Design a spreadsheet formula engine
PremiumMachine codingHard
- Dependency graph
- Topological sort
- Cycle detection
Quick answer
This machine coding interview question asks you to design a spreadsheet formula engine. 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 spreadsheet formula engine, the calculation core behind spreadsheet apps. Cells hold numbers, text or formulas such as =A1*2+SUM(B1:B3), and changing one cell updates every cell that depends on it. The hard parts are parsing formulas, recalculating only what changed in the right order and catching cycles.
There is no grid UI. Code calls set and get on the engine directly, and a small driver prints cell values.
What must a spreadsheet formula engine do?
- Set and get: set(cell, input) stores a number, text or a formula starting with =; get(cell) returns the computed value or an error value.
- Recalculation: After a set, recompute only cells that depend on it, directly or indirectly, each one once, in dependency order.
How well must it do it?
- Incremental: Changing a cell with 10 dependents recomputes about 10 cells, not the whole sheet.
- No stack overflow: A chain through all 26,000 cells recomputes without deep recursion.
What scale should you design for?
Use these numbers in your estimates.
The scale numbers unlock with this questionUnlock
Key concepts
- Dependency graph
- Topological sort
- Cycle detection
- Recursive descent
- AST evaluation
- Incremental recalculation
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 spreadsheet formula engine and explain your choices the way you would in an interview.
- Is this question asked at entry level?
- It is marked Hard. It suits engineers preparing for senior interviews, where you are expected to drive the discussion and go deep on trade-offs.
- 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