How do you evaluate a postfix expression using a Stack?

0-2 years experience

1. 1We are building a simple command-line calculator that takes space-separated postfix strings like '3 4 + 2 *'. Could you walk me through how you'd use a stack to parse and evaluate this, and show me how you'd handle the order of operands during subtraction or division?
2. 2Imagine we run your postfix evaluator on the input '5 0 /'. How would you write the evaluation logic to catch this division-by-zero error and return a clean error state instead of crashing the application?

2-5 years experience

1. 1We're adding a feature to a spreadsheet app that evaluates user-defined formulas. Users are complaining that when they enter invalid formulas, the app either freezes or throws unhelpful stack underflow errors. How would you refactor a standard stack-based evaluator to validate the expression on the fly and return precise error messages like 'Missing operand at position X'?
2. 2Our legacy postfix evaluator only handles single-digit integers. We need to upgrade it to support multi-digit numbers, decimals, and variable names (like 'x y +'). How would you design the tokenizer and stack evaluator to handle these token types safely?

5-8 years experience

1. 1We are building a high-throughput IoT telemetry rules engine that evaluates millions of incoming postfix-like threshold expressions per second. Memory allocation is our primary bottleneck. How would you optimize a stack-based evaluator to run with zero-allocation or minimal garbage collection overhead in this pipeline?
2. 2Instead of just evaluating the expression to a single value, we now need to compile these postfix expressions into an Abstract Syntax Tree (AST) for optimization before execution. How would you adapt your stack-based approach to output a tree structure, and how would you handle operator precedence if we transition to infix inputs later?

8+ years experience

1. 1Our platform allows enterprise clients to write custom mathematical rules using a domain-specific language (DSL) that compiles to postfix. Over time, we need to support custom plugins/functions (e.g., 'USER_AGE() 18 >'). How would you design an extensible execution engine architecture that allows other engineering teams to register custom operators and functions without modifying the core stack-based evaluation engine?
2. 2We are migrating a legacy financial calculation engine from a synchronous, stack-based VM to a distributed, parallelized execution model to handle massive datasets. What are the architectural limitations of a stack-based evaluation model in a distributed environment, and how would you design the transition to a register-based or dataflow-graph execution model?