To learn Swift generics, I wrote a function that creates a tableView data source, i.e. a 2-dim (sections, rows) array of elements.
The element type should be generic, and the created data source should be initialised with unique values of the elements.
I declared a protocol that is adopted by the possible element types:
protocol UniqueInit {
associatedtype T
static func uniqueInit() -> T
}
and the dataSource function.
Here, nrRowsInSection is a variadic parameter: The number of arguments given defines the number of sections, and the values of the arguments define the number of rows in the respective section.
static func dataSource<T: UniqueInit>(nrRowsInSection: Int...) -> [[T]] {
var result: [[T]] = []
for nrRows in nrRowsInSection {
var row: [T] = []
for _ in 0 ..< nrRows {
row.append(T.uniqueInit())
}
result.append(row)
}
return result
}
This function does not compile. The statement
row.append(T.uniqueInit())
gives the errors:
Argument type 'T.T' does not conform to expected type 'UniqueInit'
Cannot convert value of type 'T.T' (associated type of protocol 'UniqueInit') to expected argument type 'T' (generic parameter of static method 'dataSource(nrRowsInSection:)')
Obviously, static func uniqueInit() is considered as wrong, but why?
And what would be the correct implementation?
The generic T in your function and the associated type T in your protocol aren't the same T. Inside the function, T is referring to the type that is implementing the protocol so the associatedtype is T.T inside of the function. Your arrays and the return value would have to be using T.T.
This also means that you will need an additional parameter to your function because the [[T.T]] return value isn't enough for the compiler to infer what type T is.
This should work (I changed the generic parameter to U because all the Ts are confusing):
func dataSource<U: UniqueInit>(initializer: U.Type, nrRowsInSection: Int...) -> [[U.T]] {
var result: [[U.T]] = []
for nrRows in nrRowsInSection {
var row: [U.T] = []
for _ in 0 ..< nrRows {
row.append(U.uniqueInit())
}
result.append(row)
}
return result
}
Alternatively, you could define your function as an extension on UniqueInit which would eliminate the need for the generics:
extension UniqueInit {
func dataSource(nrRowsInSection: Int...) -> [[T]] {
var result: [[T]] = []
for nrRows in nrRowsInSection {
var row: [T] = []
for _ in 0 ..< nrRows {
row.append(Self.uniqueInit())
}
result.append(row)
}
return result
}
}
See if the below implementation works for you.
protocol UniqueInit {
static func uniqueInit() -> Self
}
func dataSource<T: UniqueInit>(nrRowsInSection: Int...) -> [[T]] {
var result: [[T]] = []
for nrRows in nrRowsInSection {
var row: [T] = []
for _ in 0 ..< nrRows {
row.append(T.uniqueInit())
}
result.append(row)
}
return result
}
I think the T in the above dataSource implementation should be replaced with UniqueInit.
Related
I have a function that can calculate the sum of numbers in array with condition like so:
func sumOfArrayWithCondition(array: [Int], filter: (element: Int) -> Bool) -> Int {
var result = 0
for i in 0..<array.count where filter(element: array[i]) {
result += array[i]
}
return result
}
Now I want it to work with Int, Float, Double type. I have tried, but didn't work.
protocol Addable {
func +(lhs: Self, rhs: Self) -> Self
}
extension Int: Addable {}
extension Double: Addable {}
extension Float: Addable {}
func sumOfArrayWithCondition<T: Addable>(array: [T], filter: (element: T) -> Bool) -> T {
var result = 0
for i in 0..<array.count where filter(element: array[i]) {
result += array[i] // <-------- Error here
}
return result // <-------- Error here
}
But it says:
Binary operator '+=' cannot be applied to operands of type 'Int' and 'T'
So how to do it.
Any helps would be appreciated. Thanks.
First issue is that the compiler is inferring the type Int for the var result because you don't declare a type and initialize it with 0. But you need result to be of type T.
First, in order to initialize result as an instance of type T with the value 0, you need to specify that Addable is also IntegerLiteralConvertible, which is already true for Int, Double and Float. Then you can declare result as type T and go from there.
