I'm working on integrating an IDTech swiper into my app and I've gotten pretty far along, added the library, registered notifications, unregistered them, and now I'm working on a function that connects the reader. I can't seem to figure out what I'm doing wrong here when I'm attempting to switch cases based on a return value.. could someone please help me?
func displayUmRet(operation: String, returnValue: UmRet) {
var string = ""
do {
switch returnValue {
case UMRET_SUCCESS: string = ""
case UMRET_NO_READER: string="No reader attached"
case UMRET_SDK_BUSY: string="Communication with reader in progress"
case UMRET_MONO_AUDIO: string="Mono audio enabled"
case UMRET_ALREADY_CONNECTED: string="Already connected"
case UMRET_LOW_VOLUME: string="Low volume"
case UMRET_NOT_CONNECTED: string="Not connected"
case UMRET_NOT_APPLICABLE: string="Not applicable to reader type"
case UMRET_INVALID_ARG: string="Invalid argument"
case UMRET_UF_INVALID_STR: string="Invalid firmware update string"
case UMRET_UF_NO_FILE: string="Firmware file not found"
case UMRET_UF_INVALID_FILE: string="Invalid firmware file"
default: string="<unknown code>"
}
} while (0)
// var retStatus = UMRET_SUCCESS==ret
//self.textResponse.text = "\(operation), \(retStatus), \(string)"
self.hexResponse.text = "";
}
You need to put a . before your cases:
enum UmRet {
case UMRET_SUCCESS, UMRET_FAILURE
}
var string = " "
let returnValue = UmRet.UMRET_SUCCESS
switch returnValue {
case .UMRET_SUCCESS: string = "y"
case .UMRET_FAILURE: string = "n"
}
Also, 0 isn't the same as false in Swift, so:
do {
...
} while (0)
Shouldn't work either.
And you don't need semicolons at the end of a line, so this:
self.hexResponse.text = "";
can be this:
self.hexResponse.text = ""
And finally, if your switch statement has every case for every case in your enum, you don't need a default case. (that's why mine didn't have one in the example)
By the way, ~= is just the operator for the pattern-matching function, which is what Swift does in a switch statement. It works kind of like the == function, for instance, Int ~= Int is the same as Int == Int. But it's a bit more versatile: for instance Range ~= Int, eg 0...3 ~= 2 returns whether or not the Int is in the range. (So true in this case) For enums, it matches cases to cases. In my example, it'll match UMRET_SUCCESS, and string will be set to y.
Related
When I have a situation where I already know enum case statement I want to get the associated value of, is there a cleaner way than using a switch statement to pluck out the associated value?
To have to come up with a switch statement, provide multiple cases, or a default case just to extract the associated value is gaudy.
enum CircularReasoning {
case justPi(pi: Double)
case pizzaPie(howMany: Int)
}
var piInTheSky : Double
let whatLogic = CircularReasoning(pi: 3.1415926)
⬇️ 𝘸𝘢𝘯𝘵 𝘵𝘰 𝘢𝘷𝘰𝘪𝘥 ⬇️
switch whatLogic {
case .justPi(let pi):
piInTheSky = pi!
default:
break
}
You can use if case .<enum_case>(let value) as in TylerP's example,
or if case let .<enum_case>(value):
enum Foo {
case anInt(Int)
case aFloat(Float)
}
let aFoo: Foo = .anInt(9)
// Example of `if case .<enum_case)(let value)` syntax:
if case .anInt(let aValue) = aFoo {
print("aFoo = anInt(\(aValue))")
// Example of `if case let .enum_case(value)` syntax:
} else if case let .aFloat(aValue) = aFoo {
print("aFoo = aFloat(\(aValue))")
}
Both work. I'm not sure why the language includes both variants.
If you only care about one enum type, then either if syntax makes sense to me. If you are dealing with more than one possible enum value then the switch version seems cleaner.
Here's an adaptation of #DuncanC's excellent upvoted and accepted answer, as applied to a fictitious version of my real-world use case.
It illustrates a possible way to use his answer to reduce the space required to extract associated values, especially if one has a lot of one-off cases...
Note: Not implying this is appropriate or professional swift styling; it's clearly idiosyncratic, yet compact. (I usually don't compress things into one liners like this, unless they get really repetitive/redundant & produce lot of pointless vertical bloat).
enum RealmKey { case realmOfRealms, anyOldRealm, someOtherRealm }
.
.
