[SR-1504] RFE: index(of:) but starting from end

[ddunbar]

Previous ID	SR-1504
Radar	None
Original Reporter	@ddunbar
Type	New Feature
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Standard Library
Labels	New Feature
Assignee	@natecook1000
Priority	Medium

md5: 2e4233e0695a5d1edce92ed2ae1572d9

Issue Description:

It is sometimes desirable to get the forward index of an element but searching from the end of a bidirectionally iterable collection. For example, consider finding the string suffix following the last period.

This is similar to C++'s `find_last_of` or Python's `rindex`.

[ddunbar]

Possible naming suggestion: lastIndex(of: )

[ddunbar]

Possible implementation:

extension BidirectionalCollection where Iterator.Element : Equatable {
    // RFE: https://bugs.swift.org/browse/SR-1504
    public func lastIndex(of element: Iterator.Element) -> Index? {
        var i = self.endIndex
        while i != self.startIndex {
            self.formIndex(before: &i)
            if self[i] == element {
                return i
            }
        }
        return nil
    }
}

[belkadan]

This would need to go through the Swift Evolution Process.

[natecook1000]

This is implemented as of Swift 4.2.

[mattneub]

@natecook1000 The docs now say there is a `firstIndex` and a `lastIndex` but no `index`. Yet there is no compile error when using `index`. Shouldn't this be a warning at least? We should all be migrating to `firstIndex` but nothing calls out the change. I've been using Swift 4.2 since WWDC and I only just noticed this by accident.