I'm new to Swift. I'm having dictionary and i'm passing it to some method which should be able to modify it. What's the best solution for it, return new Dictionary (seems to be inefficient because of copying needed):
func method(let dict : [String:String]) -> [String:String] {
var newDictionary = [String:String]()
... // copy to newDictionary/remove existing items
}
or pass it as inout argument (is it working?):
func method(inout dict : [String:String]) {
... // modify existing dictionary
}
An in-out parameter has a value that is passed in to the function, is modified by the function, and is passed back out of the function to replace the original value.
As said here, it will modify the instance that is passed as inout parameter. So in your case it's definitively what you want to do, in my opinion.
https://developer.apple.com/library/mac/documentation/Swift/Conceptual/Swift_Programming_Language/Functions.html ctrl+f "inout"
Related
How do I convert an Observable of type Int to a Variable of type Int?
Here is what I have tried so far:
let obs: Observable<Int> = Observable.of(4)
let variable: Variable<Int> = obs
.flatMap { num in
return Variable<Int>(num)
}
Using flatMap feels like the most logical way of doing the conversion, however, I get a compiler error which essentially says that it does not like the flatMap returning anything that is not an observable. Any ideas?
EDIT:
Just to give some context as to why I need to convert the Observable into a Variable:
I need my viewModel to drive a UIPickerView inside a ViewController. Currently, I have two exposed properties in my ViewModel, items: Observable<[String]> (which is the datasource for the UIPickerView) and selectedIndex: Variable. The selectedIndex will drive the initial value of the UIPickerView but also keep track of any changes the user makes.
The reason I needed to convert from the Observable to Variable was that I needed to figure out which index of the UIPickerView should be selected initially:
items.map { days -> Int in
if let index = days.index(where: { str in
return str == selectedDay
}) {
return index
}
return 0
}
.flatMap { num in
return Variable<Int>(num)
}
I'm now wondering if I'm over complicating things, and if instead of the Variable in I should just have an Observable.
A few points here:
First, You should not use Variable as it's deprecated (gonna be entirely deprecated in Swift 5 probably). Look at BehaviorSubject and BehaviorRelay instead.
About your question, I'm not sure what is the use case for having a Variable equal to an Observable, so there are two different thoughts I have:
You can bind an Observable to a Variable/Subject/Relay (e.g. observable.bind(to: variable))
You can actually go the other way around an get an Observable from the Variable, which might make more sense. e.g. variable.asObservable()
I get cannot use map[string]MyType literal (type map[string]MyType) as type map[string]IterableWithID in argument to MapToList with the code below, how do I pass in a concrete map type to method that expects a interface type?
https://play.golang.org/p/G7VzMwrRRw
Go's interface convention doesn't quite work the same way as in, say, Java (and the designers apparently didn't like the idea of getters and setters very much :-/ ). So you've got two core problems:
A map[string]Foo is not the same as a map[string]Bar, even if Bar implements Foo, so you have to break it out a bit (use make() beforehand, then assign in a single assignment).
Interface methods are called by value with no pointers, so you really need to do foo = foo.Method(bar) in your callers or get really pointer-happy to implement something like this.
What you can do to more-or-less simulate what you want:
type IterableWithID interface {
SetID(id string) IterableWithID // use as foo = foo.SetID(bar)
}
func (t MyType) SetID(id string) IterableWithID {
t.ID = id
return t
}
...and to deal with the typing problem
t := make(map[string]IterableWithID)
t["foo"] = MyType{}
MapToList(t) // This is a map[string]IterableWithID, so compiler's happy.
...and finally...
value = value.SetID(key) // We set back the copy of the value we mutated
The final value= deals with the fact that the method gets a fresh copy of the value object, so the original would be untouched by your method (the change would simply vanish).
Updated code on the Go Playground
...but it's not particularly idiomatic Go--they really want you to just reference struct members rather than use Java-style mutators in interfaces (though TBH I'm not so keen on that little detail--mutators are supes handy to do validation).
