Remove elements from an array during enumeration

AKA: .enumerate().reversed() or .reversed().enumerate() confusion, why can’t I just filter and get the result efficiently?

A visual illustration where we remove the third element in this particular array. Nice.

Difficulty: Easy | Normal | Challenging

Prerequisites:

• None

Terminology

Enumeration: A complete, ordered listing of all the items in a collection

The problem

You want to iterate through a list and remove elements that conform to some condition. The examples below might seem quite abstract, but there are many cases where this is important and simple filters may not quite fulfil your needs.

The example here is you have an array of times (in milliseconds) and you want to return the number of elements that are within 1000 milliseconds of the last element. The array is in time order, and has at least one element.

Therefore:

[1,2,3,1000] = [1,2,3,1000]

[1000,1001,1004,2000] = 1

1. The easy solution

Filtering has a time complexity of O(n), and gives us a short and easy to understand solution

var last = input.last!

input.filter{ $0 — input.last! <= 1000}