Group Array Objects by Key | Dataford Interview Questions

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Problem

The J.P. Morgan Access dashboard receives payment records as an array of objects. Implement a function that groups these records by a specified object key, returning every record under the value found at that key.

Use a hash map to achieve linear time complexity. The groups must preserve the original order of records, and the order in which distinct groups first appear must also be preserved.

Formal Specification

Implement group_by_key(items, key).

items is a list of dictionaries.
key is a string, and every dictionary contains that key.
Each key value is hashable, such as a string, integer, or boolean.
Return a dictionary mapping each distinct key value to a list of the original dictionaries.
Do not modify items or any dictionary inside it.

Constraints

0 <= len(items) <= 10^5
1 <= len(key) <= 50
Every object contains the requested key
Each grouping value is hashable
Do not modify the input list or its dictionaries

Function Signature

def group_by_key(items, key):