Sending transactions in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ethereum transaction code snippet?

Consider this simplified Ethereum transaction sending code using web3.js. What will be printed to the console?

Blockchain / Solidity

const Web3 = require('web3');
const web3 = new Web3('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');

async function sendTx() {
  const accounts = await web3.eth.getAccounts();
  console.log(accounts.length);
}
sendTx();

A 0

B An error because getAccounts is not available on mainnet without unlocked accounts

C undefined

D A positive number representing accounts length

💡 Hint

Think about what getAccounts() returns when connected to a public node without unlocked accounts.

When connected to a public node like Infura, getAccounts() returns an empty array because the node does not manage any accounts locally. So, accounts.length is 0.

❓ Predict Output

intermediate

What is the output of this Bitcoin transaction fee calculation?

Given this code snippet calculating Bitcoin transaction fee, what is the output?

Blockchain / Solidity

const txSizeBytes = 250;
const feeRateSatsPerByte = 20;
const fee = txSizeBytes * feeRateSatsPerByte;
console.log(fee);

A 250

B 270

C 20

D 5000

💡 Hint

Multiply transaction size by fee rate per byte.

The fee is calculated by multiplying the transaction size (250 bytes) by the fee rate (20 satoshis per byte), resulting in 5000 satoshis.

🔧 Debug

advanced

Why does this Ethereum transaction sending code throw an error?

Look at this code snippet that tries to send an Ethereum transaction. Why does it throw an error?

Blockchain / Solidity

const Web3 = require('web3');
const web3 = new Web3('https://mainnet.infura.io/v3/YOUR_PROJECT_ID');

async function sendTx() {
  const tx = {
    from: '0xYourAddress',
    to: '0xRecipientAddress',
    value: web3.utils.toWei('1', 'ether'),
    gas: 21000
  };
  const receipt = await web3.eth.sendTransaction(tx);
  console.log(receipt);
}
sendTx();

A Error because gas limit is too low

B Error because the private key is not provided or account is not unlocked

C Error because value is not converted to wei correctly

D No error, transaction will succeed

💡 Hint

Think about what is needed to sign and send a transaction using a public node.

When using a public node like Infura, the account is not unlocked and private keys are not managed. You must sign the transaction locally before sending. Without signing, sendTransaction throws an error.

📝 Syntax

advanced

Which option correctly creates and signs a transaction in Ethereum using ethers.js?

Choose the code snippet that correctly creates and signs a transaction using ethers.js.

A

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: ethers.utils.parseEther('1') };
const signedTx = wallet.signTransaction(tx);
console.log(signedTx);

B

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: '1' };
const signedTx = wallet.sign(tx);
console.log(signedTx);

C

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: ethers.utils.parseEther('1') };
const signedTx = await wallet.signTransaction(tx);
console.log(signedTx);

D

const wallet = new ethers.Wallet(privateKey);
const tx = { to: recipient, value: ethers.utils.parseEther('1') };
const signedTx = wallet.sign(tx);
console.log(signedTx);

💡 Hint

Check the correct method name for signing a transaction in ethers.js.

The correct method to sign a transaction in ethers.js is signTransaction. The value must be converted to wei using parseEther. Option C uses the correct method and value format.

🚀 Application

expert

How many outputs does this Bitcoin transaction have after execution?

Given this simplified Bitcoin transaction creation code, how many outputs will the transaction have?

Blockchain / Solidity

const bitcoin = require('bitcoinjs-lib');
const keyPair = bitcoin.ECPair.makeRandom();
const { address } = bitcoin.payments.p2pkh({ pubkey: keyPair.publicKey });

const txb = new bitcoin.TransactionBuilder();
txb.addInput('some-txid', 0);
txb.addOutput('recipient-address', 50000);
txb.addOutput(address, 45000);

const tx = txb.build();
console.log(tx.outs.length);

A 2

B 1

C 3

D 0

💡 Hint

Count how many times addOutput is called.

The transaction builder adds two outputs: one to the recipient and one as change back to the sender's address. So, the transaction has 2 outputs.

Practice

1.

What is the main purpose of sending a transaction on a blockchain?

easy

A. To create a new blockchain network

B. To mine new blocks

C. To move value or data from one account to another

D. To delete data from the blockchain

Solution

1. Step 1: Understand what a transaction does

   A transaction moves value or data between accounts on the blockchain.

