I have a question regarding a Dictionary of Dictionaries. Is there a shorter way to put single elements in the Dict than I did?
var cellHeight = [ Int : [ Int : CGFloat ] ]()
// ...
if let _ = cellHeight[0] {
cellHeight[0]![2] = 0.0
} else {
cellHeight[0] = [ 2 : 0.0 ]
}
In all the tutorials I checked it's only explained how to fill/initialize the full dict-of-dict and than read from it, but not how to fill it part by part.
Simply add the element. If an entry into a dictionary does not exist yet, adding an element to that key will create one. If a key already exists, then adding an element will overwrite the old one. Reading from a dictionary key that does not exist will return nil. Reading from a dictionary returns an optional.
What you're doing is basically right. You can be slightly more elegant, like this:
let didit = cellHeight[0]?[2] = 0 // yields an Optional<Void>
if didit == nil { // that didn't work, so create the entry
cellHeight[0] = [2:0]
}
And that can be tightened up even further, without the extra variable:
if nil == (cellHeight[0]?[2] = 0) {
cellHeight[0] = [2:0]
}
If this is a recurring pattern in your code, you can of course abstract it into a function.
Related
I have defined a struct in my viewcontroller before my viewdidload
struct CustomFilterButton {
var Id : Int = 0;
var Name : String = "";
var selected : Bool = false;
}
then I create reference for it in global
var customButtons = [CustomFilterButton]();
then in my viewdidload I appended some customFilterButton objects in customButtons array
customButtons.append(CustomFilterButton.init(Id: 1, Name: "A", selected: false))
customButtons.append(CustomFilterButton.init(Id: 2, Name: "B", selected: false))
customButtons.append(CustomFilterButton.init(Id: 3, Name: "C", selected: true))
customButtons.append(CustomFilterButton.init(Id: 4, Name: "D", selected: false))
in viewdidload or in any other function when I try to get an object in array using first or filter and change it, but it doesnt work.
print(customButtons[0].selected);
print("--")
var bt = customButtons.first{
$0.Id == 1
}
bt?.selected = true;
print(bt?.selected);
print(customButtons[0].selected);
here is the result
false
--
Optional(true)
false
the same applies to filter also!
Am I missing something or am I doing something wrong?
Note: I need to get the object that first or filter found and change it, not the hard copy of it
When you deal with a Struct, you have to understand that it is a value type.
So, that means that everytime you pass a value around, it's a COPY of the value and not the reference
And when you do this:
var bt = customButtons.first{
$0.Id == 1
}
You're asking the compiler to retrieve a copy of the CustomFilterButton whose Id is 1 and assign the copy to your bt variable.
To tackle this, you can access the element by its index and directly modify its value without passing it around (assigning to a new variable)
// Get the index of the element you're trying to modify
if let indexYouWantToModify = customButtons.firstIndex(where: {$0.Id == 1}){
// Modify it directly through its index
customButtons[indexYouWantToModify].selected = true
}
Oh, and though changing your Struct to a Class works for you, I think it's unnecessary just for this little use case. Structs and Classes hold their own benefits and trade-offs. I'm not sure what you're planning to do over this CustomFilterButton over the long run so I suggest you to read this article and decide for yourselves!
var bt = customButtons.first{
$0.Id == 1
}
At this moment bt has no any relation to your customButtons[0], its value was copied.
Access to item by index
print(customButtons[0].selected);
print("--")
var offset = 0
var bt = customButtons.enumerated().first{
if $0.element.Id == 1 {
offset = $0.offset
return true
}
return false
}
customButtons[offset].selected = true;
print(customButtons[0].selected);
You should use class instead of struct.
This is a little hard to explain, but I'll try my best. I am trying to update a Dictionary inside another Dictionary properly. The following code almost does what I need.
var dictionary = Dictionary<String, [Int : Int]>()
func handleStatsValue(tag: Int ) {
let currentValue: Int = dictionary["Score"]?[tag] ?? 0
dictionary["Score"] = [
tag : currentValue + 1
]
}
However, it seems the dictionary is overridden when the tag value changes (e.g. 1 to 2). I need Dictionary to have multiple dictionaries inside of it. Any tips or suggestions are deeply appreciated.
