I am new to Swift and am trying to compare my Error description name with different String constants to show the user different results, based on the error.
I am using:
let errorName = errors.first?["name"].debugDescription
The value of errorName comes as "Optional(AlreadyActiveUser)" and when i compare this to my constant string "AlreadyActiveUser", i get false.
I have tried many things, but i am not able to get the value of the string inside the optional.
Someone, please help.
You can use optional binding in this case...
guard let errorName = errors.first?["name"].debugDescription as? String {
print("value is not present...")
return
}
print(errorName)
//here you can compare errorName == "AlreadyActiveUser"
you can use this
if let errorName = errors.first?["name"] as? String {
print(errorName)
//here you can compare errorName == "AlreadyActiveUser"
}
else
{
print("value is not present...")
}
try let errorName = errors.first!["name"].debugDescription
Notes that I forced wrapping first with ! instead of ?.
Related
I am getting the error message Cannot assign value of type 'String?' to type 'Int'
I have browsed through other questions like this but it still shows the error.
if sunscreenName.text != nil && reapplyTime.text != nil {
sunscreen = sunscreenName.text!
reApplyTime = reapplyTime.text
//Some sort of message such as Progress hud
}
Thanks in advance!
I got your problem, actually what happens here Swift is is type safe langauge
So what you are doing is is to store a String value in Int which will not happen automatically you need to convert it to Int
like this
Int(sunscreenName.text)
But there is a catch there not all string are convertible to Int type, fo e.g.
let name = "roshan"
if you try to convert it to Int it will give you a nil
let a = Int(name)
So its better you do a optional Binding here provided by Swift
if let sunValue = Int(sunscreenName.text),let reApplyValue = Int(reapplyTime.text) {
sunscreen = sunValue
reApplyTime = reApplyValue
}
I recommend reading through The Swift Programming Language to get a better understanding of Swift and its fundamental concepts, since this question is fairly basic.
You make several mistakes:
if sunscreenName.text != nil && reapplyTime.text != nil {
This is wrong. In Swift, if you plan to use the value later, you should use if let rather than comparing to nil. Comparing to nil leaves the values optional, but if let unwraps them. So, do this instead:
if let sunscreenText = sunscreenName.text, let reapplyText = reapplyTime.text {
Now you have the sunscreenText and reapplyText variables, which are typed String, not String? (i.e. they are not optional).
Now, there's these two lines.
sunscreen = sunscreenName.text!
reApplyTime = reapplyTime.text
You don't say which one is giving the error, but the issue is the same in either case. First, use our unwrapped sunscreenText and reapplyText variables instead of sunscreenName.text! and reapplyTime.text. Next, if one of these is meant to be an Int instead of a String, cast it. Swift is not like JavaScript, in that it won't automatically convert values from one type to another, so if something is a string and we need an integer, we have to convert it ourselves.
(assuming reapplyTime was the line that was giving the error:)
if let reapplyInt = Int(reapplyText) {
reapplyTime = reapplyInt
}
The reason we have to unwrap is because Int(String) can return nil if the string is something that can't be converted to an integer. Alternately, we could just provide a default value:
reapplyTime = Int(reapplyText) ?? 0 // sets to 0 if it can't parse the string as an integer
I am using Vapor for Swift backend. Following is the code i am working with.
drop.post("postTodo") { request in
var jsonContent: JSON?
if let contentType = request.headers["Content-Type"], contentType.contains("application/json"), let jsonData = request.json {
jsonContent = jsonData
print("Got JSON: \(jsonContent)")
}
guard let id = jsonContent?.node.object?["id"]?.string
else {
return JSON(["message": "Please include mandatory parameters"])
}
let tempId = Int(id)!
I am getting "id" as optional string for eg: Optional("123") for jsonContent?.node.object?["id"]?.string
When I try to convert it to int using Int(id)! i get back nil
If i try to do let tempId = Int(id!) it gives error.
But when i do the same thing in playground i get proper int value.
let id: String?
id = "1234"
let myInt = Int(id!)
Why Optional string to Int is not working properly in my Vapor app ?
Any idea.
If "id" is an optional string, then you probably don't want to be force unwrapping it with the "!".
