Mapping array with the access to previous and next element

msmialko · 2021-07-14T14:40:51.271Z

Is there any algorithm in Swift Core or Swift Algorithms Package that would allow mapping collection's elements based on the tuple (previousElement, currentElement, nextElement) ?

Example:

let a = [1, 2, 3, 4].withNeighborElements()
// [ (nil, 1, 2), (1, 2, 3), (2, 3, 4), (3, 4, nil) ]

If no, what would be the best way to implement this efficiently?

jjatie · 2021-07-14T15:03:48.020Z

windows(ofCount: 3) should work for you

github.com

apple/swift-algorithms/blob/main/Guides/Windows.md

# Windows

[[Source](https://github.com/apple/swift-algorithms/blob/main/Sources/Algorithms/Windows.swift) | 
 [Tests](https://github.com/apple/swift-algorithms/blob/main/Tests/SwiftAlgorithmsTests/WindowsTests.swift)]

Break a collection into overlapping subsequences where
elements are slices from the original collection.

The `windows(ofCount:)` method takes a size as a parameter and returns a
collection of subsequences. Each element of the returned collection is a
successive overlapping slice of the given size.

```swift
let swift = "swift"

let windowed = swift.windows(ofCount: 2) 
// Array(windowed) == [ "sw", "wi", "if", "ft" ]
```

## Detailed Design

This file has been truncated. show original

msmialko · 2021-07-14T15:21:35.496Z

I think windows are not the best fit here.

[1].windows(ofCount: 3) would equal []. wheres we'd like to have [nil, 1, nil]
windows will never return nil for next / previous element. So we'd need to manually add the "first" and "last" window. Something like this:
[nil, array[0], array[1]] + array.windows(ofCount: 3) + [array[lastIdx-1] array[lastIdx], nil]

jjatie · 2021-07-14T15:46:15.760Z

If you do need the empty neigbours at the start/end of the collection, it shouldn't be too much work to update the implementation for your use case. You can also look at AdjacentPairs.

Jon_Shier · 2021-07-14T16:03:30.404Z

You can do this in Combine using Zip3 with nil + sequence.dropLast(), sequence, and sequence.dropFirst() + nil.