As Rob pointed out, you also need to add the += function to your protocol if you want to be able to use it.
So the final code that achieves what you are looking for is:
protocol Addable : IntegerLiteralConvertible {
func +(lhs: Self, rhs: Self) -> Self
func +=(inout lhs: Self, rhs: Self)
}
extension Int: Addable {}
extension Double: Addable {}
extension Float: Addable {}
func sumOfArrayWithCondition<T: Addable>(array: [T], filter: (element: T) -> Bool) -> T {
var result:T = 0
for i in 0..<array.count where filter(element: array[i]) {
result += array[i]
}
return result
}
As Rob said, result is an Int. But there's no need to create that method at all. You're wanting to call that like so, based on your method signature:
let sum = sumOfArrayWithCondition(myArray, filter: myFilter)
instead, all you have to do is use existing methods provided by swift:
let sum = myArray.filter(myFilter).reduce(0) { $0 + $1 }
The following function finds the second index of a given item in Array of Int:
func secondIndexOf(item: Int, inArray array: Array<Int>) -> Int? {
if let firstIndex: Int = array.indexOf(item) {
let slice: ArraySlice<Int> = array.suffixFrom(firstIndex + 1)
return slice.indexOf(item)
}
return nil
}
However, when I attempt to create a generic version of this function to find the second Equatable item, I get an error:
func secondIndexOf<T: Equatable>(item: T, inArray array: Array<T>) -> T? {
if let firstIndex: Int = array.indexOf(item) {
let slice: ArraySlice<T> = array.suffixFrom(firstIndex + 1)
return slice.indexOf(item) // Cannot invoke 'indexOf' with an argument list of type '(T)'
}
return nil
}
Why is this not valid Swift code, and what is the expected argument list if not (T)? Xcode autocomplete shows indexOf(element: Comparable) with which T should be compatible.
The compiler is giving you a confusing error message here—it isn't actually concerned about the argument. The return value is the source of the problem, since you aren't returning a value of type T, but an index of the array. You just need to change your return type to Int?:
func secondIndexOf<T: Equatable>(item: T, inArray array: Array<T>) -> Int? {
if let firstIndex: Int = array.indexOf(item) {
let slice: ArraySlice<T> = array.suffixFrom(firstIndex + 1)
return slice.indexOf(item)
}
return nil
}
Working with Swift generics, I have the following question:
This function works as expected with the type Int:
func + (number: Int, vector: [Int]) -> [Int] {
var resArray:[Int]=[]
for x:Int in vector {
resArray.append(number+x)
}
return resArray
}
I want to make it work with any type where addition makes sense.
I have tried the following:
func +<T:NSNumber> (number: T.Type, vector: [T.Type]) -> [T.Type] {
var resArray:[T.Type]=[]
for x:T.Type in vector {
resArray.append(number+x)
}
return resArray
}
But the line:
resArray.append(number+x)
hits a problem because number and x should obvious support addition.
How should I change my code? I suppose I need to add a constraint on the type. I don't quite know how.
You can define such a constraint as you said, as a protocol like AdditiveSemigroup below.
protocol AdditiveSemigroup {
typealias Out = Self
static func + (a: Self, b: Self) -> Out
}
func +<T: AdditiveSemigroup where T.Out == T> (value: T, vector: [T]) -> [T] {
return vector.map { $0 + value }
}
To make a type conform the protocol above, just define extension on that type.
extension String: AdditiveSemigroup {}
"A" + ["A", "B", "C"] // ==> ["AA", "AB", "AC"]
For the NSNumber, there've been no built-in + operatator, so you have to define it by hand.
extension NSNumber: AdditiveSemigroup {
typealias This = NSNumber
}
func + (a: NSNumber, b: NSNumber) -> NSNumber {
return NSNumber(double: a.doubleValue + b.doubleValue)
}
Now you could apply your special + operator to values of NSNumber.
NSNumber(double: 3) + [NSNumber(double: 5)] // ==> 8
I wondered why map() and filter() in SequenceType return both an Array.
Actually, I don't think that's necessary. Returning a sequence again feels much more sensible to me.
