The Map Reducer

A medium Python interview practice problem on DataDriven. Write and execute real python code with instant grading.

Domain

Python

Difficulty

medium

Seniority

L3

Problem

Given a list of `values`, a `mapper`, and a `reducer`, return the single value produced by applying `mapper` to every element and then reducing the mapped results left-to-right with `reducer`. The catch: `mapper` and `reducer` arrive as SOURCE STRINGS of lambda expressions (for example the string "lambda x: x * 2" and the string "lambda a, b: a + b"), so you must `eval` each string into a real callable before you can call it. `mapper` takes one argument and returns a transformed value; `reducer` takes two arguments (accumulator, next) and returns one combined value, applied cumulatively from left to right like `functools.reduce`. Return the final reduced value. If `values` is empty, return `None`.

Summary

Map it. Reduce it. One answer.

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