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iOS Generics in Swift Generic Functions, Parameters and Constraints CSS Background Blend Modes Challenge

error: type '(T) -> U' has no subscript members return transformation[array]

no really sure what they're asking

generics.swift
func map<T,U>(array: [T], transformation: (T) -> U) -> [U] {

return transformation[array]
}

6 Answers

Alex Koumparos
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Alex Koumparos
Python Development Techdegree Student 36,861 Points

Hi John,

I'm not sure why you've changed your function signature since that was correct in your original question. The function signature you just posted is different from what the question asks for and so even if your code worked perfectly, it wouldn't conform to the requirements of the question.

Let's just assume you've fixed your function signature. As I said before, the body of your function has to apply transformation to every element in array. Your function totally ignores transformation and multiplies the value in array by itself. That's fine (though unnecessary) if transformation just happens to be a squaring function but since this is challenge is explicitly a test of generics, you've got to be able to handle any function that takes a single input and produces a single output. The reason you have transformation as an argument to your function is so that you can use it in your function body.

In the Multiple Type Parameters video (the video immediately before this challenge), Pasan explains and demonstrates the exact thing you are being asked to do to every element in your array (his transform function). You should make sure you understand what this function is doing, and why because you are going to see functions of this form all the time in Swift. If after rewatching the video you still don't fully understand this form of generics, then describe what part of the concept is not making sense. Right now it's impossible to give you any really useful pointers because it's not clear what you understand and what you don't.

You should carefully read this Stack Overflow article on how to ask a good question. It will really enable you to articulate the specifics of where you are struggling so that we can provide you with useful help.

Alex Koumparos
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Alex Koumparos
Python Development Techdegree Student 36,861 Points

Hi John,

Your function signature is correct, so you have to think about what kind of behaviour the function should have that would turn your inputs into your output.

You know that you are going to be taking an array that contains elements of type T. You also know that you you are taking in a function that itself takes T and returns U. Finally, you know that your output is going to need to be an array filled with elements of type U.

Put another way, you want to write a function that will take each element of your input array, then you want to apply transformation to that element, stick the result of that back into an array, and then when you've gone through all the elements in your input array and thus finished building up your output array, return that array.

So what programming construct would you use to repeatedly perform a identical action on each element in array? How about a for loop?

Hopefully that will give you a nudge in the right direction. If you're still stuck, let me know.

Cheers

Alex

thks Alex I know all that, my question is why I can't pass the challenge with this answer.

Best

Alex Koumparos
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Alex Koumparos
Python Development Techdegree Student 36,861 Points

Because you haven't actually implemented the function. All you've done is write a function signature and a return statement (and once you've implemented the substance of the function, you'll see why your return statement isn't right).

Like I said before, you need to fill in the body of the function with something that will actually work through each element in your input array, applying transformation to it, building up an output array, then returning the output array.

Hi,

func trans<T,U>(array:T, numb:(T) -> [U]) -> [U] {

var numb:[Int] = []
for multiplier in array {
    let result = multiplier*multiplier

    numb.append(result)



}
return numb as! [U]

}

cheers

when I'm trying to pass the array into the function

func map<T,U>(array: [T], transformation: (T) -> U) -> [U] {

let value = transformation(array) return value }

it gives me that

swift_lint.swift:5:13: error: '([T]) -> U' is not convertible to '(T) -> U' let value = transformation(array)

Alex Koumparos
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Alex Koumparos
Python Development Techdegree Student 36,861 Points

Right.

Because transformation takes T and you are giving it [T].

You need to apply transformation to every element T in the array of [T].

What programming construct do you use to iterate through all the elements of an array? Hint: I said it in my original answer to your question.

Inside that programming construct you will have an element from the array of type [T] and that element will be of type T. And function transformation can take a T.