Implement Left Join Using Python Dictionaries Efficiently

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Q: What difficulty level is this coding question?

This is a medium difficulty Data Manipulation (SQL/Python) 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

Orders

+---------+----------+--------+
| order_id| customer | amount |
+---------+----------+--------+
|    101  |    C1    |  250   |
|    102  |    C2    |  300   |
|    103  |    C3    |  150   |

​

Customers

+----------+-----------+
| customer | city      |
+----------+-----------+
|   C1     |  Seattle  |
|   C3     |  Boston   |
|   C4     |  Austin   |

##### Scenario

Performing a left join in pure Python without external libraries.

##### Question

Write Python code (no third-party packages) to left-join two lists of dictionaries on key "customer"; discuss an O(N+M) hashing solution.

##### Hints

Contrast nested loops with dict-based look-ups; handle missing matches gracefully.

PracHub · Accepted Answer

This question evaluates competency in tabular data manipulation and join semantics using Python dictionaries, emphasizing understanding of hashing-based lookups and algorithmic complexity such as O(N+M).

Quick Overview

Implement Left Join Using Python Dictionaries Efficiently