Why blockchain exists - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the main purpose of blockchain.

Blockchain / Solidity

purpose = "Blockchain exists to provide [1] and transparency."

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

Aspeed

Bcomplexity

Csecurity

Dcentralization

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing speed or centralization as the main purpose.

Blockchain is mainly used to provide security and transparency in transactions.

2fill in blank

medium

Complete the code to show what blockchain removes from traditional systems.

Blockchain / Solidity

blockchain_removes = "[1] trust in a central authority"

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

Aremoves

Bignores

Cincreases

Drequires

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'increases' or 'requires' trust in central authority.

Blockchain removes the need to trust a central authority by using decentralized verification.

3fill in blank

hard

Fix the error in the statement about blockchain's data structure.

Blockchain / Solidity

blockchain_structure = "Blockchain stores data in [1] that are linked together."

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

Afiles

Bblocks

Cdatabases

Dfolders

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'files' or 'databases' which are not specific to blockchain.

Blockchain stores data in blocks that are linked in a chain.

4fill in blank

hard

Fill both blanks to complete the sentence about blockchain's verification process.

Blockchain / Solidity

Verification in blockchain is done by [1] and recorded in [2].

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

Anodes

Bminers

Cblocks

Dusers

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing nodes or users with miners, or folders with blocks.

Blockchain transactions are verified by miners and recorded in blocks.

5fill in blank

hard

Fill all three blanks to complete the code describing blockchain's key features.

Blockchain / Solidity

features = {"[1]": "decentralized", "[2]": "immutable", "[3]": "transparent"}

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

Asecurity

Bconsensus

Cdata

Dledger

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up ledger with security or consensus with data.

The blockchain ledger is consensus-based, data is immutable and transparent.

Practice

1. Why was blockchain technology created?

easy

A. To make computers run faster

B. To allow people to trust data without needing a middleman

C. To store large video files efficiently

D. To replace all traditional databases

Solution

1. Step 1: Understand the main purpose of blockchain

   Blockchain was designed to create trust in data without relying on a central authority or middleman.

2. Step 2: Compare options with blockchain's purpose

   Options B, C, and D do not match blockchain's core goal of trust and decentralization.

3. Final Answer:

   To allow people to trust data without needing a middleman -> Option B

4. Quick Check:

   Blockchain = Trust without middleman [OK]

Hint: Blockchain = trust data without middleman [OK]

Common Mistakes:

• Thinking blockchain is just a faster database
• Confusing blockchain with file storage
• Believing blockchain replaces all databases

2. Which of the following is the correct way to describe a blockchain?

easy

A. A chain of blocks where each block links to the previous one

B. A single large block storing all data

C. A random collection of unrelated data blocks

D. A block that changes data easily

Solution

1. Step 1: Recall blockchain structure

   Blockchain is made of blocks linked in order, each pointing to the previous block.

2. Step 2: Eliminate incorrect descriptions

   Options A, B, and D do not describe the linked and secure nature of blockchain blocks.

3. Final Answer:

   A chain of blocks where each block links to the previous one -> Option A

4. Quick Check:

   Blockchain = linked blocks [OK]

Hint: Blockchain links blocks in order securely [OK]

Common Mistakes:

• Thinking blockchain is one big block
• Believing blocks are unrelated
• Assuming blocks can be changed easily

3. Consider this simple blockchain data structure in Python-like pseudocode:

blocks = [
  {'index': 1, 'data': 'A', 'prev_hash': '0'},
  {'index': 2, 'data': 'B', 'prev_hash': 'hash1'},
  {'index': 3, 'data': 'C', 'prev_hash': 'hash2'}
]
print(blocks[1]['data'])

What will be the output?

medium

A. A

B. hash1

C. C

D. B

Solution

1. Step 1: Identify the list index accessed

   blocks[1] accesses the second item in the list, which has data 'B'.

2. Step 2: Access the 'data' key of that block

   blocks[1]['data'] returns 'B'.

3. Final Answer:

   B -> Option D

4. Quick Check:

   blocks[1]['data'] = B [OK]

Hint: List index 1 is second item, data is 'B' [OK]

Common Mistakes:

• Confusing list index starting at 0
• Mixing data and prev_hash values
• Assuming index key affects list indexing

4. This code tries to add a new block to a blockchain list but has an error:

blockchain = [{'index': 1, 'data': 'A', 'prev_hash': '0'}]
new_block = {'index': 2, 'data': 'B', 'prev_hash': blockchain['index']}
blockchain.append(new_block)
print(blockchain)

What is the error?

medium

A. Cannot use blockchain['index'] because blockchain is a list, not a dict

B. Missing 'data' key in new_block

C. append() method does not exist for lists

D. Index value should be a string, not a number

Solution

1. Step 1: Identify the type of 'blockchain'

   blockchain is a list, so blockchain['index'] is invalid syntax.

2. Step 2: Understand correct way to get last block's index

   To get last block's index, use blockchain[-1]['index'], not blockchain['index'].

3. Final Answer:

   Cannot use blockchain['index'] because blockchain is a list, not a dict -> Option A

4. Quick Check:

   List indexing needs integer, not string key [OK]

Hint: List uses numeric indexes, not string keys [OK]

Common Mistakes:

• Using string keys on a list
• Forgetting to access last element with -1
• Confusing append() with other methods

5. You want to create a simple blockchain that stores ownership records. Which feature of blockchain helps ensure records cannot be changed without everyone noticing?

hard

A. Blocks can be edited by anyone at any time

B. Blocks are stored in a single central database

C. Blocks are linked by hashes that depend on previous blocks

D. Blocks are deleted after 24 hours automatically

Solution

1. Step 1: Understand blockchain's security feature

   Each block contains a hash that depends on the previous block's data, linking them securely.

2. Step 2: Explain why this prevents unnoticed changes

   If any block changes, its hash changes, breaking the chain and alerting everyone.

3. Final Answer:

   Blocks are linked by hashes that depend on previous blocks -> Option C

4. Quick Check:

   Hash linking = tamper detection [OK]

Hint: Hash links blocks to detect changes [OK]

Common Mistakes:

• Thinking blockchain is centralized
• Believing blocks can be freely edited
• Assuming blocks are temporary