Function declaration and syntax in Blockchain / Solidity - Mini Project: Build & Apply

Function Declaration and Syntax in Blockchain Smart Contracts

📖 Scenario: You are creating a simple blockchain smart contract to store and retrieve a favorite number. This contract will help you understand how to declare and use functions in Solidity, the language used for Ethereum smart contracts.

🎯 Goal: Build a smart contract with a function to set a favorite number and another function to get that number.

📋 What You'll Learn

Create a state variable to store a number

Declare a function called setFavoriteNumber that takes a number as input and stores it

Declare a function called getFavoriteNumber that returns the stored number

Use correct Solidity function syntax including visibility and return types

💡 Why This Matters

🌍 Real World

Smart contracts on blockchains store and manage data securely. Functions let users interact with this data.

💼 Career

Understanding function declaration and syntax is essential for blockchain developers writing smart contracts.

1

Create a state variable

Create a public state variable called favoriteNumber of type uint to store the number.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    // Declare a public uint variable named favoriteNumber
    
    // Your code here
}

Hint

Use uint public favoriteNumber; inside the contract to create the variable.

2

Declare the setter function

Add a public function called setFavoriteNumber that takes a uint parameter named _number and sets favoriteNumber to this value.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    uint public favoriteNumber;

    // Declare setFavoriteNumber function here
    
    // Your code here
}

Hint

Define the function with function setFavoriteNumber(uint _number) public and assign favoriteNumber = _number;.

3

Declare the getter function

Add a public view function called getFavoriteNumber that returns a uint and returns the value of favoriteNumber.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    uint public favoriteNumber;

    function setFavoriteNumber(uint _number) public {
        favoriteNumber = _number;
    }

    // Declare getFavoriteNumber function here
    
    // Your code here
}

Hint

Use function getFavoriteNumber() public view returns (uint) and return the variable.

4

Test the contract output

Print the value returned by getFavoriteNumber() after setting it to 42 using setFavoriteNumber(42). Use comments to show the expected output.

Blockchain / Solidity

pragma solidity ^0.8.0;

contract FavoriteNumber {
    uint public favoriteNumber;

    function setFavoriteNumber(uint _number) public {
        favoriteNumber = _number;
    }

    function getFavoriteNumber() public view returns (uint) {
        return favoriteNumber;
    }
}

// Simulate setting and getting the favorite number
// Your code here

Hint

In Solidity, you cannot print directly. Use comments to show calling setFavoriteNumber(42) then getFavoriteNumber() returns 42.

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