Function modifiers in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function modifier named onlyOwner.

Blockchain / Solidity

modifier [1]() {
    _;
}

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

AcheckOwner

BownerOnly

ConlyOwner

DownerModifier

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different modifier name than declared.

Forgetting the modifier keyword.

The modifier name is onlyOwner as declared in the code.

2fill in blank

medium

Complete the code to require that the caller is the owner inside the onlyOwner modifier.

Blockchain / Solidity

modifier onlyOwner() {
    require(msg.sender [1] owner, "Not owner");
    _;
}

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

A!=

B>

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using != which would reject the owner.

Using comparison operators like < or > which are invalid for addresses.

The require statement checks if msg.sender is equal to owner.

3fill in blank

hard

Fix the error in the modifier usage in the function declaration.

Blockchain / Solidity

function withdraw() public [1] {
    // withdraw logic
}

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

AonlyOwner

BonlyOwner()

ConlyOwner{}

DonlyOwner;

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses after the modifier name when it has no parameters.

Using braces or semicolons after the modifier name.

Modifiers are used without parentheses if they have no parameters.

4fill in blank

hard

Fill both blanks to create a modifier that checks if the caller is the owner and then executes the function.

Blockchain / Solidity

modifier [1]() {
    require(msg.sender [2] owner, "Caller is not owner");
    _;
}

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

AonlyOwner

B==

C!=

DownerOnly

Attempts:

3 left

💡 Hint

Common Mistakes

Using != which rejects the owner.

Using a wrong modifier name.

The modifier name is onlyOwner and the require condition uses == to check ownership.

5fill in blank

hard

Fill all three blanks to create a modifier named onlyOwner that requires the caller to be the owner and then executes the function.

Blockchain / Solidity

modifier [1]() {
    require(msg.sender [2] owner, "Not owner");
    [3];
}

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

AownerOnly

B==

C_

DonlyOwner

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong modifier names.

Forgetting the _; statement.

The modifier is named onlyOwner, uses == to check ownership, and _; to continue execution.

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