itemsby

Itemsby is a term used in data processing and programming to describe the operation of grouping a collection of items by a key derived from each item. It is not a universal built-in function across programming languages; its exact form and return type depend on the environment or library in use. Typically, itemsby is implemented as a higher-order construct that accepts a collection and a key function, producing a mapping from each key value to the list of items that share that key.

Common usage involves organizing data to enable per-group processing, counting, or aggregation. For example, itemsby(people, p => p.country) would produce a map from country names to the list of people from that country. Similarly, itemsby(orders, o => o.productId) groups orders by product, facilitating product-level summaries.

Relation to related concepts: itemsby is similar in purpose to SQL's GROUP BY and to functional patterns such as Python’s groupby (often requiring pre-sorting) or generic partitioning. Different libraries may expose the concept under different names or APIs, but the underlying idea is to partition a collection into homogeneous subcollections for downstream operations.

Implementation notes: Because there is no standard specification, implementations vary in return type and semantics. Most produce a dictionary, map, or iterable of groups, with the key being the grouping attribute. The efficiency of grouping depends on the data structure used and the cost of evaluating the key function. Handling of missing or inconsistent keys also varies by platform.

Limitations: Expensive key computations, very large or highly diverse key spaces, and mutable data sources can complicate grouping results. See also: Group by, Grouping, Partition, Map, Reduce.