occurrencesIfC

occurrencesIfC is a conceptual function used in programming and formal discussions to identify or count occurrences in a collection that satisfy a given predicate C. The term is often used in pseudocode or language-agnostic explanations to illustrate predicate-based selection and counting.

Definition and variants: Given a sequence S = [s1, s2, ..., sn] and a predicate C(x) that returns true or false, occurrencesIfC can return different results depending on the goal. Common variants include:

- a list of indices i where C(si) is true,

- a list of elements si that satisfy C,

- or just the count of elements that satisfy C, i.e., |{ i : C(si) is true }|.

Some descriptions differentiate occurrencesIfC (indices), elementsIfC (values), and countIfC (a number), with exact naming varying by language or library.

Implementation notes: In functional languages, you would typically filter elements or map to indices. In imperative languages, a single pass can accumulate either indices, elements, or a running count. The choice affects memory usage and performance: counting may be done in one pass with O(1) extra space, while collecting requires additional storage.

Example (pseudo):

function occurrencesIfC(sequence, C):

result = []

for i from 1 to length(sequence):

if C(sequence[i]):

result.append(i)

return result

Applications: predicate-based filtering, data analysis, text processing, event streams, and any scenario that requires locating elements that meet a condition.

See also: count, filter, findAll, predicate, sequence processing.