You can't do what you want to do because the two map types are different. It doesn't matter that the element type of one is a type that implements the interface which is the element type of the other. The map type that you pass into the function has to be map[string]IterableWithID. You could create a map of that type, assign values of type MyType to the map, and pass that to the function.
See https://play.golang.org/p/NfsTlunHkW
Also, you probably don't want to be returning a pointer to a slice in MapToList. Just return the slice itself. A slice contains a reference to the underlying array.
I have a method that iterates over an array and call other method with every element as argument. If I declare this method as:
func didFinishedListFiles(files: [FileModel]) {
for var fileData in files {
self.downloadSingleFile(NSUUID(UUIDString: fileData.uuid!)!);
}
}
Xcode shows a warning:
Variable 'fileData' was never mutated; consider changing to 'let' constant
But if I change var to let:
func didFinishedListFiles(files: [FileModel]) {
for let fileData in files {
self.downloadSingleFile(NSUUID(UUIDString: fileData.uuid!)!);
}
}
Xcode shows an error:
'let' pattern cannot appear nested in an already immutable context
How is a correct way to implement it without any warnings/errors?
The for-in pattern implicitly uses a constant binding (in the scope it creates. That is, your fileData binding is automatically a local let, and therefore constant for each pass through the loop.
So the following:
for fileData in files { /*...*/ }
...is equivalent to :
var index = 0
while index < files.count {
let fileData = files[index]
//...
index += 1
}
You'd want to add var to the for-in binding only when you want to mutate that binding -- that is, if it's an object reference that you want to be able to point at something else during a single pass through the loop, or a value type that you want to be able to change. But it doesn't look like you're doing either of those things, so using var for this binding would be superfluous here.
(Swift 3 got rid of a lot of the places where you could make implicitly immutable bindings mutable, but left for var as an exception — it's still possible if you want to change something during a loop.)
I noticed that when a variable is captured by a closure in Swift, the closure can actually modify the value. This seems crazy to me and an excellent way of getting horrendous bugs, specially when the same var is captured by several closures.
var capture = "Hello captured"
func g(){
// this shouldn't be possible!
capture = capture + "!"
}
g()
capture
On the other hand, there's the inout parameters, which allow a function or closure to modify its parameters.
What's the need for inout, even captured variables can already be modified with impunity??!!
Just trying to understand the design decisions behind this...
Variables from an outer scope that are captured aren't parameters to the routine, hence their mutablility is inherited from context. By default actual parameters to a routine are constant (let) and hence can't be modified locally (and their value isn't returned)
Also note that your example isn't really capturing capture since it's a global variable.
var global = "Global"
func function(nonmutable:Int, var mutable:Int, inout returnable:Int) -> Void {
// global can be modified here because it's a global (not captured!)
global = "Global 2"
// nomutable can't be modified
// nonmutable = 3
// mutable can be modified, but it's caller won't see the change
mutable = 4
// returnable can be modified, and it's caller sees the change
returnable = 5
}
var nonmutable = 1
var mutable = 2
var output = 3
function(nonmutable, mutable, &output)
println("nonmutable = \(nonmutable)")
println("mutable = \(mutable)")
println("output = \(output)")
Also, as you can see, the inout parameter is passed differently so that it's obvious that on return, the value may be different.
David's answer is totally correct, but I thought I'd give an example how capture actually works as well:
func captureMe() -> (String) -> () {
// v~~~ This will get 'captured' by the closure that is returned:
var capturedString = "captured"
return {
// The closure that is returned will print the old value,
// assign a new value to 'capturedString', and then
// print the new value as well:
println("Old value: \(capturedString)")
capturedString = $0
println("New value: \(capturedString)")
}
}
let test1 = captureMe() // Output: Old value: captured
println(test1("altered")) // New value: altered
// But each new time that 'captureMe()' is called, a new instance
// of 'capturedString' is created with the same initial value:
let test2 = captureMe() // Output: Old value: captured
println(test2("altered again...")) // New value: altered again...
// Old value will always start out as "captured" for every
// new function that captureMe() returns.
