I have this static dictionary created as so:
static var pictures = Dictionary<Int, Array<UIImage>>()
I want to populate it with images. At the moment when I am creating it I don't know how many key/value pairs I need to create. I have to fetch from the internet the data, but after that I am doing this to populate, but still my dictionary is empty:
for i in 0...Fetching.numberOfAliveListings - 1 {
for _ in 0...AdsCollectionView.listings[i].photos.count - 1 {
AdsCollectionView.pictures[i]?.append(UIImage(named: "noimage")!)
}
}
pictures is initially empty. So any attempt to access a value for a given key will result in a nil value. Since the value (the array) is nil, the optional chaining skips the call to append.
One solution is to provide a default array when looking up the value for a given Int.
AdsCollectionView.pictures[i, default: []].append(UIImage(named: "noimage")!)
You may also wish to consider alternate syntax when declaring pictures:
static var pictures = [Int: [UIImage]]()
Related
This is a fairly simple issue, but one I would like to solve, as it MAY help with performance.
I want to find out if Swift has a way to create a Dictionary, specifying ONLY keys, and maybe no values, or a single value that is set in each entry.
In other words, I want to create a Dictionary object, and "preload" its keys. Since this is Swift, the values could be 0 or nil (or whatever is a default empty).
The reason for this, is so that I can avoid two loops, where I go through once, filling a Dictionary with keys and empty values, and a second one, where I then set those values (There's a practical reason for wanting this, which is a bit out of the scope of this question).
Here's sort of what I'm thinking:
func gimme_a_new_dictionary(_ inKeyArray:[Int]) -> [Int:Int] {
var ret:[Int:Int] = [:]
for key in inKeyArray {
ret[key] = 0
}
return ret
}
let test1 = gimme_a_new_dictionary([4,6,1,3,0,1000])
But I'm wondering if there's a quicker way to do the same thing (as in "language construct" way -I could probably figure out a faster way to do this in a function).
UPDATE: The first solution ALMOST works. It works fine in Mac/iOS. However, the Linux version of Swift 3 doesn't seem to have the uniqueKeysWithValues initializer, which is annoying.
func gimme_a_new_dictionary(_ inKeyArray:[Int]) -> [Int:Int] {
return Dictionary<Int,Int>(uniqueKeysWithValues: inKeyArray.map {($0, 0)})
}
let test1 = gimme_a_new_dictionary([4,6,1,3,0,1000])
For Swift 4, you can use the dictionary constructor that takes a sequence and use map to create the sequence of tuples from your array of keys:
let dict = Dictionary(uniqueKeysWithValues: [4,6,1,3,0,1000].map {($0, 0)})
I presume you could optimize your code in terms of allocation by specifying the minimum capacity during the initialization. However, one liner may be the above answer, it's essentially allocation and looping to add 0 in each position.
func gimme_a_new_dictionary(_ inKeyArray:[Int], minCapacity: Int) -> [Int:Int] {
var ret = Dictionray<Int, Int>(minimumCapacity: minCapacity)
for key in inKeyArray {
ret[key] = 0
}
return ret
}
let test1 = gimme_a_new_dictionary([4,6,1,3,0,1000])
Take a look at this official documentation:
/// Use this initializer to avoid intermediate reallocations when you know
/// how many key-value pairs you are adding to a dictionary. The actual
/// capacity of the created dictionary is the smallest power of 2 that
/// is greater than or equal to `minimumCapacity`.
///
/// - Parameter minimumCapacity: The minimum number of key-value pairs to
/// allocate buffer for in the new dictionary.
public init(minimumCapacity: Int)
I am getting image from a url using SDWebImage and assigning it to an array like so...
let imgUrl = arrProduct?[indexPath.section].images[indexPath.row].url
let placeholderImage = UIImage(named: "appLogo.jpg")
cell.prdImgView.sd_setImage(with:imgUrl,
placeholderImage:placeholderImage,
options: []) { (image, error, imageCacheType, imageUrl) in
arrayOfSelectedImages.append(image!)
}
Now I just don't want to add to an array like this. Instead, after adding the image to arrayOfSelectedImages I want to update this array value in didSet and empty the arrayOfSelectedImages array so that every time the array gets a new value, it updates that value in didSet and & arrayOfSelectedImages is emptied. So finally my array in didSet will have all the images I need and I can pass those images on to some other view...How can I achieve this..?
Not entirely sure if this is what you want, but didSet will fire on a property that is an array if you modify the array, not just if you assign the array. Here is an example:
struct A
{
var anArray = [1, 2, 3]
{
didSet
{
print("Hi, there!")
anArray.remove(at: 0)
}
}
}
var a = A()
a.anArray.append(4)
// Prints Hi there!
print(a.anArray)
// prints [2, 3, 4]
The task is quite straight-forward to accomplish. You need a valid criteria to compare appended objects and, what's important, criteria you apply before appending object to an array, not after that. Using didSet to verify appended object and delete it if unsuitable, is bad design.
If your UIImage objects are not encapsulated within any other object or struct to uniquely id these objects and if you don't have an option whether or not particular image should be downloaded at all (which is the best and most proper practice), you can compare two UIImage objects by comparing underlying image data. This could previously be accomplished by obtaining PNG representation of an image and comparing that data, but now there's a good simple method.
