Smart contract concept in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a smart contract?

A smart contract is a self-executing program stored on a blockchain that automatically enforces rules and agreements without needing a middleman.

beginner

How does a smart contract work?

It runs code on the blockchain when certain conditions are met, like a digital vending machine that releases a product only after payment.

beginner

Name one key benefit of smart contracts.

They reduce the need for trust because the code runs automatically and transparently, so no one can cheat or change the rules secretly.

beginner

What is a common use case for smart contracts?

Examples include automatic payments, digital identity verification, and managing ownership of digital assets like NFTs.

intermediate

Why are smart contracts considered 'immutable'?

Once deployed on the blockchain, their code cannot be changed, ensuring the rules stay the same and everyone follows them.

What triggers a smart contract to execute?

A When the blockchain is restarted

B When predefined conditions are met

C At a fixed time every day

D When a user manually runs it

Smart contracts run automatically when their coded conditions are fulfilled.

Which of these is NOT a feature of smart contracts?

A Automatic execution

B Transparency on blockchain

C Immutability after deployment

D Requires a trusted third party

Smart contracts remove the need for trusted third parties by running code automatically.

Smart contracts are stored on which technology?

A Blockchain

B Cloud server

C Local computer

D Mobile app

Smart contracts live on the blockchain to ensure security and transparency.

What happens if a smart contract's conditions are not met?

A It sends an error to the user

B It executes anyway

C It does not execute

D It deletes itself

Smart contracts only run when their conditions are true; otherwise, they stay inactive.

Which is a real-world analogy for a smart contract?

A A vending machine

B A paper contract

C A bank teller

D A phone call

A vending machine automatically gives a product after payment, like a smart contract executes code after conditions.

Explain what a smart contract is and how it works in simple terms.

Think of it like a digital vending machine that works by itself.

List three benefits or use cases of smart contracts.

Consider how smart contracts help people avoid needing a trusted third party.

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