I was working on a Swift tutorial and found that Swift has a strange way to handle multi-line statement.
First, I defined some extension to the standard String class:
extension String {
func replace(target: String, withString: String) -> String {
return self.stringByReplacingOccurrencesOfString(target, withString: withString)
}
func toLowercase() -> String {
return self.lowercaseString
}
}
This works as expected:
let str = "HELLO WORLD"
let s1 = str.lowercaseString.replace("hello", withString: "goodbye") // -> goodbye world
This doesn't work:
let s2 = str
.lowercaseString
.replace("hello", withString: "goodbye")
// Error: could not find member 'lowercaseString'
If I replace the reference to the lowercaseString property with a function call, it works again:
let s3 = str
.toLowercase()
.replace("hello", withString: "goodbye") // -> goodbye world
Is there anything in the Swift language specifications that prevent a property to be broken onto its own line?
Code at Swift Stub.
This is definitely a compiler bug. Issue has been resolved in Xcode 7 beta 3.
This feels like a compiler bug, but it relates to the fact that you can define prefix, infix, and postfix operators in Swift (but not the . operator, ironically enough). I don't know why it only gives you grief on the property and not the function call, but is a combination of two things:
the whitespace before and after the . (dot) operator for properties (only)
some nuance of this ever growing language that treats properties differently than function calls (even though functions are supposed to first class types).
I would file a bug to see what comes out of it, Swift is not supposed to by pythonic this way. That said, to work around it, you can either not break the property from the type, or you can add a white space before and after the . .
let s2 = str.lowercaseString
.replace("hello", withString: "goodbye")
let s3 = str
. lowercaseString
.replace("hello", withString: "goodbye")
Using semicolons is not mandatory in swift. And I think that the problems with multiline statements in swift are because of optional semicolons.
Note that swift does not support multiline strings. Check here: Swift - Split string over multiple lines
So maybe swift cannot handle multiline statements. I am not sure about this and this could be one of the reasons so I would appreciate if anyone else can help regarding this issue.
Related
var t = "13,45,344"
print(t.prefix(upTo: t.firstIndex(of: ",")))
results in error: value of optional type 'String.Index?' must be unwrapped to a value of type 'String.Index'
print(t.prefix(upTo: t.firstIndex(of: ",")))
SWIFT makes me crazy
You need to define the behavior you want when there is no comma. I assume you would like to print the whole thing. In that case you don't need indexes at all. You want "prefix as long as the current character is not comma:"
print(t.prefix(while: { $0 != "," }))
Minimal no crashing version of your code with supporting for NO COMMA situation:
if let prefix = t.firstIndex(of: ",") {
print(t.prefix(upTo: prefix))
} else {
print(t)
}
Or even more simpler mentioned by #rmaddy:
print(t.prefix(upTo: t.firstIndex(of: ",") ?? t.endIndex))
I find this amusing more than anything. I've fixed it, but I'm wondering about the cause. Here is the error: DataManager.swift:51:90: Expression was too complex to be solved in reasonable time; consider breaking up the expression into distinct sub-expressions. Why is it complaining? It seems like one of the most simple expressions possible.
The compiler points to the columns + ");"; section
func tableName() -> String { return("users"); }
func createTableStatement(schema: [String]) -> String {
var schema = schema;
schema.append("id string");
schema.append("created integer");
schema.append("updated integer");
schema.append("model blob");
var columns: String = ",".join(schema);
var statement = "create table if not exists " + self.tableName() + "(" + columns + ");";
return(statement);
}
the fix is:
var statement = "create table if not exists " + self.tableName();
statement += "(" + columns + ");";
this also works (via #efischency) but I don't like it as much because I think the ( get lost:
var statement = "create table if not exists \(self.tableName()) (\(columns))"
I am not an expert on compilers - I don't know if this answer will "change how you think in a meaningful way," but my understanding of the problem is this:
It has to do with type inference. Each time you use the + operator, Swift has to search through all of the possible overloads for + and infer which version of + you are using. I counted just under 30 overloads for the + operator. That's a lot of possibilities, and when you chain 4 or 5 + operations together and ask the compiler to infer all of the arguments, you are asking a lot more than it might appear at first glance.
That inference can get complicated - for example, if you add a UInt8 and an Int using +, the output will be an Int, but there's some work that goes into evaluating the rules for mixing types with operators.
