Returns the number of elements matching the closure predicate.

Sequence+CountWhere.swift

import Swift

extension Sequence {
    /// Returns the number of elements matching the closure predicate.
    ///
    /// The `isIncluded` closure is called sequentially comparing each
    /// element to determine the number of matches found.
    /// This example shows how to find the number of elements matching
    /// the given predicate.
    ///
    ///     let numbers = [1, 2, 3, 4]
    ///     let count = numbers.count { $0 < 3 }
    ///     // count == 2
    ///
    /// If the sequence has no elements, `isIncluded` is never executed
    /// and the result will be zero.
    ///
    /// - Parameters:
    ///   - isIncluded: The closure to execute for each element to determine if
    ///     its a match
    /// - Returns: The number of elements matching the closure predicate
    ///
    /// - Complexity: O(*n*), where *n* is the length of the sequence.
    @inlinable func count(_ isIncluded: (Element) throws -> Bool) rethrows -> Int {
        try reduce(0, { $0 + (try isIncluded($1) ? 1 : 0) })
    }
}