L1 · FoundationsList basics~1 min · 4 tests

Average of a List of Numbers

Compute the mean of a Python list safely, returning 0.0 for an empty list instead of crashing with ZeroDivisionError. Simple but often fumbled — run it.

The problem

Return the average of nums as a float. Return 0.0 for an empty list.

Examples

Example 1
Input
```
average([1, 2, 3, 4])
```
Expected output
```
2.5
```
Example 2
Input
```
average([10])
```
Expected output
```
10.0
```

+ 2 hidden tests on Submit.

Edge cases to ask about

Empty list

Hints

0/3

How an interviewer scores this

0/9

1. Understand the problemRestate it, ask about edge cases and input size.
2. Write a brute-force solutionCorrect first, clever later.
3. Get the correct outputautoAll visible examples pass.
4. Analyse time complexityautoCommit to a Big-O in the Complexity Lab.
5. Analyse space complexityautoExtra memory, not the input.
6. Identify the bottleneckautoMeasure your code — where does it bend?
7. OptimiseUsually: a set or dict instead of a scan.
8. Explain why it worksSay it out loud to Solvi.
9. Test the edge casesautoSubmit passes every hidden test.

Python 3.13 · average

def average(nums):
    """Mean of nums as a float; 0.0 if empty."""
    # Write your solution here
    pass

⌘/Ctrl + Enter runs the examples

Your code runs in real CPython inside your browser — nothing is sent anywhere. The first run downloads the interpreter (about 6 MB, once). Your code is saved on this device as you type.

Complexity Lab

What does this cost as n grows?

Interviewers score the analysis as much as the code. Commit to an answer first — then check it, and read why.

Time complexity of the optimal solution

Space complexity (extra memory)

Pick both to reveal the answer.

From brute force to optimal

The progression an interviewer wants to hear, one step at a time.

Approach	Time	Space	Idea
sum / len	O(n)	O(1)	Guard against dividing by zero.

Walkthrough of the optimal approach (try it yourself first)

sum(nums) / len(nums) — with a guard, because an empty list would raise ZeroDivisionError. In Python 3, / is true division, so the result is a float. statistics.mean also exists.

Complexity: O(n) time, O(1) space. sum() makes one pass; len() is O(1).

Reveal the reference solution

def average(nums):
    if not nums:
        return 0.0
    return sum(nums) / len(nums)

Follow-ups interviewers ask

Running average of a stream without storing it.

Frequently asked interview questions

Core interview concepts, complexities, and follow-ups scored by hiring teams.

What is the time complexity of Average of a List of Numbers in Python?

The optimal solution runs in O(n) time and O(1) auxiliary space. sum() makes one pass; len() is O(1).

What follow-up questions do interviewers ask about Average of a List of Numbers?

Running average of a stream without storing it.