And when you are using literals, like the String literals in your example, the compiler doing the work of converting the String literal to a String, and then doing the work of infering the argument and return types for the + operator, etc.
If an expression is sufficiently complex - i.e., it requires the compiler to make too many inferences about the arguments and the operators - it quits and tells you that it quit.
Having the compiler quit once an expression reaches a certain level of complexity is intentional. The alternative is to let the compiler try and do it, and see if it can, but that is risky - the compiler could go on trying forever, bog down, or just crash. So my understanding is that there is a static threshold for the complexity of an expression that the compiler will not go beyond.
My understanding is that the Swift team is working on compiler optimizations that will make these errors less common. You can learn a little bit about it on the Apple Developer forums by clicking on this link.
On the Dev forums, Chris Lattner has asked people to file these errors as radar reports, because they are actively working on fixing them.
That is how I understand it after reading a number of posts here and on the Dev forum about it, but my understanding of compilers is naive, and I am hoping that someone with a deeper knowledge of how they handle these tasks will expand on what I have written here.
This is almost same as the accepted answer but with some added dialogue (I had with Rob Napier, his other answers and Matt, Oliver, David from Slack) and links.
See the comments in this discussion. The gist of it is:
+ is heavily overloaded (Apple seems to have fixed this for some cases)
The + operator is heavily overloaded, as of now it has 27 different functions so if you are concatenating 4 strings ie you have 3 + operators the compiler has to check between 27 operators each time, so that's 27^3 times. But that's not it.
There is also a check to see if the lhs and rhs of the + functions are both valid if they are it calls through to core the append called. There you can see there are a number of somewhat intensive checks that can occur. If the string is stored non-contiguously, which appears to be the case if the string you’re dealing with is actually bridged to NSString. Swift then has to re-assemble all the byte array buffers into a single contiguous buffer and which requires creating new buffers along the way. and then you eventually get one buffer that contains the string you’re attempting to concatenate together.
In a nutshell there is 3 clusters of compiler checks that will slow you down ie each sub-expression has to be reconsidered in light of everything it might return. As a result concatenating strings with interpolation ie using " My fullName is \(firstName) \(LastName)" is much better than "My firstName is" + firstName + LastName since interpolation doesn't have any overloading
Swift 3 has made some improvements. For more information read How to merge multiple Arrays without slowing the compiler down?. Nonetheless the + operator is still overloaded and it's better to use string interpolation for longer strings
Usage of optionals (ongoing problem - solution available)
In this very simple project:
import UIKit
class ViewController: UIViewController {
let p = Person()
let p2 = Person2()
func concatenatedOptionals() -> String {
return (p2.firstName ?? "") + "" + (p2.lastName ?? "") + (p2.status ?? "")
}
func interpolationOptionals() -> String {
return "\(p2.firstName ?? "") \(p2.lastName ?? "")\(p2.status ?? "")"
}
func concatenatedNonOptionals() -> String {
return (p.firstName) + "" + (p.lastName) + (p.status)
}
func interpolatedNonOptionals() -> String {
return "\(p.firstName) \(p.lastName)\(p.status)"
}
}
struct Person {
var firstName = "Swift"
var lastName = "Honey"
var status = "Married"
}
struct Person2 {
var firstName: String? = "Swift"
var lastName: String? = "Honey"
var status: String? = "Married"
}
The compile time for the functions are as such:
21664.28ms /Users/Honey/Documents/Learning/Foundational/CompileTime/CompileTime/ViewController.swift:16:10 instance method concatenatedOptionals()
2.31ms /Users/Honey/Documents/Learning/Foundational/CompileTime/CompileTime/ViewController.swift:20:10 instance method interpolationOptionals()
0.96ms /Users/Honey/Documents/Learning/Foundational/CompileTime/CompileTime/ViewController.swift:24:10 instance method concatenatedNonOptionals()
0.82ms /Users/Honey/Documents/Learning/Foundational/CompileTime/CompileTime/ViewController.swift:28:10 instance method interpolatedNonOptionals()
Notice how crazy high the compilation duration for concatenatedOptionals is.
This can be solved by doing:
let emptyString: String = ""
func concatenatedOptionals() -> String {
return (p2.firstName ?? emptyString) + emptyString + (p2.lastName ?? emptyString) + (p2.status ?? emptyString)
}
which compiles in 88ms
The root cause of the problem is that the compiler doesn't identify the "" as a String. It's actually ExpressibleByStringLiteral
The compiler will see ?? and will have to loop through all types that have conformed to this protocol, till it finds a type that can be a default to String.