Edit: I'm trying to have multiple dictionaries nested inside a dictionary. It seems whenever the tag value is changed, the dictionary is overridden.
One way to write this would be:
func handleStatsValue(tag: Int) {
dictionary["Score", default: [:]][tag, default: 0] += 1
}
or, written without [_:default:]
func handleStatsValue(tag: Int) {
var scoreDictionary = dictionary["Score"] ?? [:]
scoreDictionary[tag] = (scoreDictionary[tag] ?? 0) + 1
dictionary["Score"] = scoreDictionary
}
However, it's not a good idea to use nested dictionaries to keep your data. Use a custom struct instead and try to avoid tags too:
struct DataModel {
var score: [Int: Int] = [:]
}
I think you need something like this to either increase the value for an existing tag or add a new tag if it doesn't exist
func handleStatsValue(tag: Int ) {
if var innerDict = dictionary["Score"] {
if let value = innerDict[tag] {
innerDict[tag] = value + 1
} else {
innerDict[tag] = 1
}
dictionary["Score"] = innerDict
}
}
Although the code looks a bit strange with the hardcoded key "Score", maybe it would be better to have multiple simple dictionaries instead, like
var score: [Int, Int]()
or if you prefer
var score = Dictionary<Int, Int>()
I've looked through the methods here but I can't quite find what I'm looking for. I'm new-ish to Swift. I would like to extract a subset from a Dictionary based on a Set of key values, preferably without a loop.
For example, if my key Set is of type Set<String> and I have a Dictionary of type Dictionary<String, CustomObject>, I would like to create a new Dictionary of type Dictionary<String, CustomObject> that contains only the key-value pairs associated with the keys in the Set of Strings.
I can see that I could do this with for loop, by initializing a new Dictionary<String, CustomObj>(), checking if the original Dictionary contains a value at each String in the set, and adding key-value pairs to the new Dictionary. I am wondering if there is a more efficient/elegant way to do this however.
I'd be open to finding the subset with an Array of Strings instead of a Set if there is a better way to do it with an Array of keys.
Many thanks!
Swift 5 - You can do this very simply:
let subsetDict = originalDict.filter({ mySet.contains($0.key)})
The result is a new dictionary with the same type as the original but which only contains the key-value pairs corresponding to the keys in mySet.
Your assumption is correct, there is a more concise/swift-ish way to accomplish what you need.
For example you can do it via reduce, a functional programming concept available in Swift:
let subDict = originalDict.reduce([String: CustomObject]()) {
guard mySet.contains($1.key) else { return $0 }
var d = $0
d[$1.key] = $1.value
return d
}
Or, in two steps, first filtering the valid elements, and then constructing back the dictionary with the filtered elements:
let filteredDict = originalDict.filter { mySet.contains($0.key) }
.reduce([CustomObject]()){ var d = $0; d[$1.key]=$1.value; return d }
forEach can also be used to construct the filtered dictionary:
var filteredDict = [CustomObject]()
mySet.forEach { filteredDict[$0] = originalDict[$0] }
, however the result would be good it it would be immutable:
let filteredDict: [String:CustomObject] = {
var result = [String:CustomObject]()
mySet.forEach { filteredDict2[$0] = originalDict[$0] }
return result
}()
Dummy type:
struct CustomObject {
let foo: Int
init(_ foo: Int) { self.foo = foo }
}
In case you'd like to mutate the original dictionary (instead of creating a new one) in an "intersect" manner, based on a given set of keys:
let keySet = Set(["foo", "baz"])
var dict = ["foo": CustomObject(1), "bar": CustomObject(2),
"baz": CustomObject(3), "bax": CustomObject(4)]
Set(dict.keys).subtracting(keySet).forEach { dict.removeValue(forKey: $0) }
print(dict) // ["foo": CustomObject(foo: 1), "baz": CustomObject(foo: 3)]
I'm saving lists in a dictionary. These lists need to be updated. But when searching for an item, I need [] operator. When I save the result to a variable, a copy is used. This can not be used, to change the list itself:
item = dicMyList[key]
if item != nil {
// add it to existing list
dicMyList[key]!.list.append(filename)
// item?.list.append(filename)
}
I know, that I need the uncommented code above, but this accesses and searches again in dictionary. How can I save the result, without searching again? (like the commented line)
I want to speed up the code.
