Design and implement a simple expression evaluator for financial rules
Last updated: August 6, 2025
Quick Overview
Build an evaluator for a simple DSL used to define financial rules. Support arithmetic operations, comparisons, logical operators, and variable references. Example: 'credit_score >= 600 AND debt_to_income < 0.4 AND loan_amount <= max_approved'.
Affirm
August 6, 20256
8
4,026 solved
Build an evaluator for a simple DSL used to define financial rules. Support arithmetic operations, comparisons, logical operators, and variable references. Example: 'credit_score >= 600 AND debt_to_income < 0.4 AND loan_amount <= max_approved'.
Affirm's credit decisioning and eligibility rules are configured as expressions that non-engineers (risk analysts) can modify. This problem tests your ability to build a parser/evaluator, which is relevant to Affirm's rules engine infrastructure.
What the Interviewer Expects
- Parse the expression into an abstract syntax tree (AST)
- Evaluate the AST against a context dictionary of variable values
- Handle operator precedence correctly (AND/OR, comparisons, arithmetic)
- Return clear error messages for invalid expressions or missing variables
- Write clean, extensible code that makes adding new operators easy
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 add support for function calls like max(a, b) or round(amount, 2)?
- How would you optimize evaluation for rules that are checked millions of times?
- How would you validate that a rule expression is safe before allowing it to be saved?
- How would you add support for list operations like 'state IN [CA, NY, TX]'?
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Practice DSA ProblemsSample Answer
Problem Analysis
The problem at hand requires us to parse a domain-specific language (DSL) that describes financial rules using various operators and variable names. Given the nature of the expression evaluation invol...
Approach
- Tokenization: First, we need to transform the input expression string into a list of tokens. For example, the input 'credit_score >= 600 AND debt_to_income < 0.4 AND loan_amount <= max_approved...
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