By Using emptyString which is hardcoded to String, the compiler no longer needs to loop through all conforming types of ExpressibleByStringLiteral
To learn how to log compilation times see here or here
Other similar answers by Rob Napier on SO:
Why string addition takes so long to build?
How to merge multiple Arrays without slowing the compiler down?
Swift Array contains function makes build times long
This is quite ridiculous no matter what you say! :)
But this gets passed easily
return "\(year) \(month) \(dayString) \(hour) \(min) \(weekDay)"
I had similar issue:
expression was too complex to be solved in reasonable time; consider breaking up the expression into distinct sub-expressions
In Xcode 9.3 line goes like this:
let media = entities.filter { (entity) -> Bool in
After changing it into something like this:
let media = entities.filter { (entity: Entity) -> Bool in
everything worked out.
Probably it has something to do with Swift compiler trying to infer data type from code around.
Great news - this seems to be fixed in the upcoming Xcode 13.
I was filing radar reports for this:
http://openradar.appspot.com/radar?id=4962454186491904
https://bugreport.apple.com/web/?problemID=39206436
... and Apple has just confirmed that this is fixed.
I have tested all cases that I have with complex expressions and SwiftUI code and everything seems to work great in Xcode 13.
Hi Alex,
Thanks for your patience, and thanks for your feedback. We believe this issue is resolved.
Please test with the latest Xcode 13 beta 2 release and update your feedback report with your results by logging into https://feedbackassistant.apple.com or by using the Feedback Assistant app.
In Swift 4, new method .split(separator:) is introduced by apple in String struct. So to split a string with whitespace which is faster for e.g..
let str = "My name is Sudhir"
str.components(separatedBy: " ")
//or
str.split(separator: " ")
Performance aside, there is an important difference between split(separator:) and components(separatedBy:) in how they treat empty subsequences.
They will produce different results if your input contains a trailing whitespace:
let str = "My name is Sudhir " // trailing space
str.split(separator: " ")
// ["My", "name", "is", "Sudhir"]
str.components(separatedBy: " ")
// ["My", "name", "is", "Sudhir", ""] ← Additional empty string
To have both produce the same result, use the omittingEmptySubsequences:false argument (which defaults to true):
// To get the same behavior:
str.split(separator: " ", omittingEmptySubsequences: false)
// ["My", "name", "is", "Sudhir", ""]
Details here:
https://developer.apple.com/documentation/swift/string/2894564-split
I have made sample test with following Code.
var str = """
One of those refinements is to the String API, which has been made a lot easier to use (while also gaining power) in Swift 4. In past versions of Swift, the String API was often brought up as an example of how Swift sometimes goes too far in favoring correctness over ease of use, with its cumbersome way of handling characters and substrings. This week, let’s take a look at how it is to work with strings in Swift 4, and how we can take advantage of the new, improved API in various situations. Sometimes we have longer, static strings in our apps or scripts that span multiple lines. Before Swift 4, we had to do something like inline \n across the string, add an appendOnNewLine() method through an extension on String or - in the case of scripting - make multiple print() calls to add newlines to a long output. For example, here is how TestDrive’s printHelp() function (which is used to print usage instructions for the script) looks like in Swift 3 One of those refinements is to the String API, which has been made a lot easier to use (while also gaining power) in Swift 4. In past versions of Swift, the String API was often brought up as an example of how Swift sometimes goes too far in favoring correctness over ease of use, with its cumbersome way of handling characters and substrings. This week, let’s take a look at how it is to work with strings in Swift 4, and how we can take advantage of the new, improved API in various situations. Sometimes we have longer, static strings in our apps or scripts that span multiple lines. Before Swift 4, we had to do something like inline \n across the string, add an appendOnNewLine() method through an extension on String or - in the case of scripting - make multiple print() calls to add newlines to a long output. For example, here is how TestDrive’s printHelp() function (which is used to print usage instructions for the script) looks like in Swift 3
"""
var newString = String()
for _ in 1..<9999 {
newString.append(str)
}
var methodStart = Date()
_ = newString.components(separatedBy: " ")
print("Execution time Separated By: \(Date().timeIntervalSince(methodStart))")
methodStart = Date()
_ = newString.split(separator: " ")
print("Execution time Split By: \(Date().timeIntervalSince(methodStart))")
I run above code on iPhone6 , Here are the results
Execution time Separated By: 8.27463299036026
Execution time Split By: 4.06880903244019
Conclusion : split(separator:) is faster than components(separatedBy:).
