Design a shopping cart with discount rules
PremiumMachine codingMedium
- Discount rules
- Strategy pattern
- Money in minor units
Quick answer
This machine coding interview question asks you to design a shopping cart with discount rules. Aim for a design that meets its requirements at the stated scale, then explain its main components, data flow and trade-offs. Interviewers want to see how you scope the problem, size it and defend each choice.
The problem
Build a shopping cart with discount rules, like the cart page of an online store. It holds items and quantities, applies the store's active rules and returns an itemised bill. The hard parts are rules that interact, a fixed order of application and money that never drifts by a paisa.
Rules are set by the store, not typed in by the shopper. Shopper-entered coupon codes are a separate engine and out of scope here.
What must a shopping cart with discount rules do?
- Catalog: addProduct(sku, name, category, pricePaise) registers products; prices are integers in paise.
- Cart operations: add(sku, qty), remove(sku, qty), setQty(sku, qty) and clear(). Quantity 0 removes the line; unknown SKUs and negative quantities are rejected.
How well must it do it?
- Exact money: All amounts are integer paise. Percent discounts round half up per line, and the bill adds up exactly.
- Deterministic: The same cart and rules give the same bill, whatever order the rules were added in.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Discount rules
- Strategy pattern
- Money in minor units
- Rule ordering
- Rounding
- Buy x get y
How you’ll practise
- Requirements
- Entities and interfaces
- Core implementation
- Edge cases and tests
- Extensions
- Wrap-up
Lala, personalized to you, gives a hint when you ask and reviews your design when you are ready.
What do interviewers look for?
In any machine coding design round.
- Clarifies the requirements before drawing anything
- Uses a few numbers that actually change the design
- Draws a clear path from the client to storage
- Picks storage and caching with reasons, not by habit
- Explains what breaks first under load and how to fix it
- States trade-offs openly instead of claiming a perfect design
Questions people ask about this interview question
- Which design type is this question?
- It is a machine coding question. You design a shopping cart with discount rules and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Medium. Entry-level engineers can practise it, but it expects you to reason about requirements and trade-offs, not only list components.
- What should I cover first?
- Start with the requirements: what the system must do, how well it must do it and which numbers change the design. Agree on scope with the interviewer before you design anything.
- Can I practise it for free?
- It is a premium question. Sign up and unlock it once for 10 credits; it then stays unlocked. When you finish, Lala, your personalized AI mentor, can review your design for 30 credits.
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Updated · Written by the SevyDevy team