I'm trying to access an element a dictionary element and downcast it to a type other than AnyObject but keep getting the same compiler error: Could not find an overload for 'subscript' that accepts the supplied arguments.
I know I can just do this using two if statements like so:
if let x = dict["key"] {
if let y = x as? String {
// ...
}
}
But I feel there has to be a more elegant solution than this. The format that makes the most sense to me is:
if let x = dict["key"] as? String {
// ...
}
But this just results in the error, mentioned above. I've tried dozens of variations of this, but none of it seems to make any difference. Is this something that just can't be done in Swift?
The reason your desired formulation isn't working is that you're trying to unwrap two Optionals with a single as?. There are two Optionals because both the subscripting of your dictionary and the attempted cast to String return optional values. There isn't a way to do it in one if statement that will be runtime-safe, but there is a way:
if let x = dict["key"]! as? String {
println(x)
}
The problem is that if dict["key"] ends up being nil, the forced unwrapping will crash your app. Better to do it this way (you can skip the first if), even if it is an extra step:
let x: AnyObject? = dict["key"]
if let y = x as? String {
println(y)
}
The extra step is just the cost of working with a Dictionary<String, AnyObject> - if you can get your dictionary type more specific, you won't have to do it any more.
I was also struggling with this thing but then i fount out that optional chaining is the solution. Just use
if let x = dict["key"]? as? String {
println(x)
}
It gives you both the safety and compactness.
In the end I had to use this one-line solution:
if let name = jsonDict["name"] as AnyObject? as? String {
println("name is \(name)")
} else {
println("property was nil")
}
Related
I'm learning Rust and would like to know how I can improve the code below.
I have a vector of tuples of form (u32, String). The u32 values represent line numbers and the Strings are the text on the corresponding lines. As long as all the String values can be successfully parsed as integers, I want to return an Ok<Vec<i32>> containing the just parsed String values, but if not I want to return an error of some form (just an Err<String> in the example below).
I'm trying to learn to avoid mutability and use functional styles where appropriate, and the above is straightforward to do functionally if that was all that was needed. Here's what I came up with in this case:
fn data_vals(sv: &Vec<(u32, String)>) -> Result<Vec<i32>, String> {
sv.iter()
.map(|s| s.1.parse::<i32>()
.map_err(|_e| "*** Invalid data.".to_string()))
.collect()
}
However, the small catch is that I want to print an error message for every invalid value (and not just the first one), and the error messages should contain both the line number and the string values in the offending tuple.
I've managed to do it with the following code:
fn data_vals(sv: &Vec<(u32, String)>) -> Result<Vec<i32>, String> {
sv.iter()
.map(|s| (s.0, s.1.parse::<i32>()
.or_else(|e| {
eprintln!("ERROR: Invalid data value at line {}: '{}'",
s.0, s.1);
Err(e)
})))
.collect::<Vec<(u32, Result<i32, _>)>>() // Collect here to avoid short-circuit
.iter()
.map(|i| i.1
.clone()
.map_err(|_e| "*** Invalid data.".to_string()))
.collect()
}
This works, but seems rather messy and cumbersome - especially the typed collect() in the middle to avoid short-circuiting so all the errors are printed. The clone() call is also annoying, and I'm not really sure why it's needed - the compiler says I'm moving out of borrowed content otherwise, but I'm not really sure what's being moved. Is there a way it can be done more cleanly? Or should I go back to a more procedural style? When I tried, I ended up with mutable variables and a flag to indicate success and failure, which seems less elegant:
fn data_vals(sv: &Vec<(u32, String)>) -> Result<Vec<i32>, String> {
let mut datavals = Vec::new();
let mut success = true;
for s in sv {
match s.1.parse::<i32>() {
Ok(v) => datavals.push(v),
Err(_e) => {
eprintln!("ERROR: Invalid data value at line {}: '{}'",
s.0, s.1);
success = false;
},
}
}
if success {
return Ok(datavals);
} else {
return Err("*** Invalid data.".to_string());
}
}
Can someone advise me on the best way to do this? Should I stick to the procedural style here, and if so can that be improved? Or is there a cleaner functional way to do it? Or a blend of the two? Any advice appreciated.
