functions
• Confusing modifiers with return type changes
• Believing modifiers execute functions themselves

2. Which of the following is the correct syntax to define a function modifier in Solidity?

easy

A. modifier onlyOwner() => { require(msg.sender == owner); _; }

B. modifier onlyOwner() : { require(msg.sender == owner); _; }

C. modifier onlyOwner() -> { require(msg.sender == owner); _; }

D. modifier onlyOwner() { require(msg.sender == owner); _; }

Solution

1. Step 1: Recall Solidity modifier syntax

   Modifiers use the keyword 'modifier', name, parentheses, and curly braces with '_;' inside.

2. Step 2: Identify correct syntax

   modifier onlyOwner() { require(msg.sender == owner); _; } matches the correct Solidity syntax with braces and '_;'. Others use invalid symbols like '=>', '->', or ':'.

3. Final Answer:

   modifier onlyOwner() { require(msg.sender == owner); _; } -> Option D

4. Quick Check:

   Modifier syntax uses braces and '_;' [OK]

Hint: Modifiers use braces { } and _; inside [OK]

Common Mistakes:

• Using arrows (=>, ->) instead of braces
• Omitting the '_;' statement
• Using colon ':' instead of braces

3. What will be the output of the following Solidity code snippet?

contract Test {
    address owner;
    modifier onlyOwner() {
        require(msg.sender == owner, "Not owner");
        _;
    }
    function setOwner(address _owner) public {
        owner = _owner;
    }
    function secureAction() public onlyOwner {
        // action code
    }
}

If msg.sender is not the owner and calls secureAction(), what happens?

medium

A. The function runs normally

B. The owner address changes

C. The transaction reverts with error 'Not owner'

D. The function ignores the modifier

Solution

1. Step 1: Understand the modifier behavior

   The modifier checks if msg.sender equals owner; if not, it calls require which reverts with "Not owner".

2. Step 2: Analyze the call with wrong sender

   If msg.sender is not owner, require fails and the transaction reverts with the error message.

3. Final Answer:

   The transaction reverts with error 'Not owner' -> Option C

4. Quick Check:

   require fails if sender not owner [OK]

Hint: require reverts if condition false inside modifier [OK]

Common Mistakes:

• Assuming function runs even if require fails
• Thinking owner changes on secureAction call
• Ignoring the require statement effect

4. Identify the error in this Solidity code using a function modifier:

contract Sample {
    address owner;
    modifier onlyOwner() {
        require(msg.sender == owner);
        _
    }
    function changeOwner(address newOwner) public onlyOwner {
        owner = newOwner;
    }
}

medium

A. Missing semicolon after '_' in modifier

B. Incorrect function visibility for changeOwner

C. owner variable not initialized

D. Modifier name should be 'OnlyOwner' with capital O

Solution

1. Step 1: Check modifier syntax

   The modifier has '_', but it misses the semicolon '_;' which is required to mark function execution point.

2. Step 2: Verify other parts

   Function visibility 'public' is correct, owner can be uninitialized but not a syntax error, and modifier names are case-sensitive but not required to be capitalized.

3. Final Answer:

   Missing semicolon after '_' in modifier -> Option A

4. Quick Check:

   Modifier must have '_;' not just '_' [OK]

Hint: Modifier must end '_' with semicolon ';' [OK]

Common Mistakes:

• Forgetting semicolon after '_' in modifier
• Thinking function visibility causes error here
• Assuming variable initialization causes syntax error

5. You want to create a modifier that allows a function to run only if the contract is not paused. The contract has a boolean paused. Which modifier code correctly implements this logic?

hard

A. modifier whenNotPaused() { require(!paused, "Paused"); _; }

B. modifier whenNotPaused() { if(paused) { revert("Paused"); } }

C. modifier whenNotPaused() { require(paused, "Paused"); _; }

D. modifier whenNotPaused() { if(!paused) { _; } }

Solution

1. Step 1: Understand the paused logic

   The function should run only if paused is false, so require(!paused) ensures this.

2. Step 2: Check modifier completeness

   modifier whenNotPaused() { require(!paused, "Paused"); _; } uses require with negation and includes '_;' to run the function. modifier whenNotPaused() { if(paused) { revert("Paused"); } } lacks '_;', so function never runs. modifier whenNotPaused() { require(paused, "Paused"); _; } requires paused to be true, which is opposite. modifier whenNotPaused() { if(!paused) { _; } } runs function only if not paused but lacks else handling and is less clear.

3. Final Answer:

   modifier whenNotPaused() { require(!paused, "Paused"); _; } -> Option A

4. Quick Check:

   Require !paused with '_;' runs function [OK]

Hint: Use require(!paused) and include '_;' to run function [OK]

Common Mistakes:

• Reversing paused condition in require
• Omitting '_;' causing function not to run
• Using if without else or revert