Every Trace

A medium Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
medium
Seniority
L3

Problem

A monitoring feed records one reading per slot. You're auditing for a specific value: return the slots where that value appears, earliest first.

It is in there somewhere. Where exactly?

Example 1

Input:

target = 20, readings = [10,20,30,20,40,20]

Output:

[1,3,5]

Example 2

Input:

target = 3, readings = [1,2,3,4,5]

Output:

[2]

Worked solution and explanation

What this really is

Under the audit-log costume this is just 'give me every index where the value matches', and the tell is that people reach for list.index out of habit. It hands back the FIRST hit and nothing else, so the moment the target repeats you have silently dropped positions; call it in a loop to find the rest and you re-scan from the front each time, turning an O(n) walk into O(n^2). The skill being probed is walking position and value together and collecting as you go, plus not forgetting that 'nothing matched' is a real, empty answer.

Break down the requirements

Step 1: Walk with the index in hand

Use enumerate (or range(len(readings))) so each step gives you both the position and the value at once.

Step 2: Keep every position that matches

Compare each reading to the target with == and append the index when they match. Because you scan left to right, the collected indices are already ascending.

The solution

Single-pass scan with equality check
def index_of_all(readings, target):
    result = []
    for i, value in enumerate(readings):
        if value == target:
            result.append(i)
    return result

Common follow-up questions