In case you needn't verify whether the inner list was actually existing or not prior to adding element fileName, you could use a more compact solution making use of the nil coalescing operator.
// example setup
var dicMyList = [1: ["foo.sig", "bar.cc"]] // [Int: [String]] dict
var key = 1
var fileName = "baz.h"
// "append" (copy-in/copy-out) 'fileName' to inner array associated
// with 'key'; constructing a new key-value pair in case none exist
dicMyList[key] = (dicMyList[key] ?? []) + [fileName]
print(dicMyList) // [1: ["foo.sig", "bar.cc", "baz.h"]]
// same method used for non-existant key
key = 2
fileName = "bax.swift"
dicMyList[key] = (dicMyList[key] ?? []) + [fileName]
print(dicMyList) // [2: ["bax.swift"], 1: ["foo.sig", "bar.cc", "baz.h"]]
Dictionaries and arrays are value types. So if you change an entry you'll need to save it back into the dictionary.
if var list = dicMyList[key] {
list.append(filename)
dicMyList[key] = list
} else {
dicMyList[key] = [filename]
}
It's a little bit late, but you can do something like this:
extension Optional where Wrapped == Array<String> {
mutating func append(_ element: String) {
if self == nil {
self = [element]
}
else {
self!.append(element)
}
}
}
var dictionary = [String: [String]]()
dictionary["Hola"].append("Chau")
You can try this in the Playground and then adapt to your needs.
Just simple task. I've got a dictionary var types = [Int : String]() which inits like an empty and after some user actions it fills with data. According to emptiness or some specific data in this dictionary I enable/disable a button in UI.
Check for emptiness is easy, but how to check if dictionary contains certain value?
Compiler suggested me a placeholder with predicate:
types.contains(predicate: ((Int, String)) throws -> Bool>)
Since you only want to check for existance of a given value, you can apply the contains method for the values properties of your dictionary (given native Swift dictionary), e.g.
var types: [Int : String] = [1: "foo", 2: "bar"]
print(types.values.contains("foo")) // true
As mentioned in #njuri: answer, making use of the values property of the dictionary can seemingly yield an overhead (I have not verified this myself) w.r.t. just checking the contains predicate directly against the value entry in the key-value tuple of each Dictionary element. Since Swift is fast, this shouldn't be an issue, however, unless you're working with a huge dictionary. Anyway, if you'd like to avoid using the values property, you could have a look at the alternatives given in the forementioned answer, or, use another alternative (Dictionary extension) as follows:
extension Dictionary where Value: Equatable {
func containsValue(value : Value) -> Bool {
return self.contains { $0.1 == value }
}
}
types.containsValue("foo") // true
types.containsValue("baz") // false
I wrote a function which is using contains method on dictionary.
Your specific case:
let dic : [Int : String] = [1 : "a", 2 : "b"]
func dictionary(dict : [Int : String], containsValue value : String)->Bool{
let contains = dict.contains { (_,v) -> Bool in
return v == value
}
return contains
}
let c = dictionary(dic, containsValue: "c") // false
let a = dictionary(dic, containsValue: "a") // true
Generic:
extension Dictionary{
func containsValue<T : Equatable>(value : T)->Bool{
let contains = self.contains { (k, v) -> Bool in
if let v = v as? T where v == value{
return true
}
return false
}
return contains
}
}
I've tested this function against dictionary.values.contains() and it is roughly two times faster.
If you want to check if already contains a value this would be the way:
if !yourDictionary.values.contains("Zero") {
yourDictionary[newItemKey] = newItemValue; //addNewItem
}
else {
print("this value already exists");
}
And this one if you want to check if the key exists:
You get the item to add to your dictionary.
Check if the item's key already exists
If it doesn't, append the item or enable the button.
//1
let newItemKey = 0
let newItemValue = "Zero"
//2
let keyExists = yourDictionary[newItemKey] != nil
//3
if !keyExists {
yourDictionary[newItemKey] = newItemValue; //addNewItem
}
else {
print("This key already exists");
}
The dictionary getter returns an optional value.
let dictionary = ["ben": "says hi"]
let containsAlpha = dictionary["alpha"] != nil
let containsBen = dictionary["ben"] != nil