Filter array based on next object - arrays

Question

I'm writing some code to filter out driving trips from the motion sensor. I figured out the best way to do this is to add a subarray to a nested array based on the following:
Detect the first occurrence of a confident automotive event
Add all the following motion events to the same array of the events until the first confident observation that says otherwise.
For example
automotive confidence 2 //Add
automotive confidence 2 //Add
automotive confidence 2 //Add
walking confidence 2 //Add the sub-array to the master array and start over on the next confident automotive event.
Currently i'm doing it this way:
//Remove all uncertain values.
let confidentActivities = activities!.filter{$0.confidence.rawValue == 2}
var needsNew = true
var automotiveActivities:Array<Array<CMMotionActivity>> = Array() //Master array to contain subarrays of automotiveactivity arrays
var automotiveActivitySession:Array<CMMotionActivity> = Array()
for activity in confidentActivities {
if activity.automotive && (!activity.cycling && !activity.running && !activity.walking){
if needsNew {
needsNew = false
}
automotiveActivitySession.append(activity)
} else {
if !needsNew {
//If user is no longer in car, store a cpoy of the session and reset the array
automotiveActivities.append(Array(automotiveActivitySession))
automotiveActivitySession = []
needsNew = true
}
}
}
This solution is not very elegant.
Is there any way to use Swift's Array.filter{} to make this sorting prettier?

Answer 1

Filter is not going to do it, but you can use reduce.
The example below shows how to collect runs of consecutive "A"s (denoting an automotive event) into arrays inside an array of arrays:
let data = ["A","A","A","B","A","A","B","A","A","A","A","B","B","B","A","B","A","A","A","A","A","A","B","A"]
var res = [[String]]()
_ = data.reduce("") { (last: String, current: String) in
if current == "A" {
if last != "A" {
res.append([String]())
}
res[res.count-1].append(current)
}
return current
}
print(res)
The prior value is passed to reduce's function as the first parameter. This makes it possible for the function to decide whether to append to the current list or to start a new list.
The result of this run is as follows:
[ ["A", "A", "A"]
, ["A", "A"]
, ["A", "A", "A", "A"]
, ["A"]
, ["A", "A", "A", "A", "A", "A"]
, ["A"]]

Answer 2

If you're after a pretty solution, you can use split to do this. You just have to provide it with a condition on what should be considered a separator. In your case, this will be any motion event that isn't an automotive one.
let arr = ["A","A","A","B","A","A","B","A","A","C","A","B","D","B","A","B","A","E","A","A","F","A","B","A","B"]
let split = arr.split {$0 != "A"} // insert your condition for whether the given element should be considered a 'seperator'
$0 here is the anonymous closure argument for an element in the array (as it iterates through). You can always expand the closure in order to make the namings more explicit, although it looks less elegant. For example:
let split = arr.split {element in
return element != "A"
}
This will return an array of ArraySlices like so:
[
ArraySlice(["A", "A", "A"]),
ArraySlice(["A", "A"]),
ArraySlice(["A", "A"]),
ArraySlice(["A"]),
ArraySlice(["A"]),
ArraySlice(["A"]),
ArraySlice(["A", "A"]),
ArraySlice(["A"]),
ArraySlice(["A"])
]
If you want them to be explicit Arrays, you can simply use map afterwards:
let split = arr.split {$0 != "A"}.map{Array($0)}
Returns:
[
["A", "A", "A"],
["A", "A"],
["A", "A"],
["A"], ["A"],
["A"],
["A", "A"],
["A"],
["A"]
]