Implement a payment installment calculator
Last updated: August 6, 2025
Quick Overview
Given a purchase amount, number of installments, and APR, calculate the monthly payment, total interest, and generate an amortization schedule. Handle edge cases like zero-interest plans and rounding to cents.
Affirm
August 6, 20259
5
3,244 solved
Given a purchase amount, number of installments, and APR, calculate the monthly payment, total interest, and generate an amortization schedule. Handle edge cases like zero-interest plans and rounding to cents.
This coding problem directly mirrors work at Affirm. It tests your ability to implement financial calculations correctly, handle decimal precision, and produce clean Python code. Rounding errors in financial calculations can cause real monetary discrepancies.
What the Interviewer Expects
- Implement the standard amortization formula correctly
- Handle decimal precision using the decimal module, not floating point
- Generate a full schedule showing principal and interest breakdown per payment
- Handle edge cases: zero APR, single installment, very small amounts
- Write clean, well-tested Python code with proper function decomposition
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- How would you handle the case where rounding causes the last payment to be slightly different?
- How would you extend this to support biweekly payments?
- What if the consumer makes an extra payment? How does the schedule change?
- How would you test this function to ensure accuracy to the penny?
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Problem Analysis
The problem requires us to calculate monthly payments, total interest, and generate an amortization schedule based on provided parameters: purchase amount, number of installments, and APR. Given the n...
Approach
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Input Handling: First, we will read the input values:
principal(purchase amount),num_installments(number of payments), andapr(annual percentage rate). -
**Edge Case for Zero Insta...
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