I think that's what partition_map() from itertools is for:
use itertools::{Either, Itertools};
fn data_vals<'a>(sv: &[&'a str]) -> Result<Vec<i32>, Vec<(&'a str, std::num::ParseIntError)>> {
let (successes, failures): (Vec<_>, Vec<_>) =
sv.iter().partition_map(|s| match s.parse::<i32>() {
Ok(v) => Either::Left(v),
Err(e) => Either::Right((*s, e)),
});
if failures.len() != 0 {
Err(failures)
} else {
Ok(successes)
}
}
fn main() {
let numbers = vec!["42", "aaaezrgggtht", "..4rez41eza", "55"];
println!("{:#?}", data_vals(&numbers));
}
In a purely functional style, you have to avoid side-effects.
Printing errors is a side-effect. The preferred style would be to return an object of the style:
Result<Vec<i32>, Vec<String>>
and print the list after the data_vals function returns.
So, essentially, you want your processing to collect a list of integers, and a list of strings:
fn data_vals(sv: &Vec<(u32, String)>) -> Result<Vec<i32>, Vec<String>> {
let (ok, err): (Vec<_>, Vec<_>) = sv
.iter()
.map(|(i, s)| {
s.parse()
.map_err(|_e| format!("ERROR: Invalid data value at line {}: '{}'", i, s))
})
.partition(|e| e.is_ok());
if err.len() > 0 {
Err(err.iter().filter_map(|e| e.clone().err()).collect())
} else {
Ok(ok.iter().filter_map(|e| e.clone().ok()).collect())
}
}
fn main() {
let input = vec![(1, "0".to_string())];
let r = data_vals(&input);
assert_eq!(r, Ok(vec![0]));
let input = vec![(1, "zzz".to_string())];
let r = data_vals(&input);
assert_eq!(r, Err(vec!["ERROR: Invalid data value at line 1: 'zzz'".to_string()]));
}
Playground Link
This uses partition which does not depend on an external crate.
Side effects (eprintln!) in an iterator adapter are definitely not "functional". You should accumulate and return the errors and let the caller deal with them.
I would use fold here. The goal of fold is to reduce a list to a single value, starting from an initial value and augmenting the result with every item. This "single value" can very well be a list, though. Here, though, there are two possible lists we might want to return: a list of i32 if all values are valid, or a list of errors if there are any errors (I've chosen to return Strings for errors here, for simplicity.)
fn data_vals(sv: &[(u32, String)]) -> Result<Vec<i32>, Vec<String>> {
sv.iter().fold(
Ok(Vec::with_capacity(sv.len())),
|acc, (line_number, data)| {
let data = data
.parse::<i32>()
.map_err(|_| format!("Invalid data value at line {}: '{}'", line_number, data));
match (acc, data) {
(Ok(mut acc_data), Ok(this_data)) => {
// No errors yet; push the parsed value to the values vector.
acc_data.push(this_data);
Ok(acc_data)
}
(Ok(..), Err(this_error)) => {
// First error: replace the accumulator with an `Err` containing the first error.
Err(vec![this_error])
}
(Err(acc_errors), Ok(..)) => {
// There have been errors, but this item is valid; ignore it.
Err(acc_errors)
}
(Err(mut acc_errors), Err(this_error)) => {
// One more error: push it to the error vector.
acc_errors.push(this_error);
Err(acc_errors)
}
}
},
)
}
fn main() {
println!("{:?}", data_vals(&[]));
println!("{:?}", data_vals(&[(1, "123".into())]));
println!("{:?}", data_vals(&[(1, "123a".into())]));
println!("{:?}", data_vals(&[(1, "123".into()), (2, "123a".into())]));
println!("{:?}", data_vals(&[(1, "123a".into()), (2, "123".into())]));
println!("{:?}", data_vals(&[(1, "123a".into()), (2, "123b".into())]));
}
The initial value is Ok(Vec::with_capacity(sv.len())) (this is an optimization to avoid reallocating the vector as we push items to it; a simpler version would be Ok(vec![])). If the slice is empty, this will be fold's result; the closure will never be called.