The upshot of that is that you don't have to worry about the closure altering the captured value - yes, it can alter it, but only for that particular instance of the returned closure. All other instances of the returned closure will get their own, independent copy of the captured value that they, and only they, can alter.
Here are a couple of use cases for closures capturing variables outside their local context, that may help see why this feature is useful:
Suppose you want to filter duplicates out of an array. There’s a filter function that takes a filtering predicate and returns a new array of only entries matching that predicate. But how to pass the state of which entries have already been seen and are thus duplicates? You’d need the predicate to keep state between calls – and you can do this by having the predicate capture a variable that holds that state:
func removeDupes<T: Hashable>(source: [T]) -> [T] {
// “seen” is a dictionary used to track duplicates
var seen: [T:Bool] = [:]
return source.filter { // brace marks the start of a closure expression
// the closure captures the dictionary and updates it
seen.updateValue(true, forKey: $0) == nil
}
}
// prints [1,2,3,4]
removeDupes([1,2,3,1,1,2,4])
It’s true that you could replicate this functionality with a filter function that also took an inout argument – but it would be hard to write something so generic yet flexible as the possibilities with closures. (you could do this kind of filter with reduce instead of filter, since reduce passes state from call to call – but the filter version is probably clearer)
There is a GeneratorOf struct in the standard library that makes it very easy to whip up sequence generators of various kinds. You initialize it with a closure, and that closure can capture variables to use for the state of the generator.
Suppose you want a generator that serves up a random ascending sequence of m numbers from a range 0 to n. Here’s how to do that with GeneratorOf:
import Darwin
func randomGeneratorOf(#n: Int, #from: Int) -> GeneratorOf<Int> {
// state variable to capture in the closure
var select = UInt32(n)
var remaining = UInt32(from)
var i = 0
return GeneratorOf {
while i < from {
if arc4random_uniform(remaining) < select {
--select
--remaining
return i++
}
else {
--remaining
++i
}
}
// returning nil marks the end of the sequence
return nil
}
}
var g = randomGeneratorOf(n: 5, from: 20)
// prints 5 random numbers in 0..<20
println(",".join(map(g,toString)))
Again, it’s possible to do this kind of thing without closures – in languages without them, you’d probably have a generator protocol/interface and create an object that held state and had a method that served up values. But closure expressions allow a flexible way to do this with minimal boiler plate.
A closure being able to modify the captured variable in the outer scope is pretty common across languages. This is the default behavior in C#, JavaScript, Perl, PHP, Ruby, Common Lisp, Scheme, Smalltalk, and many others. This is also the behavior in Objective-C if the outer variable is __block, in Python 3 if the outer variable is nonlocal, in C++ if the outer variable is captured with &
http://golang.org/pkg/sort/
This is from Go example.
// OrderedBy returns a Sorter that sorts using the less functions, in order.
// Call its Sort method to sort the data.
func OrderedBy(less ...lessFunc) *multiSorter {
return &multiSorter{
changes: changes,
less: less,
}
}
What does this do by colon? Is it mapping? Is it closure? Too much new syntax here. What should I read to understand this syntax in Go?
It's a factory function, creating and initialising a struct of type multisorter:
https://sites.google.com/site/gopatterns/object-oriented/constructors
Additionally, Go "constructors" can be written succinctly using initializers within a factory function:
function NewMatrix(rows, cols, int) *matrix {
return &matrix{rows, cols, make([]float, rows*cols)}
}
Also, it is using named parameters when initialising:
http://www.golang-book.com/9
This allocates memory for all the fields, sets each of them to their zero value and returns a pointer. (Circle) More often we want to give each of the fields a value. We can do this in two ways. Like this:
c := Circle{x: 0, y: 0, r: 5}
The `less ...lessFunc` in the func declaration means:
any number of parameters, each of type `lessFunc` can be passed here, and will be stored in the slice `less`
So it creates a `multiSorter` struct, which supports the sort interface, and calling the sort method from that interface (and implemented by multiSorter) will cause the object to use each lessFunc in turn while sorting
Does this make sense? I can expand more if needed...