Comparing Images
The isEqual(:) method is the only reliable way to
determine whether two images contain the same image data. The image
objects you create may be different from each other, even when you
initialize them with the same cached image data. The only way to
determine their equality is to use the isEqual(:) method, which
compares the actual image data. Listing 1 illustrates the correct and
incorrect ways to compare images.
https://developer.apple.com/documentation/uikit/uiimage
Usage
if !arrayOfSelectedImages.contains(where: { $0.isEqual(image) }) {
arrayOfSelectedImages.append(image)
}
Most of the posts here are on about how to convert a set into an array.But I would like to know if I could convert an array to a set.I have downloaded my array from parse and want to convert it to a set because I want the user to edit their details and the form builder I used needs to accept a set data type only.
This is what I got.The "Subjects" field is an NSArray in Parse.
let subjects = Set(currentuser?.objectForKey("Subjects"))
Basically the syntax is correct, but the compiler (and me too) doesn't know that the object for key Subjects is supposed to be an array. Use optional binding to check at least for an array of NSObject to conform to the requirement that the items in the array are hashable (thanks to dan for pointing that out):
if let userSubjects = currentuser?.objectForKey("Subjects") as? [NSObject] {
let subjects = Set(userSubjects)
}
If the real type of Subjects is more specific than [NSObject] use that instead
I have an array of structs:
struct CalendarDate {
var date: NSDate?
var selected = false
}
private var collectionData = [CalendarDate]()
Which I simply populate with a date like this:
for _ in 1...7 {
collectionData.append(CalendarDate(date: NSDate(), selected: false))
}
So when you tap on a collectionView, I simply want to loop through the data and mark them all as False.
for c in collectionData {
c.selected = false ///ERROR: Cannot assign to 'selected' in 'c'
}
Why do I get this error?
If I do this, it works fine but I want to know what I did wrong above:
for i in 0..<collectionData.count {
collectionData[i].selected = false
}
As I understand it, the iterator
for c in collectionData
returns copies of the items in collectionData - (structs are value types, not reference types, see http://www.objc.io/issue-16/swift-classes-vs-structs.html), whereas the iteration
for i in 0..<collectionData.count
accesses the actual values. If I am right in that, it is pointless to assign to the c returned from the iterator... it does not "point" at the original value, whereas the
collectionData[i].selected = false
in the iteration is the original value.
Some of the other commentators suggested
for (var c) in collectionData
but although this allows you to assign to c, it is still a copy, not a pointer to the original, and though you can modify c, collectionData remains untouched.
The answer is either A) use the iteration as you originally noted or B) change the data type to a class, rather than a struct.
because each 'c' is by default let, and this is a new instance of CalendarDate and the value of array at index copied to this for each step of for, and 'c' isn't pointer to the index of the array and it is just a copy of index, so if you set a new value to this, the new value does not apply in array.
but 'i' is used as index of array and can directly manipulate the values of array.
If you are using structs they are copies in the array. So even changing them only changes the copy, not an actual object in the array.
You have to make them a variable in the loop to be editable copy, and reassign them into the array right back.
If they are classes and not structs, than you don't have to reassign part, just do the var thing.
for (index, var c) in collectionData.enumerated() {
c.selected = false
collectionData[index] = c
}
ok so I am trying to return nil if a certain type is passed into my function. In this case im passing in an instance of my class "BlogPost" and a type within this blogpost. I also have an array called "types" and I have assigned the variable Videos to the last index of that array. If this type is passed into my function I would like to return nil (so assuming im going to need an optional here for returning a possible nil) this is what I have so far :-
so all in all I need to pass in an instance of my blog post but always return nil if a certain type is passed in. Hope this makes sense
Update:
The types array is defined as follows:
let types : [String] = ["technology", "Fashion", "Animals"]
this is the array I am referring to in the function. Basically if that last entry of the array is entered into the function I need to return nil
sure this is blogpost it does actually have an empty string for type
great so im getting there what Ive done now is change the blogpost.type to choose one at random. So now if the specfic type is chosen from this array how would I do that still getting an error. This is what I have updated to
so now all I need to do is access the 2 type in that array and if I do access it return nil. Any thoughts on that? so to drag it on thanks
I don't think you can. You can create failable initialisers which does what you need but you cannot use it with normal function.
The best solution for you would be return optional Int or String and when you call the function just check the result for nil and do what you need to do, otherwise ignore it:
func randomViews(blog : BlogPost.Type) -> Int? {
case 10:
return nil
case 10, 20 :
return 0
default:
random
}
if (randomViews(parameter) == nil) {
//function returned nil
}
You have displayed error because you compare optional blog to Videos, you have to unwrap it first, for example if you are sure the blog has always have a value use:
if blog! == Videos
if not sure is safer to use:
if let blg = blog {
if blg == Videos {
}
else {
// blog has not have a value
}
You are passing blog as a BlogPost.Type parameter. That is not correct. You should have either just passed it the String parameter, or you could pass it the BlogPost itself:
func randomViews(blog: BlogPost) {
let videos = types[2]
if blog.type == videos {
// do whatever you want
}
// carry on
}
Unrelated to your question at hand, but notice that I use let instead of var when defining videos. Always use let if the value will not (and cannot) change.
Also note that I use lowercase letter v in videos, because Cocoa naming conventions dictate that variables generally start with lowercase letters, whereas types, classes, structs, and enums generally start with uppercase letters.