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Check if optional array is empty
(8 answers)
Closed 4 years ago.
I have a array of my custom model, and I want to check if it is not nil and its size is greater then 0.
Following is my array with custom object
var listCountries : [Countries]? = nil
now In viewDIdLoad I want to make a check on it. I am new to Swift. I have good experience in working in Java.
I have read out Optional values concept and guard, if let statements. But I am unable to understand how efficiently they may be used. I have read too much SO questions but failed to figure out.
for example , if I want to check the upper given array in java I have only to do
if(listCountries != null && listCountries.size()>0){
//DO something
}
So to summarize my question:
How to make the upper given(Java code) check in to swift 4.? What is more smooth and reliable way.
What is a use of if let , guard, guard let statements. if I declare a variable (array, string) as optional I have to bear optional check like force wrapping each and every place. This is for me so making too much confusion.
Please help. I know this question has been asked in different ways. But this has some different context.
Just use ??.
if !(listCountries ?? []).isEmpty {
However, since you want to probably use listCountries in the if block, you should unwrap
if let listCountries = self.listCountries, !listCountries.isEmpty {
Ideally, if nil and empty means the same to you, don't even use an optional:
var listCountries: [Countries] = []
I would do it something like...
if let list = listCountries, !list.isEmpty { // Can also use list.count > 0
// do something
}
Even though you are not using the list inside the braces you are still using the list in the condition.
Or, like Sulthan said... make it non-optional to begin with if it makes no difference.
Obviously, I would assume that you are able to recognize the difference between nil array and empty array.
So, if we tried to implement a literal translation to your question:
I want to check if it is not nil and its size is greater then 0
For the first condition:
// "I want to check if it is not nil":
if let unwrappedList = listCountries {
// ...
}
and for the second condition:
// "I want to check if it is not nil":
if let unwrappedList = listCountries {
// "and its size is greater then 0":
if !unwrappedList.isEmpty {
// ...
}
}
However, you could combine both of the conditions by using the comma to achieve the multi-clause condition:
// I want to check if it is not nil and its size is greater then 0
if let unwrappedList = listCountries, !unwrappedList.isEmpty {
// ...
}
Or by using guard statement:
// I want to check if it is not nil and its size is greater then 0
guard let unwrappedList = listCountries, !unwrappedList.isEmpty else {
return
}
if let list = listCountries {
if(!list.isEmpty && list.count > 0) {
//value
}
}
Related
I am a new to SwiftUI, so I apologise if this a dumb question but I came upon a problem that I cant find a solution to.
I am searching something similar to this code below, but instead of finding the first element I would like to retrieve all elements that have the status == 0.
if let new = array.first(where: {$0.status == 0}) {
// do something with foo
} else {
// item could not be found
}
Swift collections have a filter method which allows you to select all elements that match a given condition. You already have a matching condition, so to adapt your example text you'd write:
let newItems = array.filter { $0.status == 0 }
I have a NSObject Subclass. Say CityWalks
class CityWalks{
var totalCount:Int?
}
How do I use this property further? Should I check the nil coalescing every time this value is accessed.
example:
let aObject =
say in one fucntion (function1()) , I need to access this value, then it would like (aObject!.totalCount ?? 0)
func function1(){
...Some Access code for the object....
(aObject!.totalCount ?? 0)
}
Similarly in every other function(function2()) , I will have to write the same code.
func function2(){
...Some Access code for the object....
(aObject!.totalCount ?? 0)
}
So, what could be a better approach for such field, considering this property might receive a value from server or might not.
If you have a default value for this property just assign this value as default value.
class YourClass {
var totalCount = 0
}
I'd recommend you avoid using an optional value if it's possible. Because optional values its a first place when you can get an error.
As stated in the comments and the other answer using an optional is not really optimal in your case. It seems like you might as well use a default value of 0.
However, to clarify, you have to check the value when unwrapping the optional.
Sometimes it's possible to pass an optional to UIElement etc and then you don't really need to do anything with them
There are pretty ways of checking for nil in optional values built into swift so you can build pretty neat code even though you work with optional.
Look in to guard let and if let if you want to know more about unwrapping values safely.
if let
if let totalWalks = aObject?.totalCount {
//operate on totalWalks
}
guard
guard let totalWalks = aObject?.totalCount else { return }
//operate on totalWalks
There are also cases where you will want to call a function on an optional value and in this case you can do so with ?
aObject?.doSomething()
Any return values this function might have will now be wrapped in an optional and you might have to unwrap them as well with an if let or guard
When working with optionals you should try to avoid forcing the unwrap with ! as even though you at the moment know that the value is not null that might after a change in the code not be true anymore.
I want to remove element of custom type value from an array.
I want to pass a variant instance to function to remove it from array, I don't want to use removeAtIndex().
var favoriteVariants: [Variant]
func removeVariant(variant: Variant)
{
}
If Variant is Equatable and you only want to remove the first one that matches:
if let idx = favoriteVariants.indexOf(variant) {
favoriteVariants.removeAtIndex(idx)
}
If it isn’t Equatable and you have some other matching criteria to find just one to remove:
let idx = favoriteVariants.indexOf {
// match $0 to variant
}
if let idx = idx {
favoriteVariants.removeAtIndex(idx)
}
(these are assuming Swift 2.0 – if 1.2, it’s find(favoriteVariants, variant) instead of indexOf, and there isn’t a version that takes a closure, though it’s not too hard to write one)
If there are multiple ones you want to remove in one go:
favoriteVariants = favoriteVariants.filter {
// criteria to _keep_ any given favorite
}
All of these could be wrapped in extensions if what you want to do is general enough to justify it.
//un-wrap string in label and cast to Int
let made = ShotsMadeLabel.text?.toInt() //shots made Int
let miss = ShotsMissedLabel.text?.toInt() //shots missed Int
// check to see if both values are not equal to zero
// add values and display answer (ans)in TotshotShotsMadelabel
if (miss != 0) || (made != 0){
var ans = made! + miss!
TotalShotsMadeLabel.text = "\(ans)"
println(ans) // check results
per request provided clarity to my question.. please let me know if there is a better way to do this..
Your code looks good but it fails to cover the case where one of the labels' text is empty or not a number. In that case, made or miss can be nil and you will crash when you force-unwrap it (with the exclamation mark).
So, you'd need to add another level of safety:
if (miss != 0) || (made != 0) {
if (miss != nil) && (made != nil) {
var ans = made! + miss!
}
}
But the truth is that you should not be doing this in the first place. Do not store state in a view like a label! This whole business of extracting the text from a label and turning it into a number is just wrong. You should have model variables in your code that are actual numbers, maintaining this information. When they change, change the labels. And that way, you can just add them. MVC, model-view-controller. Do not use View as Model, which is what you are doing.
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.