`count(where:)` on Sequence

soroush · August 5, 2018, 1:13pm

The review starts tomorrow!

apple/swift-evolution/blob/916268db7ef15250b3d842d765ed238978b9ae04/proposals/0220-count-where.md

# `count(where:)`

* Proposal: [SE-0220](0220-count-where.md)
* Authors: [Soroush Khanlou](https://github.com/khanlou)
* Review Manager: [Chris Lattner](https://github.com/lattner)
* Status: **Scheduled for review (August 6...12)**

## Introduction

While Swift's `Sequence` models brings a lot of niceties that we didn't have access to in Objective-C, like `map` and `filter`, there are other useful operations on sequences that the standard library doesn't support yet. One current missing operation is  `count(where:)`, which counts the number of elements in a `Sequence` that pass some test.

Swift-evolution thread: [`count(where:)` on Sequence](https://forums.swift.org/t/count-where-on-sequence/11186)

## Motivation

Counting the number of objects that pass a test has a wide range of uses in many domains. However, Swift currently doesn't give its users a simple way to perform this operation. While the behavior can currently be approximated with a `filter` and a `count`, this approach creates an intermediate array which it immediately discards. This is a bit wasteful.

    [1, 2, 3, -1, -2].filter({ $0 > 0 }).count // => 3

To correctly avoid a potentially expensive intermediate array, you can use the Swift's `lazy` subsystem:

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