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Citadel Quant Research Intern Interview: Moving Average in Python

Adnan

Apr 17, 2026

One of the classic interview questions you may encounter is implementing a moving average algorithm in Python. Understanding this concept and its implementation not only demonstrates your programming skills but also your grasp of essential quantitative techniques used in finance and data analysis.

Citadel Quantitative Research Intern Interview Question: Moving Average Implementation in Python

Introduction to Moving Averages
Importance of Moving Average in Quantitative Finance
Understanding the Interview Question
Moving Average: Conceptual Explanation
Mathjax Formula for Moving Average
Step-by-Step Implementation in Python
Efficient Approaches and Complexity Analysis
Edge Cases and Testing
Advanced Moving Averages
Application in Financial Data
Citadel Interview Tips
Full Python Solution
Conclusion

Introduction to Moving Averages

A moving average is a statistical technique widely used in time series analysis, signal processing, and finance. It smooths out short-term fluctuations and highlights longer-term trends or cycles by averaging data points within a fixed-size window as it progresses through the data series.

Simple Moving Average (SMA): The unweighted mean of the previous k data points.
Weighted Moving Average (WMA): Assigns different weights to each data point.
Exponential Moving Average (EMA): Applies decreasing weights exponentially.

For the purpose of this article, we focus on the Simple Moving Average (SMA), as it is the most common form encountered in quantitative interviews and the Citadel Quantitative Research Intern interview.

Importance of Moving Average in Quantitative Finance

Moving averages are foundational in quantitative finance. They are used for:

Smoothing noise: Filtering out short-term volatility in price data.
Trend detection: Identifying upward or downward trends in markets.
Technical indicators: Creating trading signals (e.g., golden cross, death cross).
Risk management: Assessing recent volatility and adjusting positions.

Given their significance, a solid understanding and efficient implementation of moving averages is a must-have skill for prospective quantitative researchers and analysts.

Understanding the Interview Question

Question: Implement a function to compute the moving average of a list with window size k.

This question assesses:

Your understanding of the moving average concept.
Your ability to write clean, efficient, and correct Python code.
Your awareness of edge cases and algorithmic complexity.

Before jumping into coding, let's break down the core components of the problem.

Moving Average: Conceptual Explanation

Given a list of numbers (e.g., prices, returns, or any time series data) and a window size k, the moving average at each position is the average of the current and previous k-1 elements.

For example, for the list: [3, 5, 8, 10, 14, 18] and k = 3:

The first moving average is the mean of [3, 5, 8]
The next is the mean of [5, 8, 10]
Continue sliding the window by one element each time until the end of the list

The result is a shorter list of averages, each representing the mean of a window of size k in the original list.

Mathjax Formula for Moving Average

The mathematical formula for the simple moving average (SMA) of a time series $ x_1, x_2, ..., x_n $ at time t with window size k is:

$$ SMA_t = \frac{1}{k} \sum_{i = t - k + 1}^{t} x_i $$

Where:

$ SMA_t $ is the moving average at position t
$ x_i $ is the value at position i in the series
$ k $ is the window size
$ t \geq k $, so the first moving average can be computed at the k-th element

The output list will have $ n - k + 1 $ elements.

Step-by-Step Implementation in Python

1. Naive Approach

The most straightforward Python implementation involves iterating over the list and, for each window, calculating the mean.


def moving_average_naive(nums, k):
    """
    Compute the moving average of a list using a naive approach.
    Args:
        nums (list of float): Input data.
        k (int): Window size.
    Returns:
        list of float: Moving averages.
    """
    if k <= 0:
        raise ValueError("Window size k must be positive")
    if len(nums) < k:
        return []  # Not enough elements for one window
    result = []
    for i in range(len(nums) - k + 1):
        window = nums[i:i+k]
        avg = sum(window) / k
        result.append(avg)
    return result

Explanation:

Iterate from i = 0 to len(nums) - k, as that’s the last valid window start.
For each window, extract the k elements, compute their sum, divide by k, and append the result.
Edge cases: If k is 0 or negative, or if the list is shorter than k, handle accordingly.

2. Optimized Approach (Sliding Window)

The naive approach recalculates the sum for each window, resulting in $O(nk)$ time complexity. This can be improved to $O(n)$ using a sliding window.


def moving_average_optimized(nums, k):
    """
    Compute the moving average using the sliding window optimization.
    Args:
        nums (list of float): Input data.
        k (int): Window size.
    Returns:
        list of float: Moving averages.
    """
    if k <= 0:
        raise ValueError("Window size k must be positive")
    n = len(nums)
    if n < k:
        return []
    result = []
    window_sum = sum(nums[:k])  # Initial window sum
    result.append(window_sum / k)
    for i in range(k, n):
        window_sum += nums[i] - nums[i - k]
        result.append(window_sum / k)
    return result

Explanation:

Compute the sum of the first window of size k.
Iterate through the rest of the list, updating the sum by adding the new element and subtracting the element that just left the window.
This reduces redundant calculations, improving efficiency, which is crucial in quantitative roles like those at Citadel.

3. Using Python’s Standard Libraries

While interviews usually prefer custom implementations, it's valuable to know Pythonic ways using libraries like itertools for conciseness.


from itertools import islice

def moving_average_itertools(nums, k):
    if k <= 0:
        raise ValueError("Window size k must be positive")
    n = len(nums)
    if n < k:
        return []
    it = iter(nums)
    window = list(islice(it, k))
    result = []
    window_sum = sum(window)
    result.append(window_sum / k)
    for num in it:
        window_sum += num - window.pop(0)
        window.append(num)
        result.append(window_sum / k)
    return result

This method is useful for interview discussions about code readability and leveraging standard libraries.

