Smart contract concept in Blockchain / Solidity - Mini Project: Build & Apply

Smart Contract Concept

📖 Scenario: You are creating a simple smart contract for a blockchain platform. This contract will store and manage a message that anyone can read or update. This is like a digital bulletin board where people can post and change a note.

🎯 Goal: Build a smart contract that stores a message string, allows updating it, and lets anyone read the current message.

📋 What You'll Learn

Create a string variable called message with an initial value

Add a function called setMessage that updates the message

Add a function called getMessage that returns the current message

Print the initial message and the updated message

💡 Why This Matters

🌍 Real World

Smart contracts are programs that run on blockchains to automate agreements and actions without middlemen.

💼 Career

Understanding smart contracts is key for blockchain developers, enabling them to build decentralized apps and services.

1

DATA SETUP: Create the initial message variable

Create a string variable called message and set it to "Hello, Blockchain!".

Blockchain / Solidity

# Create a string variable called message and set it to "Hello, Blockchain!"
# Your code here

Hint

Use a string type variable named message and assign the text inside double quotes.

2

CONFIGURATION: Add a function to update the message

Add a public function called setMessage that takes a string parameter called newMessage and sets message to newMessage.

Blockchain / Solidity

string message = "Hello, Blockchain!";
// Add a public function setMessage(string newMessage) to update message
# Your code here

Hint

Define a public function named setMessage with one string input that assigns the input to message.

3

CORE LOGIC: Add a function to read the message

Add a public function called getMessage that returns the current message string.

Blockchain / Solidity

string message = "Hello, Blockchain!";

public void setMessage(string newMessage) {
    message = newMessage;
}

// Add a public function getMessage() that returns message
# Your code here

Hint

Define a public function named getMessage that returns the message variable.

4

OUTPUT: Display the initial and updated messages

Print the initial message, then call setMessage with "Welcome to Smart Contracts!", and finally print the updated message by calling getMessage.

Blockchain / Solidity

string message = "Hello, Blockchain!";

public void setMessage(string newMessage) {
    message = newMessage;
}

public string getMessage() {
    return message;
}

// Print initial message
// Call setMessage("Welcome to Smart Contracts!")
// Print updated message using getMessage()
# Your code here

Hint

Use System.Console.WriteLine to print messages. Call setMessage with the new text before printing the updated message.

Practice

1. What is a smart contract in blockchain technology?

easy

A. A type of cryptocurrency wallet

B. A program that runs on a blockchain to automate agreements

C. A physical contract signed by two parties

D. A blockchain mining algorithm

Solution

1. Step 1: Understand the definition of smart contracts

   Smart contracts are computer programs that run on blockchains to automate and enforce agreements without middlemen.

2. Step 2: Compare options with the definition

   Only A program that runs on a blockchain to automate agreements describes a program on blockchain automating agreements. Other options describe unrelated concepts.

3. Final Answer:

   A program that runs on a blockchain to automate agreements -> Option B

4. Quick Check:

   Smart contract = automated blockchain program [OK]

Hint: Smart contracts are code on blockchain, not physical papers [OK]

Common Mistakes:

• Confusing smart contracts with physical contracts
• Thinking smart contracts are wallets or mining tools
• Assuming smart contracts require middlemen

2. Which of the following is the correct way to start a simple smart contract in Solidity?

easy

A. contract MyContract { }

B. class MyContract { }

C. function MyContract() { }

D. module MyContract { }

Solution

1. Step 1: Recall Solidity syntax for defining contracts

   In Solidity, smart contracts start with the keyword contract followed by the contract name and braces.

2. Step 2: Check each option's syntax

   contract MyContract { } uses contract keyword correctly. Options A, B, and C use keywords from other languages or incorrect syntax.

3. Final Answer:

   contract MyContract { } -> Option A

4. Quick Check:

   Solidity contracts start with 'contract' keyword [OK]

Hint: Solidity contracts start with 'contract' keyword, not class or function [OK]

Common Mistakes:

• Using 'class' instead of 'contract' keyword
• Confusing functions with contract declarations
• Using module or other unrelated keywords

3. What will be the output of this Solidity function when called?

function getValue() public pure returns (uint) { return 5 + 3; }

medium

A. Error

B. 53

C. 8

D. 0

Solution

1. Step 1: Analyze the function logic

   The function returns the sum of 5 and 3, which is 8.

2. Step 2: Understand Solidity return behavior

   The function is marked pure and returns a uint, so it will output the calculated value 8 without errors.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add numbers inside function to get output quickly [OK]

Common Mistakes:

• Thinking '5 + 3' concatenates as '53'
• Expecting runtime error due to missing state
• Assuming default return is zero

4. Identify the error in this Solidity smart contract snippet:

contract Test {
  uint value;
  function setValue(uint _value) public {
    value = _value
  }
}

medium

A. Missing semicolon after assignment

B. Incorrect function visibility

C. Variable 'value' should be public

D. Function name should be 'set_value'

Solution

1. Step 1: Check syntax line by line

   The line value = _value is missing a semicolon at the end, which is required in Solidity.

2. Step 2: Verify other parts

   Function visibility 'public' is correct, variable can be private, and function naming is valid. So only missing semicolon is error.

3. Final Answer:

   Missing semicolon after assignment -> Option A

4. Quick Check:

   Semicolons end statements in Solidity [OK]

Hint: Check for missing semicolons after statements [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking function visibility is wrong
• Assuming variable must be public

5. You want to create a smart contract that stores user balances and allows deposits only if the amount is positive. Which Solidity code snippet correctly enforces this?

hard

A. function deposit(uint amount) public {
if(amount >= 0) {
balances[msg.sender] = amount;
}
}

B. function deposit(uint amount) public {
balances[msg.sender] += amount;
}

C. function deposit(int amount) public {
balances[msg.sender] += amount;
}

D. function deposit(uint amount) public {
require(amount > 0, "Amount must be positive");
balances[msg.sender] += amount;
}

Solution

1. Step 1: Check input validation for positive amount

   function deposit(uint amount) public { require(amount > 0, "Amount must be positive"); balances[msg.sender] += amount; } uses require(amount > 0) to ensure only positive deposits are accepted.

2. Step 2: Compare other options for correctness

   function deposit(uint amount) public { if(amount >= 0) { balances[msg.sender] = amount; } } allows zero and overwrites balance instead of adding. function deposit(int amount) public { balances[msg.sender] += amount; } uses signed int allowing negative deposits. function deposit(uint amount) public { balances[msg.sender] += amount; } lacks any check.

3. Final Answer:

   function deposit(uint amount) public { require(amount > 0, "Amount must be positive"); balances[msg.sender] += amount; } -> Option D

4. Quick Check:

   Use require() to enforce positive deposits [OK]

Hint: Use require() to check conditions before updating state [OK]

Common Mistakes:

• Not validating input amounts
• Using signed integers for balances
• Overwriting balances instead of adding