For each item, the closure checks 1) whether there were any errors so far (indicated by the accumulator value being an Err) or not and 2) whether the current item is valid or not. I'm matching on two Result values simultaneously (by combining them in a tuple) to handle all 4 cases. The closure then returns an Ok if there are no errors so far (with all the parsed values so far) or an Err if there are any errors so far (with every invalid value found so far).
You'll notice I used the push method to add an item to a Vec. This is, strictly speaking, mutation, which is not considered "functional", but because we are moving the Vecs here, we know there are no other references to them, so we know we aren't affecting any other use of these Vecs.
I'm trying to convert Vec<&str> to Vec<u16> but I can't figure out a functional way to do it.
let foo: &str = "1,2,3"; // Parsing a string here
let bar: Vec<&str> = foo.split(",").collect(); // Bar is a nice vector of &str's
I need to get bar into a Vec<u16>.
There's an iterator adapter map! You'd use it like this:
let bar: Vec<u16> = foo.split(",").map(|x| x.parse::<u16>().unwrap()).collect();
parse is a library function that relies on the trait FromStr, and it can return an error, so we need to unwrap() the error type. (This is a good idea for a short example, but in real code, you will want to handle the error properly - if you have a value that's not a u16 there, your program will just crash).
map takes a closure that takes it's parameter by value and then returns the iterator obtained by lazily applying that function. You're collecting all of the values here, but if you only take(5) of them, you would only parse 5 of the strings.
You haven't fully specified your problem. Specifically, what should happen when one of the strings cannot be parsed into a number? When you parse a number from a string using parse, it can fail. That is why the function returns a Result:
fn parse<F>(&self) -> Result<F, F::Err>
where
F: FromStr,
Here's a solution that takes the vector, gets an iterator with iter, changes each item using map and ultimately returns a Result using collect. If the parsing was a success, you get an Ok. If any failed, you get an Err:
fn main() {
let input = "1,2,3";
let strings: Vec<_> = input.split(",").collect();
let numbers: Result<Vec<u16>, _> = strings.iter().map(|x| x.parse()).collect();
println!("{:?}", numbers);
}
Or you could remove failed conversions by filtering out Err values with flat_map:
fn main() {
let input = "1,2,3";
let strings: Vec<_> = input.split(",").collect();
let numbers: Vec<u16> = strings.iter().flat_map(|x| x.parse()).collect();
println!("{:?}", numbers);
}
Of course, it's a bit silly to convert the string into a vector of strings and then convert it again to a vector of integers. If you actually have a comma-separated string and want numbers, do it in one go:
fn main() {
let input = "1,2,3";
let numbers: Result<Vec<u16>, _> = input.split(",").map(|x| x.parse()).collect();
println!("{:?}", numbers);
}
See also:
Why does `Option` support `IntoIterator`?
My take as someone not really experienced in Rust yet.
fn main() {
let foo: &str = "1,2,3"; // Parsing a string here
let bar: Vec<&str> = foo.split(",").collect(); // Bar is a nice vector of &str's
// here the magic happens
let baz = bar.iter().map(|x| x.parse::<i64>());
for x in baz {
match x {
Ok(i) => println!("{}", i),
Err(_) => println!("parse failed"),
}
}
}
Note that since parse returns a Result, you have to extract the value from each parsed element. You might want to behave in a different way, e.g. filter only the succeeded results.
How do i force unwrap an optional value thats "stored" as Any?
let optionalString: String? = "optional string"
let anyString: Any = optionalString
if let unwrappedString = anyString as? String {
println(unwrappedString)
// does not recognize the anyString as an optinal String
}
How do i write an if statement that force unwraps the value stored in anyString and prints the optionalString value? I have to do it only accessing the anyString attribute.
The use case for this is to get values out of MirrorType, which stores the attribute values as Any.
Ignore answer as not really providing a solution - the problem looks like a compiler bug
I'm not deleting it by now so readers know what I tried, and how I misunderstood the real question:
How to unwrap an optional value stored in a variable of Any type
Also be sure to read comments.
