Smart contract concept in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a smart contract named MyContract.

Blockchain / Solidity

contract [1] {
    // contract code here
}

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

ASmartContract

Bmycontract

CContract

DMyContract

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase or different contract names.

The contract name should start with a capital letter and match the declaration. 'MyContract' is the correct name.

2fill in blank

medium

Complete the code to define a public state variable named count of type uint.

Blockchain / Solidity

uint public [1];

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

Acount

Bcounter

Cnumber

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using other variable names like 'counter' or 'number'.

The variable name should be 'count' as specified.

3fill in blank

hard

Fix the error in the function declaration to make it a public function named increment.

Blockchain / Solidity

function [1]() public {
    count += 1;
}

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

Aincrement

Badd

Cincrease

DIncrement

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Increment' or other names.

Function names are case-sensitive. 'increment' in lowercase is correct.

4fill in blank

hard

Fill both blanks to complete the function that returns the current count.

Blockchain / Solidity

function [1]() public view returns ([2]) {
    return count;
}

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

AgetCount

Buint

Cint

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong function names or return types like 'int'.

The function name should be 'getCount' and it returns a 'uint' type.

5fill in blank

hard

Fill both blanks to complete the constructor that sets count to zero.

Blockchain / Solidity

constructor() {
    count [1] [2];
}

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

Apublic

B=

C0

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Adding visibility like 'public' or 'private', or wrong assignment.

In modern Solidity, constructors do not use visibility modifiers. Set count to zero using '=' and '0'.

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