Why handling value is core to blockchain - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple transaction object with a value field.

Blockchain / Solidity

transaction = {"sender": "Alice", "receiver": "Bob", "[1]": 50}

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

Avalue

Bhash

Ctimestamp

Dnonce

Attempts:

3 left

💡 Hint

Common Mistakes

Using keys like 'hash' or 'nonce' instead of 'value' for the amount.

The key value represents the amount being transferred, which is essential in blockchain transactions.

2fill in blank

medium

Complete the code to add the transaction value to the sender's balance check.

Blockchain / Solidity

if sender_balance >= [1]:
    print("Transaction approved")

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

Atransaction['hash']

Btransaction['value']

Ctransaction['timestamp']

Dtransaction['nonce']

Attempts:

3 left

💡 Hint

Common Mistakes

Checking against 'hash' or 'nonce' which are unrelated to the amount.

The sender must have enough value to cover the transaction amount.

3fill in blank

hard

Fix the error in the code to correctly update the receiver's balance by adding the transaction value.

Blockchain / Solidity

receiver_balance = receiver_balance [1] transaction['value']

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

A+

B*

C/

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication instead of addition.

To increase the receiver's balance, you add the transaction value.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps each user to their transaction value if the value is greater than 10.

Blockchain / Solidity

{user: [1] for user, [2] in transactions.items() if [1] > 10}

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

Avalue

Bamount

Cval

Dbalance

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keys like 'amount' or 'val' which may not exist.

The dictionary comprehension uses the key value from each transaction to filter and map users.

5fill in blank

hard

Fill all three blanks to create a filtered dictionary of users with transaction values above 100, using uppercase usernames as keys.

Blockchain / Solidity

filtered = {user.[1](): [2] for user, [3] in transactions.items() if [2] > 100}

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

Alower

Bvalue

Ctransaction_value

Dupper

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'lower' instead of 'upper' or wrong keys for transaction amounts.

Usernames are converted to uppercase with upper(), and the transaction value is used for filtering and mapping.

Practice

1. Why is handling value considered core to blockchain technology?

easy

A. Because blockchains enable secure transfer of money or assets between people.

B. Because blockchains only store text data without any value.

C. Because blockchains are used only for social media applications.

D. Because blockchains do not require any authorization for transactions.

Solution

1. Step 1: Understand blockchain purpose

   Blockchains are designed to let people send money or assets safely.

2. Step 2: Identify role of value handling

   Handling value is essential because it allows secure and trusted transfer of assets.

3. Final Answer:

   Because blockchains enable secure transfer of money or assets between people. -> Option A

4. Quick Check:

   Value handling = secure asset transfer [OK]

Hint: Value handling means secure asset transfer on blockchain [OK]

Common Mistakes:

• Thinking blockchain only stores text data
• Assuming no authorization is needed
• Confusing blockchain with social media

2. Which of the following is the correct way to represent a transaction value in a blockchain smart contract?

easy

A. let value = 100; // as a number

B. let value = null; // as null

C. let value = true; // as a boolean

D. let value = '100'; // as a string

Solution

1. Step 1: Identify correct data type for value

   Transaction values represent amounts, so they should be numbers, not strings or booleans.

2. Step 2: Check each option

   let value = 100; // as a number uses a number type which is correct for value representation.

3. Final Answer:

   let value = 100; // as a number -> Option A

4. Quick Check:

   Value must be a number [OK]

Hint: Transaction value must be a number, not string or boolean [OK]

Common Mistakes:

• Using strings to represent numeric values
• Using booleans or null for amounts
• Confusing data types in smart contracts

3. Consider this simplified blockchain transaction code snippet:

let senderBalance = 500;
let transferAmount = 200;
if (senderBalance >= transferAmount) {
  senderBalance -= transferAmount;
  receiverBalance += transferAmount;
}
console.log(senderBalance);

What will be the output?

medium

A. 700

B. 300

C. 500

D. 200

Solution

1. Step 1: Check condition and initial values

   senderBalance is 500, transferAmount is 200, condition senderBalance >= transferAmount is true.

2. Step 2: Calculate new senderBalance

   senderBalance is reduced by 200, so 500 - 200 = 300.

3. Final Answer:

   300 -> Option B

4. Quick Check:

   500 - 200 = 300 [OK]

Hint: Subtract transfer amount from sender balance if enough funds [OK]

Common Mistakes:

• Adding instead of subtracting transfer amount
• Ignoring the condition check
• Confusing sender and receiver balances

4. The following code tries to update balances after a transaction but has a bug:

let senderBalance = 300;
let receiverBalance = 100;
let transferAmount = 400;
if (senderBalance > transferAmount) {
  senderBalance -= transferAmount;
  receiverBalance += transferAmount;
}
console.log(senderBalance);

What is the bug and how to fix it?

medium

A. The senderBalance should be increased, not decreased.

B. The transferAmount should be less than senderBalance, so no fix needed.

C. The receiverBalance update is missing a semicolon.

D. The condition should be senderBalance >= transferAmount to allow exact balance transfer.

Solution

1. Step 1: Analyze the condition

   The condition uses > which excludes the case when senderBalance equals transferAmount.

2. Step 2: Correct the condition

   Change > to >= so sender can transfer all their balance exactly.

3. Final Answer:

   The condition should be senderBalance >= transferAmount to allow exact balance transfer. -> Option D

4. Quick Check:

   Use >= to allow full balance transfer [OK]

Hint: Use >= to allow transfer of full sender balance [OK]

Common Mistakes:

• Using > excludes exact balance transfers
• Ignoring condition logic
• Misunderstanding balance updates

5. You want to create a function that safely transfers value between two accounts on a blockchain. Which approach best ensures the transaction is valid and balances update correctly?

hard

A. Add amount to receiver first, then subtract from sender without checks.

B. Subtract amount from sender without checking balance, then add to receiver.

C. Check sender balance >= amount, subtract amount from sender, add to receiver, then record transaction.

D. Record transaction first, then update balances without validation.

Solution

1. Step 1: Validate sender has enough balance

   Checking sender balance >= amount prevents overspending and errors.

2. Step 2: Update balances and record transaction

   Subtract from sender, add to receiver, then record transaction to keep system consistent.

3. Final Answer:

   Check sender balance >= amount, subtract amount from sender, add to receiver, then record transaction. -> Option C

4. Quick Check:

   Validate then update balances then record [OK]

Hint: Validate balance before transfer, then update and record [OK]

Common Mistakes:

• Updating balances without validation
• Recording transaction before balance update
• Adding to receiver before subtracting from sender