Liquidity pools in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a liquidity pool with two tokens.

Blockchain / Solidity

liquidityPool = {"tokenA": [1], "tokenB": 1000}

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

ANone

B"ETH"

C500

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of a number for token amount.

The liquidity pool needs a numeric value for tokenA amount, so 500 is correct.

2fill in blank

medium

Complete the code to calculate the total liquidity in the pool.

Blockchain / Solidity

totalLiquidity = liquidityPool["tokenA"] + [1]

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

AliquidityPool["tokenB"]

BliquidityPool.tokenB

C"tokenB"

DtokenB

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation which doesn't work for dictionary keys.

Accessing tokenB amount from the dictionary requires the key in brackets and quotes.

3fill in blank

hard

Fix the error in the code to add liquidity correctly.

Blockchain / Solidity

def addLiquidity(pool, token, amount):
    pool[token] [1] amount
    return pool

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

A+=

B-=

C*=

D/=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-=' which subtracts liquidity instead of adding.

To add liquidity, we increase the pool's token amount by the given amount using '+='.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters tokens with liquidity above 500.

Blockchain / Solidity

filteredPool = {token: pool[token] for token in pool if pool[token] [1] [2]

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

A>

B500

C<

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' which filters tokens with less liquidity.

The comprehension keeps tokens with amounts greater than 500, so '>' and 500 are correct.

5fill in blank

hard

Fill all three blanks to create a dictionary of token prices where price is amount divided by 1000.

Blockchain / Solidity

tokenPrices = { [1]: pool[[2]] / [3] for [2] in pool }

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

Atoken

C1000

Dtokens

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names or missing division.

The dictionary comprehension uses 'token' as key and divides pool[token] by 1000 for price.

Practice

1. What is the main purpose of a liquidity pool in blockchain?

easy

A. To allow users to trade tokens directly without a middleman

B. To create new tokens automatically

C. To store user passwords securely

D. To mine new blocks faster

Solution

1. Step 1: Understand liquidity pool function

   Liquidity pools let users trade tokens directly without needing a middleman like an exchange.

2. Step 2: Compare options to definition

   Only To allow users to trade tokens directly without a middleman describes this function correctly; others describe unrelated blockchain features.

3. Final Answer:

   To allow users to trade tokens directly without a middleman -> Option A

4. Quick Check:

   Liquidity pools enable direct token trading = B [OK]

Hint: Liquidity pools remove middlemen in token trading [OK]

Common Mistakes:

• Confusing liquidity pools with token creation
• Thinking liquidity pools mine blocks
• Assuming liquidity pools store passwords

2. Which of the following is the correct way to represent a liquidity pool share in code?

easy

A. shares = {'user1': 100, 'user2': 50}

B. shares = ['user1', 'user2', 100, 50]

C. shares = (100, 50, 'user1', 'user2')

D. shares = 'user1:100, user2:50'

Solution

1. Step 1: Identify data structure for mapping users to shares

   A dictionary (key-value pairs) is best to map user names to their share amounts.

2. Step 2: Check options for dictionary syntax

   shares = {'user1': 100, 'user2': 50} uses a dictionary with user keys and numeric values, which is correct syntax and logic.

3. Final Answer:

   shares = {'user1': 100, 'user2': 50} -> Option A

4. Quick Check:

   Use dictionary for user-share mapping = A [OK]

Hint: Use dictionaries to map users to their shares [OK]

Common Mistakes:

• Using lists instead of dictionaries for key-value pairs
• Incorrect tuple ordering for mapping
• Using string instead of structured data

3. Given this Python code simulating a liquidity pool token ratio update:

pool = {'tokenA': 1000, 'tokenB': 2000}
new_tokenA = 100
new_tokenB = 200
pool['tokenA'] += new_tokenA
pool['tokenB'] += new_tokenB
price_ratio = pool['tokenB'] / pool['tokenA']
print(round(price_ratio, 2))

What is the printed output?

medium

A. 1.90

B. 2.20

C. 2.0

D. 3.00

Solution

1. Step 1: Calculate updated token amounts in pool

   tokenA = 1000 + 100 = 1100; tokenB = 2000 + 200 = 2200

2. Step 2: Compute price ratio and round

   price_ratio = 2200 / 1100 = 2.0; rounded to 2 decimals is 2.0

3. Final Answer:

   2.0 -> Option C

4. Quick Check:

   2200 ÷ 1100 = 2.0 [OK]

Hint: Add tokens first, then divide for ratio [OK]

Common Mistakes:

• Dividing before adding new tokens
• Rounding incorrectly
• Mixing tokenA and tokenB values

4. This code snippet tries to update a liquidity pool but has a bug:

pool = {'tokenA': 500, 'tokenB': 1000}
new_tokenA = 50
new_tokenB = 100
pool['tokenA'] =+ new_tokenA
pool['tokenB'] =+ new_tokenB
print(pool)

What is the bug?

medium

A. The dictionary keys 'tokenA' and 'tokenB' are misspelled

B. The operator '=+' is incorrect; should be '+='

C. The print statement is missing parentheses

D. The new_token variables should be strings, not integers

Solution

1. Step 1: Identify operator usage in assignment

   The code uses '=+' which is not a valid operator; it assigns positive new_tokenA instead of adding.

2. Step 2: Correct operator for addition assignment

   The correct operator is '+=' to add new_tokenA to pool['tokenA'] and similarly for tokenB.

3. Final Answer:

   The operator '=+' is incorrect; should be '+=' -> Option B

4. Quick Check:

   Use '+=' to add values in place [OK]

Hint: Use '+=' to add and assign in one step [OK]

Common Mistakes:

• Confusing '=+' with '+=' operator
• Assuming print needs no parentheses in Python 3
• Thinking keys are misspelled

5. You want to write a function that calculates each user's share percentage in a liquidity pool given a dictionary of shares like {'Alice': 300, 'Bob': 700}. Which code correctly returns a new dictionary with user names and their share percentages rounded to 2 decimals?

hard

A. def calc_shares(shares):
total = len(shares)
return {user: amount / total for user, amount in shares.items()}

B. def calc_shares(shares):
total = sum(shares.keys())
return {user: amount / total for user, amount in shares.items()}

C. def calc_shares(shares):
total = sum(shares.values())
return [round(amount / total * 100, 2) for amount in shares.values()]

D. def calc_shares(shares):
total = sum(shares.values())
return {user: round(amount / total * 100, 2) for user, amount in shares.items()}

Solution

1. Step 1: Calculate total shares correctly

   Sum the values of the shares dictionary to get total tokens contributed.

2. Step 2: Compute percentage per user and round

   Use dictionary comprehension to divide each user's amount by total, multiply by 100, and round to 2 decimals.

3. Final Answer:

   def calc_shares(shares): total = sum(shares.values()) return {user: round(amount / total * 100, 2) for user, amount in shares.items()} -> Option D

4. Quick Check:

   Sum values, divide each, round = correct share % [OK]

Hint: Sum values, then divide each share by total and round [OK]

Common Mistakes:

• Summing keys instead of values
• Returning list instead of dictionary
• Using length instead of sum for total