Occurrencesif

Occurrencesif is a term used in computer science to describe the operation of counting how many elements in a collection satisfy a given predicate. It is not a standardized built-in operator in most mainstream languages, but it appears in pseudocode, algorithm explanations, and discussions about domain-specific languages and language design. The idea is to return the count of items x for which predicate(x) is true.

Definition: Given a collection C and a predicate P, occurrencesif(C, P) equals the cardinality of { x in C : P(x) }. It is functionally equivalent to applying a filter to C with P and taking the length, or to summing 1 for each x where P(x) holds.

Example: For the list [1, 2, 3, 4, 5] and P(x) = (x % 2 == 0), occurrencesif returns 2.

Implementation notes: In languages with a functional style, you can implement as length(filter(C, P)) or sum(1 for x in C if P(x)). Time complexity is O(n) in the size of C, and memory usage is O(1) if streaming, or O(n) if a filtered list is materialized.

Relation to related concepts: It is closely related to count_if in C++, Python’s sum with a generator, and SQL’s COUNT with a WHERE clause. It emphasizes the predicate rather than the elements themselves.

Status: The term is not widely standardized and appears mainly in informal writings and design discussions rather than official specifications.