Elementcountp

Elementcountp is a term used in programming to denote a function or operation that counts the number of elements in a collection that satisfy a given predicate p. The name is descriptive: it combines element counting with a predicate function, and it is commonly used in tutorials and pseudocode to illustrate higher-order functions.

Definition: elementcountp takes two arguments: a predicate p and a collection coll. It returns the number of elements x in coll for which p(x) evaluates to true. The predicate is typically a function that accepts a single argument and returns a boolean.

Signature examples: In a Lisp-like notation: (elementcountp pred coll). In Python-like pseudocode: def elementcountp(pred, coll): return sum(1 for x in coll if pred(x)).

Semantics and complexity: The operation is typically linear in the size of the input collection, O(n). It does not modify the input collection. It should handle empty collections by returning 0. Some implementations may support streaming collections to avoid materializing intermediate results.

Variants: Some libraries may overload elementcountp to accept a comparison value, or to count elements equal to a given value without a predicate, or to count with an index-aware predicate. There are related functions such as count, filter, and reduce, which offer alternative ways to express the same idea.

Examples: elementcountp(is_even, [1, 2, 3, 4, 5]) might return 2 in languages where is_even is a valid predicate. Different languages will have different syntax, but the underlying concept remains the same: tallying elements that satisfy a condition.

Notes: As naming varies, some languages provide built-in count-with-predicate facilities under different names. elementcountp serves as a concise, descriptive abstraction for this common data-processing operation.