Maybe a little late to answer:
split is a native swift method
components is NSString Foundation method
When you play with them, they behave a little bit different:
str.components(separatedBy: "\n\n")
This call can give you some interesting results
str.split(separator: "\n\n")
This leads to an compile error as you must provide a single character.
I'm very new to Swift; I've spent the morning reading StackOverflow and trying many strategies, in vain, to accomplish the following:
I have a string, say "12345 is your number!"
I want to extract "12345" to a variable.
In Java, I'd do something like:
String myStr = "12345 is your number!";
return myStr.substring(0, myStr.indexOf(" "));
How do I do something similar in Swift? I don't want to hard-code any assumptions about what the ending index will be. It might be 5 characters in, it might not. I just want to take the substring of everything up to the first occurrence of " ", wherever that might be.
The closest I've gotten so far is:
var myMessage = "12345 is your number!"
myMessage.endIndex.advancedBy(myMessage.characters.count - myMessage.characters.indexOf(" "))
but it doesn't compile for reasons I don't fully yet grok("Binary operator '-' cannot be applied to operands of type Distance (aka 'Int') and 'String.CharacterView.Index?'")
Any help on this is appreciated. Thank you.
Something like this should work:
myMessage.substringToIndex(myMessage.characters.indexOf(" ")!)
Note that in this code I force unwrapped the optional. If you're not guaranteed to have that space in the string, it might make more sense to have the index in a optional binding.
With optional binding, it would look something like this:
if let index = myMessage.characters.indexOf(" ") {
let result = myMessage.substringToIndex(index)
}
You can use a regex, try this code:
var myMessage = "12345 is your number!"
if let match = myMessage.rangeOfString("-?\\d+", options: .RegularExpressionSearch) {
print(myMessage.substringWithRange(match)) // 12345
let myNumber = Int(myMessage.substringWithRange(match)) // Then you can initialize a new variable
}
The advantage is that this method extracts only the numbers wherever they are in the String
Hope this help ;)
With Swift 5 you can use:
myStr.prefix(upTo: myStr.firstIndex(of: " ") ?? myStr.startIndex)
You may need to cast it back to String (String(myStr.prefix(upTo: myStr.firstIndex(of: " ") ?? myStr.startIndex))) since it returns a Substring
I'm trying to write a function that takes a parameter of type Printable:
func logMessage(message: Printable) {
// ...
}
Strangely, this doesn't work as expected when passing in Strings.
This doesn't compile:
logMessage("some string \(someVariable)")
// Neither does this:
let aString = "aString"
logMessage(aString)
This however compiles:
logMessage("A string")
// This works too:
let aString: Printable = "a string"
logMessage(aString)
This is quite confusing. It seems that in some cases String implements Printable and in others not.
In addition, it seems that string interpolation always produces a String that does not implement Printable. This crashes at runtime with a cast error:
let aString = "a string"
let interpolatedString = "contains \(aString)"
Any idea what's going on here?
You're right that String doesn't conform to Printable. The reason this compiles:
let aString: Printable = "Ceci n'est pas une String"
is that you aren't creating a String with that literal – you're creating an NSString (which is Printable).
Generally, in Swift, it’s usually better to write generic functions constrained by protocols. So instead of
func logMessage(message: Printable) {
// ...
}
you would probably be better off writing:
func logMessage<T: Printable>(message: T) {
// ...
}
This approach has a number of advantages – better type-safety and avoiding type erasure, more performant etc. You can read more about this stuff here.
But you'll still hit a problem because you can't pass in a String. You have two options here. First, just don't constraint it at all:
func logMessage<T>(message: T) {
// ...then use toString(message) to create a String if you need one,
// or use string interpolation or print()
}
This will work with String, and in fact will also work with anything that isn’t Printable as well (though you'll get quite a unhelpful output involving the mangled classname).
Or, you could use Streamable which strings do conform to:
func logMessage<T: Streamable>(message: T) {
println(message)
}
let s: String = "hello"
logMessage(s)
I think I read a while back on twitter one of the Swift team mention that the reason String doesn't conform to Printable is exactly because they didn't want people using Printable directly like this and that it’s better to always use toString or similar.