Efficient Approaches and Complexity Analysis

Approach	Time Complexity	Space Complexity	Notes
Naive	$O(nk)$	$O(n - k + 1)$	Recomputes sum for each window
Sliding Window	$O(n)$	$O(1)$ (not counting result)	Efficient; updates sum incrementally
Library-Based	$O(n)$	$O(k)$	Readable; uses built-in functions

For large time series, always prefer an $O(n)$ sliding window approach.

Edge Cases and Testing

Handling edge cases is critical in interviews and production code. Consider the following:

Empty list: Should return an empty list.
k = 0 or negative: Should raise an error.
k = 1: Result is the same as the original list.
k > len(nums): No valid window; return empty list.
Non-numeric values: Should raise an error or handle gracefully (interview dependent).

Let’s test with some examples:


# Test cases
print(moving_average_optimized([3, 5, 8, 10, 14, 18], 3))  # [5.333..., 7.666..., 10.666..., 14.0]
print(moving_average_optimized([], 3))                      # []
print(moving_average_optimized([1, 2, 3], 1))               # [1.0, 2.0, 3.0]
print(moving_average_optimized([1, 2], 3))                  # []

Advanced Moving Averages

In financial modeling and quantitative research, you may encounter other forms of moving averages:

Exponential Moving Average (EMA):
Applies more weight to recent data points.
$$ EMA_t = \alpha \cdot x_t + (1 - \alpha) \cdot EMA_{t-1} $$ where $\alpha = \frac{2}{k+1}$
Weighted Moving Average (WMA):
Each data point in the window has a different weight.
$$ WMA_t = \frac{\sum_{i=0}^{k-1} w_i x_{t-i}}{\sum_{i=0}^{k-1} w_i} $$

While the Citadel interview may primarily focus on the simple moving average, showing awareness of these variants and their use cases can help you stand out as a candidate.

Application in Financial Data

Moving averages are not just theoretical—they are widely used in real-world financial analysis. Let’s see how to apply our function to historical stock price data.

Example: Moving Average on Stock Prices with `pandas`


import pandas as pd
import yfinance as yf

# Download historical data for Apple (AAPL)
data = yf.download('AAPL', start='2023-01-01', end='2023-12-31')
closing_prices = data['Close'].tolist()

# Calculate 20-day moving average
ma_20 = moving_average_optimized(closing_prices, 20)
print(ma_20[:5])  # Print first 5 values

This demonstrates how your implementation can be directly applied to real market data, a key skill for a Citadel quant intern.

Citadel Interview Tips

Clarify the Problem: Ask about expected output for edge cases.
Discuss Complexity: Explain why you choose a certain approach (e.g., sliding window for efficiency).
Handle Edge Cases: Show awareness of empty lists, invalid k
Write Clean Code: Use clear variable names, docstrings, and comments when necessary.
Test Thoroughly: Demonstrate testing your code with a variety of input scenarios.
Communicate Clearly: Speak aloud your thought process and justify design decisions, especially around complexity and trade-offs.
Mention Extensions: Briefly mention how you could adapt your approach for weighted or exponential moving averages.

Real-World Financial Example: Trading Signals

Moving averages are also used as trading signals. For example, a simple strategy is to buy when the short-term moving average crosses above the long-term moving average (a “golden cross”) and to sell when the opposite occurs (a “death cross”).


import pandas as pd
import yfinance as yf

data = yf.download('AAPL', start='2023-01-01', end='2023-12-31')
data['MA_20'] = data['Close'].rolling(window=20).mean()
data['MA_50'] = data['Close'].rolling(window=50).mean()

data[['Close', 'MA_20', 'MA_50']].plot(figsize=(12,6))
plt.title("AAPL Closing Price with 20 & 50 Day Moving Averages")
plt.xlabel("Date")
plt.ylabel("Price")
plt.show()

This code uses pandas to compute two moving averages and plots them, illustrating how moving averages can signal changes in market trends.

Common Interview Mistakes to Avoid

Not handling edge cases: Always consider k > len(nums), k ≤ 0, or empty input.
Inefficient code: Avoid recomputing sums for each window; use a sliding window.
Incorrect windowing: Off-by-one errors are common when slicing or indexing.
No testing: Always validate your code with multiple test cases.
Not explaining complexity: Interviewers want to hear your understanding of trade-offs.

Conclusion

Implementing a moving average function is a classic quantitative research interview question for Citadel and other top financial firms. It tests your ability to:

Understand and articulate the mathematical concept of moving averages.
Write efficient, clean, and robust Python code.
Handle edge cases and validate with tests.
Discuss extensions and practical applications in quantitative finance.

By mastering both the underlying mathematics and implementation details, you demonstrate not only technical proficiency but also the analytical mindset essential for a successful career in quantitative research. Moving averages are just a starting point—showing depth in your solutions and awareness of real-world applications will help you excel in interviews at Citadel and beyond.

Remember: Always clarify requirements, optimize your code, and communicate your approach clearly. Good luck with your Citadel Quantitative Research Intern interview!

Approach	Time Complexity	Space Complexity	Notes
Naive	\(O(nk)\)	\(O(n - k + 1)\)	Recomputes sum for each window
Sliding Window	\(O(n)\)	\(O(1)\) (not counting result)	Efficient; updates sum incrementally
Library-Based	\(O(n)\)	\(O(k)\)	Readable; uses built-in functions