问题
I'm trying to implement a zip function as an extension to Array, and I'm intending for it to be used like this:
let myKeys = ["a","b","c"]
let myVars = [1,2,3]
myKeys.zip(myVars) // ["a":1,"b":2,"c":3]
Here is my attempt:
extension Array {
public func zip<T,U>(vals: [U]) -> [T:U] {
var dict: [T:U] = Dictionary()
for (i,key) in self.enumerate() {
if let k = key as? T {
dict[k] = vals[i]
}
}
return dict
}
}
let myKeys = ["a","b","c"]
let myVars = [1,2,3]
myKeys.zip(myVars) // ERROR: Cannot convert value of type '[Int]' to expected argument type '[_]'
On the last line I am getting an error that I do not completely understand. I understand it to mean that I'm passing an [Int] and it's expecting an [_]. But isn't _ just a generic placeholder here? Why would it complain about receiving an [Int]?
Also, if I implement zip as a class function it has no issues:
class func zip<T,U>(keys keys: [T], vals: [U]) -> [T:U] {
var dict: [T:U] = Dictionary()
for (i,key) in keys.enumerate() {
dict[key] = vals[i]
}
return dict
}
zip(keys: myKeys,vals: myVals) // ["a":1,"b":2,"c":3]
Any thoughts on this would be much appreciated!
回答1:
Note that Array
already has an associated type, it's called Element
, so no need to declare T
. The problem with your code is that you cannot really convert an Element
to undefined type T
(there is no connection between T
and the array element type).
extension Array where Element: Hashable {
public func zip<U>(vals: [U]) -> [Element: U] {
var dict: [Element: U] = Dictionary()
for (i, key) in self.enumerate() {
dict[key] = vals[i]
}
return dict
}
}
let myKeys = ["a","b","c"]
let myVars = [1,2,3]
let map = myKeys.zip(myVars)
print(map)
Also note that I have added Hashable
requirement for the keys.
来源:https://stackoverflow.com/questions/36553852/swift-cannot-convert-value-of-type-int-to-expected-argument-type