In several aspects, an electronic device and method index a repository of N documents by W words, by not storing between queries, N*W numbers that are specific to each word i and each document j, normally used to compute a score of relevance to a query, of each document j. Instead, the electronic device and method generate the N*W word-specific -document-specific numbers dynamically at query time, based on a set of W numbers corresponding to the W words, and one or more sets (e.g. x sets) of N numbers corresponding to the N documents. Query-time generation of word-specific-document-specific numbers reduces memory otherwise required, e.g. to store these numbers. Hence, in certain aspects W+xN numbers are maintained between queries, and these numbers are changed incrementally when a new document is added to the set or an existing document is removed. Maintaining W+xN numbers reduces processing otherwise required, to start from scratch.