2. Step 2: Compare options to the definition

   Only To move value or data from one account to another correctly describes sending a transaction.

3. Final Answer:

   To move value or data from one account to another -> Option C

4. Quick Check:

   Transaction purpose = move value/data [OK]

Hint: Transactions move value or data, not create or delete [OK]

Common Mistakes:

• Confusing transactions with mining
• Thinking transactions create blockchains
• Believing transactions delete blockchain data

2.

Which of the following is the correct way to sign a transaction before sending it?

transaction.sign(____)

easy

A. sender's public key

B. sender's private key

C. receiver's private key

D. network's public key

Solution

1. Step 1: Identify the key needed for signing

   Transactions must be signed with the sender's private key to prove ownership.

2. Step 2: Match the correct key to the method

   Only the sender's private key can sign the transaction securely.

3. Final Answer:

   sender's private key -> Option B

4. Quick Check:

   Sign with private key = sender's private key [OK]

Hint: Sign with sender's private key, never public key [OK]

Common Mistakes:

• Using public key to sign
• Using receiver's key instead of sender's
• Confusing private and public keys

3.

Consider this code snippet sending a transaction:

tx = {
  'to': '0xabc123',
  'value': 10,
  'nonce': 5,
  'gas': 21000
}
signed_tx = sign_transaction(tx, private_key)
result = send_transaction(signed_tx)
print(result)

What will print(result) most likely output if the nonce is incorrect?

medium

A. 'Nonce too low or too high error'

B. 'Transaction successful'

C. 'Gas limit exceeded error'

D. 'Invalid signature error'

Solution

1. Step 1: Understand nonce role in transactions

   Nonce must be correct and sequential to avoid replay or duplication errors.

2. Step 2: Identify error caused by wrong nonce

   An incorrect nonce causes a 'Nonce too low or too high' error during sending.

3. Final Answer:

   'Nonce too low or too high error' -> Option A

4. Quick Check:

   Wrong nonce = nonce error [OK]

Hint: Wrong nonce causes nonce error, not gas or signature errors [OK]

Common Mistakes:

• Confusing nonce error with gas error
• Assuming signature error for nonce issues
• Expecting success despite wrong nonce

4.

Given this code snippet, what is the main error preventing the transaction from sending?

tx = {
  'to': '0xdef456',
  'value': 5,
  'nonce': 3
}
signed_tx = sign_transaction(tx, private_key)
result = send_transaction(signed_tx)
print(result)

Options:

medium

A. Receiver address is invalid

B. Incorrect private key used for signing

C. Nonce value is too high

D. Missing gas field in the transaction

Solution

1. Step 1: Check required transaction fields

   Gas is required to pay for transaction processing; missing gas causes failure.

2. Step 2: Verify other fields and keys

   Nonce and receiver address look valid; no info about wrong private key.

3. Final Answer:

   Missing gas field in the transaction -> Option D

4. Quick Check:

   Missing gas = transaction fails [OK]

Hint: Always include gas field to avoid transaction failure [OK]

Common Mistakes:

• Ignoring gas field requirement
• Assuming nonce or address is wrong without evidence
• Not checking transaction structure

5.

You want to send multiple transactions quickly from the same account. Which approach ensures all transactions are accepted without nonce conflicts?

1. Use the same nonce for all transactions
2. Increment nonce by 1 for each transaction
3. Use random nonce values
4. Skip nonce and rely on network

hard

A. Increment nonce by 1 for each transaction

B. Skip nonce and rely on network

C. Use random nonce values

D. Use the same nonce for all transactions

Solution

1. Step 1: Understand nonce role in multiple transactions

   Nonce must be unique and sequential per account to avoid conflicts.

2. Step 2: Identify correct nonce handling method

   Incrementing nonce by 1 for each transaction ensures proper ordering and acceptance.

3. Final Answer:

   Increment nonce by 1 for each transaction -> Option A

4. Quick Check:

   Sequential nonce = no conflicts [OK]

Hint: Always increment nonce by 1 for each new transaction [OK]

Common Mistakes:

• Reusing same nonce causing rejection
• Using random nonce causing errors
• Assuming network assigns nonce automatically