iOS Swit 3 - filter array inside filter - ios

Question

I would like to filter array inside a filter. First I have a big array of Staff object (self.bookingSettings.staffs). Inside this array I have multiple object like this :
"staffs": [
{
"id": 1,
"name": "Brian",
"services": [
{
"id": 1
},
{
"id": 2
},
{
"id": 3
},
{
"id": 4
}
],
"pos": 1
},...
I would like to filter this array in order to have only services with id = 3.
I succeed to have if first object is equal to 3 with this code :
self.bookingSettings.staffs.filter({ $0.services.first?.id == self.bookingService.id })
but that takes only the first item.
I think I have to filter inside my filter function, something like this to loop over all object inside services :
self.bookingSettings.staffs.filter({ $0.services.filter({ $0.id == self.bookingService.id }) })
but I've the following error: Cannot convert value of type [BookingService] to closure result type Bool.
Is this a good idea ? How can I achieve this ?

Answer 1

You could use filter, which would look something like this:
self.bookingSettings.staffs.filter {
!$0.services.filter{ $0.id == self.bookingService.id }.isEmpty
}
This code is constructing an entire array of filtered results, only to check if its empty and immediately discard it. Since filter returns all items that match the predicate from the list, it won't stop after it finds a match (which is really what you're looking for). So even if the first element out of a list of a million elements matches, it'll still go on to check 999,999 more elements. If the other 999,999 elements also match, then they will all be copied into filter's result. That's silly, and can use way more CPU and RAM than necessary in this case.
You just need contains(where:):
self.bookingSettings.staffs.filter {
$0.services.contains(where: { $0.id == self.bookingService.id })
}
contains(where:) short-circuits, meaning that it won't keep checking elements after a match is found. It stops and returns true as soon as find a match. It also doesn't both copying matching elements into a new list.