Function declaration and syntax in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple Solidity function

What is the output of this Solidity function when called with input 5?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    function multiplyByTwo(uint x) public pure returns (uint) {
        return x * 2;
    }
}

A 10

B 5

C 0

D Compilation error

💡 Hint

The function returns the input multiplied by 2.

The function multiplyByTwo takes an unsigned integer and returns its value multiplied by 2. So input 5 returns 10.

❓ Predict Output

intermediate

Return value of a function with multiple returns

What does this Solidity function return when called?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    function getValues() public pure returns (uint, bool) {
        return (42, true);
    }
}

A (42, true)

B 42

C true

D Compilation error

💡 Hint

The function returns two values as a tuple.

The function returns two values: an unsigned integer 42 and a boolean true as a tuple.

❓ Predict Output

advanced

Output of a function with state variable update

What is the value of the state variable after calling this function once?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Counter {
    uint public count = 0;
    function increment() public {
        count += 1;
    }
}

A Compilation error

B 0

C 1

D Undefined

💡 Hint

The function increases the count by 1.

The increment function adds 1 to the state variable count. Initially 0, after one call it becomes 1.

❓ Predict Output

advanced

Error type from incorrect function syntax

What error does this Solidity code produce?

Blockchain / Solidity

pragma solidity ^0.8.0;
contract Test {
    function wrongSyntax() public returns (uint) {
        return 5;
    }
}

A TypeError: Return type mismatch

B Runtime error

C No error, compiles fine

D SyntaxError: Missing semicolon

💡 Hint

Check the end of the return statement.

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

🧠 Conceptual

expert

Function visibility and access control

Which function visibility keyword allows a function to be called only from within the contract or derived contracts?

A public

B internal

C external

D private

💡 Hint

Think about which keyword restricts access to the contract and its children.

The 'internal' keyword restricts function access to the contract itself and contracts that inherit from it.

Practice

1. What is the correct way to declare a function named transferFunds in a blockchain smart contract?

easy

A. transferFunds function() { }

B. function transferFunds() { }

C. func transferFunds() { }

D. def transferFunds() { }

Solution

1. Step 1: Identify the common function declaration keyword

   In many blockchain languages like Solidity, functions start with the keyword function.

2. Step 2: Check the syntax structure

   The correct syntax is function functionName() { }, so function transferFunds() { } is correct.

3. Final Answer:

   function transferFunds() { } -> Option B

4. Quick Check:

   Function declaration uses 'function' keyword [OK]

Hint: Look for 'function' keyword before name [OK]

Common Mistakes:

• Using 'def' instead of 'function'
• Placing 'function' after the name
• Using 'func' which is not standard here

2. Which of the following is the correct syntax to declare a function with one parameter named amount?

easy

A. function send(amount) { }

B. function send amount { }

C. function send(amount) => { }

D. function send[amount] { }

Solution

1. Step 1: Recognize parameter syntax in function declaration

   Parameters are placed inside parentheses separated by commas if multiple.

2. Step 2: Validate each option's syntax

   function send(amount) { } uses parentheses correctly: function send(amount) { }. Others misuse brackets, arrows, or omit parentheses.

3. Final Answer:

   function send(amount) { } -> Option A

4. Quick Check:

   Parameters go inside parentheses [OK]

Hint: Parameters always go inside parentheses () [OK]

Common Mistakes:

• Omitting parentheses around parameters
• Using square brackets instead of parentheses
• Using arrow syntax incorrectly

3. What will be the output of this function call in a blockchain smart contract?

function multiply(x, y) { return x * y; }
multiply(3, 4);

medium

A. 7

B. Error

C. 34

D. 12

Solution

1. Step 1: Understand the function logic

   The function multiplies two inputs x and y and returns the product.

2. Step 2: Calculate the result for inputs 3 and 4

   3 multiplied by 4 equals 12.

3. Final Answer:

   12 -> Option D

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply inputs as given to get output [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Assuming syntax error without checking code

4. Identify the error in this function declaration:

function deposit(amount) {
  balance += amount
}

medium

A. Missing semicolon after statement

B. Function name is invalid

C. No return statement in function

D. Missing parentheses around parameter

Solution

1. Step 1: Check syntax inside the function body

   Statements inside functions usually require semicolons to end lines.

2. Step 2: Identify missing semicolon

   The line balance += amount lacks a semicolon at the end.

3. Final Answer:

   Missing semicolon after statement -> Option A

4. Quick Check:

   Statements need semicolons [OK]

Hint: Check for missing semicolons inside function body [OK]

Common Mistakes:

• Ignoring semicolon requirements
• Thinking return is mandatory
• Assuming function name is wrong

5. You want to create a function calculateFee that takes a transaction amount and returns 2% fee only if the amount is above 1000. Which is the correct function?

hard

A. function calculateFee(amount) { if amount > 1000 then return amount * 0.02 else return 0; }

B. function calculateFee(amount) { return amount * 0.02 if amount > 1000 else 0; }

C. function calculateFee(amount) { if (amount > 1000) { return amount * 0.02; } else { return 0; } }

D. function calculateFee(amount) { if (amount > 1000) return amount * 0.02; }

Solution

1. Step 1: Understand conditional return logic

   The function must check if amount is greater than 1000 and return 2% fee; otherwise, return 0.

2. Step 2: Validate syntax correctness

   function calculateFee(amount) { if (amount > 1000) { return amount * 0.02; } else { return 0; } } uses proper if-else syntax with braces and returns both cases correctly. Others have syntax errors or missing else return.

3. Final Answer:

   function calculateFee(amount) { if (amount > 1000) { return amount * 0.02; } else { return 0; } } -> Option C

4. Quick Check:

   Use if-else with braces for conditional returns [OK]

Hint: Use full if-else with braces for clear returns [OK]

Common Mistakes:

• Missing else return causes undefined output
• Incorrect if syntax without parentheses or braces
• Using Python-style if-else in blockchain code