Implement Infinite Fibonacci Generator Using Lazy Evaluation

Q: Implement Infinite Fibonacci Generator Using Lazy Evaluation

This question evaluates understanding of lazy evaluation and Python generator semantics, testing a candidate's ability to work with iterative sequence production and manage state across iterations.

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Use PracHub's coding console to write, test, and debug your solutions in Python or JavaScript. View hints, test against sample inputs, and compare with official solutions.

Q: What difficulty level is this coding question?

This is a Medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Citadel.

Q: What role is this question designed for?

This question is commonly asked for Data Scientist candidates at Citadel during technical interviews.

Question

##### Scenario

Testing understanding of Python lazy evaluation and generators.

##### Question

Explain what lazy evaluation means in Python and implement a generator using "yield" that produces an infinite Fibonacci sequence.

##### Hints

Show how state is preserved between yields and why values are computed only when requested.

PracHub · Accepted Answer

from typing import List

def lazy_fibonacci(n: int) -> List[int]:
    if n < 0:
        raise ValueError("n must be non-negative")

def fib_gen():
        a, b = 0, 1
        while True:
            yield a
            a, b = b, a + b

gen = fib_gen()
    out: List[int] = []
    for _ in range(n):
        out.append(next(gen))
    return out

The inner generator fib_gen maintains state in variables a and b. Each call to next(gen) yields the current value a and then advances the state to the next pair (b, a+b). This is lazy: numbers are computed only when next is called. The outer function calls next exactly n times and collects the results into a list.

Quick Overview

Implement Infinite Fibonacci Generator Using Lazy Evaluation

Constraints

Solution

Explanation

Hints