The safest approach would be something like:
if let id = id
{
let myIdAsInt = Int(id)
}
The reason it "works" in the playground, is you are definitely assigning a non-nil value to the string (therefore you get away with the force unwrap).
String!might contain a string, or it might contain nil. It’s like a regular optional, but Swift lets you access the value directly without the unwrapping safety. If you try to do it, it means you know there’s a value there – but if you’re wrong your app will crash.
var optionalString: String? = "123"
// first check if it doesn't contain nil
if let str = optionalString {
// string to -> Int
if let id = Int(str) {
print(id) // work with id
}
} else {
// optionalString contains nil
}
what i found is in my iOS code i had a struct with optional properties coz of which when mapped to Dict gave object with optional values to keys.
If I make properties non optional and send it to vapor backend after it works fine.
So basically it was the case of using Optionals properly.
I am retrieving a number value from my Firebase database (JSON db) and then displaying this number into a textField, although I get this error when I try to display it.
Could not cast value type '__NSCFNumber' to 'NSString'
How can I properly convert the retrieved value to a String, taking into consideration that this value maybe change between a String and a Number when I retrieve it.
Here is my code:
let quantity = child.childSnapshot(forPath: "quantity").value // Get value from Firebase
// Check if the quantity exists, then add to object as string.
if (!(quantity is NSNull) && ((quantity as! String) != "")) {
newDetail.setQuantity(quantity: quantity as! String)
}
The error is saying that your quantity is Number and you cannot directly convert number to String, try like this.
newDetail.setQuantity(quantity: "\(quantity)")
Or
if let quantity = child.childSnapshot(forPath: "quantity").value as? NSNumber {
newDetail.setQuantity(quantity: quantity.stringValue)
}
else if let quantity = child.childSnapshot(forPath: "quantity").value as? String {
newDetail.setQuantity(quantity: quantity)
}
Or With Single if statement
if let quantity = child.childSnapshot(forPath: "quantity").value,
(num is NSNumber || num is String) {
newDetail.setQuantity(quantity: "\(quantity))
}
Using second and third option there is no need to check for nil.
Swift 4:
let rollNumber:String = String(format: "%#", rollNumberWhichIsANumber as! CVarArg)
You may convert your NSNumber value to string like this too, its more traditional and basic approach to format a string
newDetail.setQuantity(String(format: "%#", quantity))
Swift 4.
newDetail.setQuantity(String(describing: rollNumberWhichIsANumber))
In swift 4.1 and Xcode 9.4.1
newDetail.setQuantity(quantity: "(quantity)")
We can convert to string like this "\(quantity)"
let session_id : Int32 = (jsonObject.value(forKey: "id") as! NSNumber).int32Value
Swift 5:
I manage to fix it buy using .description when interpolating the UserDefaults value into a label converting from Int to String.
working code
highScoreLabel?.text = "HiScore: \(hiScoreValue.description)"
old code
highScoreLabel?.text = "HiScore: \(String(describing: hiScoreValue))"
I'm new to learning Swift so I decided I might as well learn Swift 2 instead. Everything has made sense to me so far except for the following code snippet. Hopefully someone can shed some light on this for me.
//: Playground - noun: a place where people can play
import UIKit
//Works
let possibleNumber="2"
if let actualNumber = Int(possibleNumber) {
print("\'\(possibleNumber)\' has an integer value of \(actualNumber)")
}
else {
print("could not be converted to integer")
}
//Doesn't Work and I'm not sure why
let testTextField = UITextField()
testTextField.text = "2"
let numberString = testTextField.text //I know this is redundant
if let num = Int(numberString) {
print("The number is: \(num)")
}
else {
print("Could not be converted to integer")
}
The top section of the code is straight from Apple's Swift 2 ebook and it makes sense to me how it uses optional binding to convert the string to an int. The second piece of code is basically the same except that the string comes from the text property of a UITextField. The bottom part of the code gives the following error:
Playground execution failed: /var/folders/nl/5dr8btl543j51jkqypj4252mpcnq11/T/./lldb/843/playground21.swift:18:18: error: value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?
if let num = Int(numberString) {
I fixed the problem by using this line:
if let num = Int(numberString!) {
I just want to know why the second example needs the ! and the first doesn't. I'm sure the problem has to do with the fact that I'm getting the string from a textfield. Thanks!