However, I got stuck when trying to add sequential versions. Here's my attempt with map:
extension SequenceType {
func seqMap<T, S: SequenceType where S.Generator.Element == T>(
transform: Self.Generator.Element -> T) -> S
{
var sourceGen = generate()
let tGen: AnyGenerator<T> = anyGenerator {
if let el = sourceGen.next() {
return transform(el)
} else {
return nil
}
}
return AnySequence { tGen }
}
}
XCode tells me at the last return statement the following error:
cannot invoke initializer for type 'AnySequence<T>' with an argument list of type '(() -> AnyGenerator<T>)'
note: overloads for 'AnySequence<T>' exist with these partially matching parameter lists: (S), (() -> G)
Actually, my tGen is of type () -> G, so why does XCode think it is ambiguous?
The problem becomes more apparent if you split the return statement:
let tSeq = AnySequence { tGen }
return tSeq // error: cannot convert return expression of type 'AnySequence<T>' to return type 'S'
The compiler would infer the placeholder type S from the context
of a method call, and that could be any sequence
type with element type T, and not necessarily an AnySequence.
Here is a simple example demonstrating the same problem:
protocol MyProtocol { }
struct MyType { }
extension MyType : MyProtocol { }
func foo<P : Protocol>() -> P {
return MyType() // error: cannot convert return expression of type 'MyType' to return type 'P'
}
To solve the problem, change the return type to AnySequence<T>
and drop the generic type S:
extension SequenceType {
func seqMap<T>(transform: Self.Generator.Element -> T) -> AnySequence<T>
{
var sourceGen = generate()
let tGen: AnyGenerator<T> = anyGenerator {
if let el = sourceGen.next() {
return transform(el)
} else {
return nil
}
}
return AnySequence { tGen }
}
}
which can be written more compactly as
extension SequenceType {
func seqMap<T>(transform: Self.Generator.Element -> T) -> AnySequence<T>
{
var sourceGen = generate()
return AnySequence(anyGenerator {
sourceGen.next().map(transform)
})
}
}
using the map() method of the Optional type.
But note that SequenceType already has a lazy method which returns
a LazySequenceType:
/// A sequence containing the same elements as a `Base` sequence, but
/// on which some operations such as `map` and `filter` are
/// implemented lazily.
///
/// - See also: `LazySequenceType`
public struct LazySequence<Base : SequenceType>
and you can use
someSequence.lazy.map { ... }
to get a (lazily evaluated) sequence of the mapped values.
As an exercise, I'm implementing a map function that takes an array and a function and applies the function to all elements of the array, but I don't know how to declare it such that it works for any type of array.
I can do something like
func intMap(var arr: [Int], fun: (Int) -> Int) -> [Int] {
for i in 0 ..< arr.count {
arr[i] = fun(arr[i])
}
return arr
}
intMap([1,2,3], {x in return x * x})
But this only works for int.
What is the type signature for Swift's built-in map?
Edit:
So I was missing the fact that I can declare param type signatures without declaring their types explicitly.
func myMap<T>(var arr: [T], fun: (T) -> T) -> [T] {
for i in 0 ..< arr.count {
arr[i] = fun(arr[i])
}
return arr
}
myMap([1,2,3], fun: {
x in return x * x
})
Create a new Playground
Just under where it has import UIKit type import Swift
Command click on the word Swift
This will open the Swift library and you can see all the type definitions there.
And you can see:
extension CollectionType {
/// Return an `Array` containing the results of mapping `transform`
/// over `self`.
///
/// - Complexity: O(N).
#warn_unused_result
#rethrows public func map<T>(#noescape transform: (Self.Generator.Element) throws -> T) rethrows -> [T]
Edited to add
Alternatively, you can write a more generalised map
func myMap<T, U>(var arr: [T], fun: T -> U) -> [U] {
var a: [U] = []
for i in 0 ..< arr.count {
a.append(fun(arr[i]))
}
return a
}
Which returns a new array, of a possibly different type, which you can see for yourself by putting this in your playground.
let a = [1, 2, 3]
let b = myMap(a, fun: { x in Double(x) * 2.1 })
a
b