Swift iOS -Compare elements in 1 array by property - ios

Question

I have an array of objects and I want to compare the objects based on property to find out if the properties are all the same. Right now I loop through all the objects, place all values of the properties in a separate array, and then use filterArr.allSatisfy { $0 == filterArr.last } to detemermine wether the properties are all the same or not.
This method works but I know there has to be a more elegant way then what I'm doing.
I actually went looking for an answer to this but every single thing I came across was comparing the elements of 2 different arrays instead of 1.
class IceCream {
var flavor: String?
var price: Double?
}
let iceCream1 = IceCream()
iceCream1.flavor = "vanilla"
iceCream1.price = 1.5
let iceCream2 = IceCream()
iceCream2.flavor = "chocolate"
iceCream2.price = 2.0
let iceCream3 = IceCream()
iceCream3.flavor = "vanilla"
iceCream3.price = 1.5
let iceCream4 = IceCream()
iceCream4.flavor = "strawberry"
iceCream4.price = 2.5
let iceCreams = [iceCream1, iceCream2, iceCream3, iceCream4]
var filterArr = [String]()
for iceCream in iceCreams {
filterArr.append(iceCream.flavor ?? "")
}
let areItemsEqual = filterArr.allSatisfy { $0 == filterArr.last }
print(areItemsEqual) // prints false

Answer 1

You can avoid having to initialize and then assign the properties on separate lines with a struct.
struct IceCream {
let flavor: String?
let price: Double?
}
let iceCreams: [IceCream] = [
IceCream(flavor: "vanilla", price: 1.5),
IceCream(flavor: "chocolate", price: 2.0),
IceCream(flavor: "vanilla", price: 1.5),
IceCream(flavor: "strawberry", price: 2.5)
]
Using the generics sugar provided by #Alexander and #matt, we have a nice looking extension.
extension Collection {
func allEqual<T: Equatable>(by key: KeyPath<Element, T>) -> Bool {
return allSatisfy { first?[keyPath:key] == $0[keyPath:key] }
}
}
print(iceCreams.allEqual(by: \.flavor))
Alternatively, you could specify an IceCream be equal to one another by flavor.
extension IceCream: Equatable {
static func == (lhs: IceCream, rhs: IceCream) -> Bool {
return lhs.flavor == rhs.flavor
}
}
extension Collection where Element: Equatable {
func allEqual() -> Bool {
return allSatisfy { first == $0 }
}
}
print(iceCreams.allEqual())

Answer 2

Here's a pretty Swifty way to do it. I define an extension on Collection that checks for equality among the collection's items, according to a given predicate:
extension Collection {
func allEqual<T: Equatable>(by deriveKey: (Element) -> T) -> Bool {
guard let firstElement = self.first else {
return true // empty lists are all-equal
}
let sampleKey = deriveKey(firstElement)
return self.dropFirst().allSatisfy{ deriveKey($0) == sampleKey }
}
}
struct IceCream {
let flavor: String
let price: Double
}
let iceCreams = [
IceCream(flavor: "vanilla", price: 1.5),
IceCream(flavor: "chocolate", price: 2.0),
IceCream(flavor: "vanilla", price: 1.5),
IceCream(flavor: "strawberry", price: 2.5)
]
let allItemsEqualByFlavour = iceCreams.allEqual(by: { $0.flavor})
print(allItemsEqualByFlavour) // false
let vanillaOnlyIceCreams = iceCreams.filter{ $0.flavor == "vanilla" }
print(vanillaOnlyIceCreams.allEqual(by: { $0.flavor})) // true

Answer 3

Here's an elegant way to make sure your ice creams are the same along any arbitrary axis, i.e. either flavor or price or any other equatable property you may later inject:
extension Array {
func allSameForProperty<T:Equatable> (_ p:KeyPath<Element,T>) -> Bool {
return self.isEmpty || self.allSatisfy{
self.first![keyPath:p] == $0[keyPath:p]
}
}
}
Let's test it. First, some initial conditions:
struct Icecream {
let flavor : String
let price : Double
}
let arr = [
Icecream(flavor: "vanilla", price: 1.5),
Icecream(flavor: "vanilla", price: 1.75)
]
And now the actual test:
arr.allSameForProperty(\Icecream.flavor) // true
arr.allSameForProperty(\Icecream.price) // false