.
enum SymbolToken {
case realm (key: RealmKey?)
case space (key: SpaceKey?)
case area (key: AreaKey?)
case region (key: RegionKey?)
case preserve (key: PeserveKey?)
case openParen
case closeParen
case logicalAnd
case logicalOr
case logicalNot
var realmKey : RealmKey? { if case .realm (let key) = self { return key } else { return nil } }
var spaceKey : SpaceKey? { if case .space (let key) = self { return key } else { return nil } }
var areaKey : AreaKey? { if case .area (let key) = self { return key } else { return nil } }
var regionKey : RegionKey? { if case .region (let key) = self { return key } else { return nil } }
var preserveKey : PreserveKey? { if case .preserve (let key) = self { return key } else { return nil } }
}
let realm = SymbolToken.realm(.realmOfRealms)
let realmKey = realm.realmKey
Trying to make a func that will count characters in between two specified char like:
count char between "#" and "." or "#" and ".com"
If this is only solution could this code be written in a simple way with .count or something less confusing
func validateEmail(_ str: String) -> Bool {
let range = 0..<str.count
var numAt = Int()
numDot = Int()
if str.contains("#") && str.contains(".") && str.characters.first != "#" {
for num in range {
if str[str.index(str.startIndex, offsetBy: num)] == "#" {
numAt = num
print("The position of # is \(numAt)")
} else if
str[str.index(str.startIndex, offsetBy: num)] == "." {
numDot = num
print("The position of . is \(numDot)")
}
}
if (numDot - numAt) > 1 {
return true
}
}
return false
}
With help from #Βασίλης Δ. i made a direct if statement for func validateEmail that check if number of char in between are less than 1
if (str.split(separator: "#").last?.split(separator: ".").first!.count)! < 1{
return false
}
It could be usefull
There are many edge cases to what you're trying to do, and email validation is notoriously complicated. I recommend doing as little of it as possible. Many, many things are legal email addresses. So you will need to think carefully about what you want to test. That said, this addresses what you've asked for, which is the distance between the first # and the first . that follows it.
func lengthOfFirstComponentAfterAt(in string: String) -> Int? {
guard
// Find the first # in the string
let firstAt = string.firstIndex(of: "#"),
// Find the first "." after that
let firstDotAfterAt = string[firstAt...].firstIndex(of: ".")
else {
return nil
}
// Return the distance between them (not counting the dot itself)
return string.distance(from: firstAt, to: firstDotAfterAt) - 1
}
lengthOfFirstComponentAfterAt(in: "rob#example.org") // Optional(7)
There's a very important lesson about Collections in this code. Notice the expression:
string[firstAt...].firstIndex(of: ".")
When you subscript a Collection, each element of the resulting slice has the same index as in the original collection. The returned value from firstIndex can be used directly to subscript string without offsetting. This is very different than how indexes work in many other languages, and allows powerful algorithms, and also creates at lot of bugs when developers forget this.
Say you need to determine the actual associated type of an enum.
So, a situation like
enum MessageItem: Decodable {
case a(Images)
case b(Text)
case c(Reply)
...
}
I used to have code like this
xSome = x.filter {
switch $0 {
case .a(_):
return false
case .b(_):
return true
case .c(_):
return true
}
}
But then it was possible to have code like this
xSome = x.filter {
if case .a = $0 { return false }
return true
}
Is there now some way it Swift to compare against associated type producing a boolean?
So, something like:
xSome = x.filter {
return (case .a = $0)
}
So, something like anEnum.is( .someCase )
Is anything like this now in Swift?
(Naturally, I mean without adding a var in the enum, which of course you can do.)
Enums with associated values are equatable if you declare them Equatable. Here's the state of play:
enum MyEnum : Equatable {
case hey
case ho
case heyNonnyNo(String)
}
let e = MyEnum.hey
e == .hey // true
e == .ho // false
// e == .heyNonnyNo // blap, illegal
let e2 = MyEnum.heyNonnyNo("hello")
e2 == .heyNonnyNo("hello") // true
e2 == .heyNonnyNo("goodbye") // true
Why is e == .heyNonnyNo illegal? Because it's unclear what it can mean. This case has an associated value; its value is the associated value. So we can check whether two instances of this case have the same associated value, but we can't just ask (using ==) whether an instance is some associated value of this case.
So if that's what we want to know, we are back to if case:
if case .heyNonnyNo = e2 {
print("it's a hey nonny no")
}
But you can't say that without if (for use in a conditional) because if case is the keyword; case can't exist by itself. If you really need a Bool, you could write it out like this:
let ok : Bool = {
switch e2 {
case .heyNonnyNo: return true
default: return false
}
}()
this is my code:
enum SymptomPeriod {
case Day
case Night
}
enum SymptomType {
case Breathing(SymptomPeriod)
case Breathlessness(SymptomPeriod)
case Opression(SymptomPeriod)
case Cough(SymptomPeriod)
case ActivityLimited()
case SecureTreatment()
}
struct Symptom {
let type: SymptomType
let date: NSDate
}
And i have an array of symptoms.
let symptomList: [Symptom] = ...