The difference is that in the first case possibleNumber is not an optional variable. It is definitely a string. It cannot be nil.
In the second case textField.text returns an optional string and so numberString is an optional variable. It could be nil.
Now... The conversion Int("") returns an optional int. if the string is "abc" then it cannot return a number so returns nil. This is what you are unwrapping with the if let... statement.
However, in the second case your string is also optional and the Int() will not accept an optional. So you are force unwrapping it. This is dangerous as it could crash the app if the string is nil.
What you could do instead is this...
if let numberString = textFeidl.text,
number = Int(numberString) {
// use the number
}
This will unwrap the text first and if it's available then use it to. Get the number. If that is not nil then you enter the block.
In Swift 2 you could use the guard let function here also.
Just seen that you are using Swift 2.
You can do it this way also...
func getNumber() -> Int {
guard let numberString = textField.text,
number = Int(numberString)
else {
return 0
}
return number
}
I am doing what I believe to be a very simple task. I'm trying to get a value out of a dictionary if the key exists. I am doing this for a couple keys in the dictionary and then creating an object if they all exist (basically decoding a JSON object). I am new to the language but this seems to me like it should work, yet doesn't:
class func fromDict(d: [String : AnyObject]!) -> Todo? {
let title = d["title"]? as? String
// etc...
}
It gives me the error: Operand of postfix ? should have optional type; type is (String, AnyObject)
HOWEVER, if I do this, it works:
class func fromDict(d: [String : AnyObject]!) -> Todo? {
let maybeTitle = d["title"]?
let title = maybeTitle as? String
// etc...
}
It appears to be basic substitution but I may be missing some nuance of the language. Could anyone shed some light on this?
The recommended pattern is
if let maybeTitle = d["title"] as? String {
// do something with maybeTitle
}
else {
// abort object creation
}
It is possibly really a question of nuance. The form array[subscript]? is ambiguous because it could mean that the whole dictionary (<String:AnyObject>) is optional while you probably mean the result (String). In the above pattern, you leverage the fact that Dictionary is designed to assume that accessing some key results in an optional type.
After experimenting, and noticing that the ? after as is just as ambiguous, more, here is my solution:
var dictionary = ["one":"1", "two":"2"]
// or var dictionary = ["one":1, "two":2]
var message = ""
if let three = dictionary["three"] as Any? {
message = "\(three)"
}
else {
message = "No three available."
}
message // "No three available."
This would work with all non-object Swift objects, including Swift Strings, numbers etc. Thanks to Viktor for reminding me that String is not an object in Swift. +
If you know the type of the values you can substitute Any? with the appropriate optional type, like String?
There are a few of things going on here.
1) The ? in d["title"]? is not correct usage. If you're trying to unwrap d["title"] then use a ! but be careful because this will crash if title is not a valid key in your dictionary. (The ? is used for optional chaining like if you were trying to call a method on an optional variable or access a property. In that case, the access would just do nothing if the optional were nil). It doesn't appear that you're trying to unwrap d["title"] so leave off the ?. A dictionary access always returns an optional value because the key might not exist.
2) If you were to fix that:
let maybeTitle = d["title"] as? String
The error message changes to: error: '(String, AnyObject)' is not convertible to 'String'
The problem here is that a String is not an object. You need to cast to NSString.
let maybeTitle = d["title"] as? NSString
This will result in maybeTitle being an NSString?. If d["title"] doesn't exist or if the type is really NSNumber instead of NSString, then the optional will have a value of nil but the app won't crash.
3) Your statement:
let title = maybeTitle as? String
does not unwrap the optional variable as you would like. The correct form is:
if let title = maybeTitle as? String {
// title is unwrapped and now has type String
}
So putting that all together:
if let title = d["title"] as? NSString {
// If we get here we know "title" is a valid key in the dictionary, and
// we got the type right. title has now been unwrapped and is ready to use
}
title will have the type NSString which is what is stored in the dictionary since it holds objects. You can do most everything with NSString that you can do with String, but if you need title to be a String you can do this:
if var title:String = d["title"] as? NSString {
title += " by Poe"
}
and if your dictionary has NSNumbers as well:
if var age:Int = d["age"] as? NSNumber {
age += 1
}