Balance calculation algorithm in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the balance variable correctly.

LLD

balance = [1]

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

A0

BNone

C''

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing balance as None causes errors when adding amounts.

Using a negative number as initial balance is incorrect.

The balance should start at zero to correctly accumulate transactions.

2fill in blank

medium

Complete the code to add a transaction amount to the balance.

LLD

balance = balance [1] transaction_amount

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

A/

B+

C*

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition reduces the balance incorrectly.

Using multiplication or division is not appropriate here.

Adding the transaction amount updates the balance correctly.

3fill in blank

hard

Fix the error in the condition to check if the balance is negative.

LLD

if balance [1] 0:
    print('Balance is negative')

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

A>

B==

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than or equal to checks for positive or zero balances.

Using equality checks only for zero balance.

The balance is negative if it is less than zero.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps account IDs to their balances if balance is positive.

LLD

positive_balances = {account_id: [1] for account_id, [2] in accounts.items() if balance > 0}

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

Abalance

Caccount_balance

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causes errors or undefined variables.

Not matching the variable in the condition with the loop variable.

The dictionary comprehension uses 'balance' as the value and 'account_balance' as the variable name for balances.

5fill in blank

hard

Fill all three blanks to create a function that calculates the total balance from a list of transactions.

LLD

def calculate_total_balance(transactions):
    total = [1]
    for [2] in transactions:
        total [3]= transaction
    return total

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

A0

Btransaction

C+

Dbalance

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing total to None or other types causes errors.

Using incorrect loop variable names or operators.

The function initializes total to zero, iterates over each transaction, adds it to total, and returns the total balance.

Practice

1. What is the main purpose of a balance calculation algorithm in a financial system?

easy

A. To add credits and subtract debits from an initial amount

B. To sort transactions by date

C. To encrypt user data

D. To generate random transaction IDs

Solution

1. Step 1: Understand the role of balance calculation

   The balance calculation algorithm updates the current balance by adding credits and subtracting debits.

2. Step 2: Compare with other options

   Sorting transactions, encrypting data, and generating IDs are unrelated to balance calculation.

3. Final Answer:

   To add credits and subtract debits from an initial amount -> Option A

4. Quick Check:

   Balance calculation = add credits - subtract debits [OK]

Hint: Balance means adding credits and subtracting debits [OK]

Common Mistakes:

• Confusing balance calculation with sorting or encryption
• Thinking balance calculation generates IDs
• Ignoring subtraction of debits

2. Which of the following code snippets correctly initializes a balance variable to zero in a balance calculation algorithm?

easy

A. balance := 0

B. balance = 0

C. balance == 0

D. balance = 'zero'

Solution

1. Step 1: Identify correct assignment syntax

   In most programming languages, = assigns a value. So balance = 0 sets balance to zero.

2. Step 2: Check other options for errors

   := is not standard in many languages, == is a comparison, and assigning a string 'zero' is incorrect for numeric balance.

3. Final Answer:

   balance = 0 -> Option B

4. Quick Check:

   Use = for assignment, not == or := [OK]

Hint: Use single = to assign values in most languages [OK]

Common Mistakes:

• Using == instead of = for assignment
• Using := which is not common in many languages
• Assigning string instead of numeric zero

3. Consider this pseudocode for balance calculation:

balance = 100
transactions = [20, -10, 30, -5]
for t in transactions:
    balance += t
print(balance)

What will be the printed balance?

medium

A. 135

B. 145

C. 155

D. 125

Solution

1. Step 1: Calculate sum of transactions

   Sum = 20 + (-10) + 30 + (-5) = 20 - 10 + 30 - 5 = 35

2. Step 2: Add sum to initial balance

   Initial balance 100 + 35 = 135

3. Final Answer:

   135 -> Option A

4. Quick Check:

   100 + (20 -10 +30 -5) = 135 [OK]

Hint: Add all transactions to initial balance [OK]

Common Mistakes:

• Adding absolute values instead of signed values
• Forgetting to add initial balance
• Miscalculating sum of transactions

4. Identify the bug in this balance calculation code snippet:

balance = 50
transactions = [10, -20, 15]
for t in transactions:
    balance = t
print(balance)

medium

A. The initial balance is not set

B. The transactions list is empty

C. The balance is overwritten instead of updated

D. The loop variable is incorrect

Solution

1. Step 1: Analyze the loop operation

   Inside the loop, balance = t overwrites balance each time instead of adding.

2. Step 2: Understand correct update

   It should be balance += t to add each transaction to balance.

3. Final Answer:

   The balance is overwritten instead of updated -> Option C

4. Quick Check:

   Use += to update balance, not = [OK]

Hint: Use += to add transactions, not = [OK]

Common Mistakes:

• Using = instead of += inside loop
• Assuming transactions list is empty
• Ignoring initial balance

5. You need to design a balance calculation system that handles millions of transactions per day with real-time updates. Which design choice best supports scalability and accuracy?

hard

A. Ignore debits and only add credits to simplify calculation

B. Use a single database transaction to update balance after each transaction

C. Store all transactions in memory and recalculate balance on every request

D. Batch transactions and update balance periodically with distributed processing

Solution

1. Step 1: Consider scalability needs

   Millions of transactions require efficient processing; single updates per transaction cause bottlenecks.

2. Step 2: Evaluate batch processing benefits

   Batching with distributed processing reduces load and maintains accuracy by processing groups of transactions.

3. Step 3: Reject other options

   Single DB transactions cause slowdowns, in-memory recalculation is memory-heavy and slow, ignoring debits causes incorrect balances.

4. Final Answer:

   Batch transactions and update balance periodically with distributed processing -> Option D

5. Quick Check:

   Batch + distributed processing = scalable & accurate [OK]

Hint: Batch and distribute processing for large-scale balance updates [OK]

Common Mistakes:

• Updating balance per transaction causing bottlenecks
• Recalculating balance on every request wasting memory
• Ignoring debits leading to wrong balances