My need is to filter the list of symptoms with the SymptomPerion criteria, i trying to do like this:
let daySymtoms = symptomList.filter { (symptom) -> Bool in
return symptom.type = ???
}
My problem is in the filter function.
(My goal is to use a filter function and not a loop)
A few suggestions
Use your struct as namespace
Instead of repeating the word Symptom (e.g. SymptomPeriod, SymptomType) you should put your enums into you Symptom struct
Rename SymptomType as Kind
Once you moved SymptomType into Symptom you can drop the Symptom part of the name. However using Type as name will create a conflict so you should rename it Kind.
Add the period computed property to Kind
This will make the filtering much easier
Here's the code
struct Symptom {
enum Period {
case Day
case Night
}
enum Kind {
case Breathing(Period)
case Breathlessness(Period)
case Opression(Period)
case Cough(Period)
case ActivityLimited()
case SecureTreatment()
var period: Period? {
switch self {
case Breathing(let period): return period
case Breathlessness(let period): return period
case Opression(let period): return period
case Cough(let period): return period
default: return nil
}
}
}
let kind: Kind
let date: NSDate
}
The solution
Now the filtering has become very easy
let symptoms: [Symptom] = ...
let filtered = symptoms.filter { $0.kind.period == .Day }
This is how i am doing it:
let daySymtoms = symtoms.filter { (symptom) -> Bool in
switch symptom.type {
case .Breathing(.Day), .Breathlessness(.Day), .Opression(.Day), .Cough(.Day):
return true
default:
return false
}
}
Let me know if you have more simple way to do it.
I want to have a constant using let that may be one of several values.
For instance:
if condition1 {
constant = "hi"
}
else if condition2 {
constant = "hello"
}
else if condition3 {
constant = "hey"
}
else if condition4 {
constant = "greetings"
}
I'm not sure how to do this with Swift and the let feature. But I'm inclined to believe it's possible, as this is in the Swift book:
Use let to make a constant and var to make a variable. The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once.
How would I accomplish this?
As pointed out in the other answers you can't directly do this. But if you're looking to just variably set the initial value of a constant, then yes, that is possible. Here's an example with a computed property.
class MyClass {
let aConstant: String = {
if something == true {
return "something"
} else {
return "something else"
}
}()
}
I think you are looking for variable which will be assigned later inside switch-case:
let constant :String
switch conditions {
case condition1:
constant = "hi"
case condition2:
constant = "hello"
case condition3:
constant = "hey"
case condition4:
constant = "greetings"
default:
constant = "salute"
}
One option would be something like this, using a closure:
let constant: String = ({ value in
if conditionOne {
return "Hi"
} else if conditionTwo {
return "Bye"
}
return "Oops!"
})(myData /*needed for condition*/)
Or, for another twist, using generics:
func fancySwitch<S, T>(val: S, fn: S -> T) -> T {
return fn(val)
}
let x: String = fancySwitch(3) { val in
if val == 2 {
return "Hi"
} else if val < 5 {
return "Bye"
}
return "Oops"
}
let y: String = fancySwitch((3, 4)) { (a, b) in
if a == 2 {
return "Hi"
} else if b < 5 {
return "Bye"
}
return "Oops"
}
I understand what you're looking for. In Scala and some other functional languages this can be done using the match statement (kind of like switch) because the entire statement resolves to a value like this:
val b = true
val num = b match {
case true => 1
case false => 0
}
This is unfortunately not directly possible in Swift because there is no way to get a value from a branch statement. As stated in the Swift book, "Swift has two branch statements: an if statement and a switch statement." Neither of these statements resolve to a value.
The closest code structure I can think of is to first use a variable to retrieve the correct value and then assign it to a constant to be used in any later code:
let b = true
var num_mutable: Int
switch b {
case true:
num_mutable = 1
default:
num_mutable = 0
}
let num = num_mutable
Just add the line let constant: String before your if/else statement.
Below, an excerpt from Swift 1.2 and Xcode 6.3 beta - Swift Blog - Apple Developer elaborates.
let constants are now more powerful and consistent — The new rule is
that a let constant must be initialized before use (like a var), and
that it may only be initialized, not reassigned or mutated after
initialization. This enables patterns like:
let x : SomeThing
if condition {
x = foo()
} else {
x = bar()
}
use(x)
This formerly required the use of a var even though there is no
mutation taking place. Properties have been folded into this model to
simplify their semantics in initializers as well.
I found the Swift blog post above from the article "Let It Go: Late Initialization of Let in Swift", which I found by googling: swift let constant conditional initialize.