In your code you are using optional binding. Forced unwrapping instead uses the postfix ! operator:
println(optionalString!)
but if the optional string contains a nil value, that throws a runtime exception. So unless you are 100% sure it's not nil, I recommend using optional binding:
if let unwrappedString = optionalString {
println(unwrappedString)
}
That said, if you have an optional string stored in an Any variable, using optional downcasting and optional binding you can extract as follows:
var anyString: Any? = "optional string" as String?
if let unwrappedString = anyString as? String {
println(unwrappedString)
}
If you want to use a forced downcast, that's simply:
println(anyString as String)
but as in forced unwrapping, this causes a runtime exception if anyString is nil.
If the anyString variable is not optional, things are even simpler, because there's no optional binding, just optional downcasting, although the related code looks exactly the same:
var anyString: Any = "optional string"
if let downcastedString = anyString as? String {
println(downcastedString)
}
If you have an Any then you can test if it's optional and it it's nil by using something as:
var anyValue: String? = "The string"
var theValue: Any = anyValue
let mi:MirrorType = reflect(theValue)
if mi.disposition == .Optional {
if mi.count == 0 { return NSNull() } // Optional.None
let (name,some) = mi[0]
theValue = some.value
}
Now theValue will just be a String and not an Optional String. So you can cast it and use it as a String.
I am trying to do this:
var dictArray = [String:[String]]()
dictArray["test"] = [String]()
dictArray["test"]! += "hello"
But I am getting the weird error NSString is not a subtype of 'DictionaryIndex<String, [(String)]>'.
I just want to be able to add objects to an array inside a dictionary.
Update: Looks like Apple considers this a "known issue" in Swift, implying it will work as expected eventually. From the Xcode 6 Beta 4 release notes:
...Similarly, you cannot modify the underlying value of a mutable
optional value, either conditionally or within a force-unwrap:
tableView.sortDescriptors! += NSSortDescriptor(key: "creditName", ascending: true)
Workaround: Test the optional value explicitly and then assign the
result back:
if let window = NSApplication.sharedApplication.mainWindow {
window.title = "Currently experiencing problems"
}
tableView.sortDescriptors = tableView.sortDescriptors!
You can only do this
var dictArray = [String:[String]]()
dictArray["test"] = [String]()
var arr = dictArray["test"]!;
arr += "hello"
dictArray["test"] = arr
because dictArray["test"] give you Optional<[String]> which is immutable
6> var test : [String]? = [String]()
test: [String]? = 0 values
7> test += "hello"
<REPL>:7:1: error: '[String]?' is not identical to 'UInt8'
append also won't work due to the same reason, Optional is immutable
3> dictArray["test"]!.append("hello")
<REPL>:3:18: error: '(String, [(String)])' does not have a member named 'append'
dictArray["test"]!.append("hello")
^ ~~~~~~
BTW the error message is horrible...
You may use NSMutableArray instead of [String] as a value type for your dictionary:
var dictArray: [String: NSMutableArray] = [:]
dictArray["test"] = NSMutableArray()
dictArray["test"]!.addObject("hello")
This is still an issue in Swift 3. At least I was able to create method that can handle it for you.
func appendOrCreate(newValue: Any, toArrayAt key: String, in existingDictionary: inout [AnyHashable:Any]) {
var mutableArray = [Any]()
if let array = existingDictionary[key] as? [Any]{
//include existing values in mutableArray before adding new value
for existingValue in array {
mutableArray.append(existingValue)
}
}
//append new value
mutableArray.append(newValue)
//save updated array in original dictionary
existingDictionary[key] = mutableArray
}
The problem is that we want class semantics here but have to use structs. If you put class objects into the dictionary, you get what you want!
So, if you have¹ to have mutable values, you can wrap them in a class and perform updates with a closure:
class MutableWrapper<T> {
var rawValue: T
init(_ val: T) {
self.rawValue = val
}
func update(_ with: (inout T) -> Void) {
with(&self.rawValue)
}
}
Example:
func foo() {
var dict = [String: MutableWrapper<[String]>]()
dict["bar"] = MutableWrapper(["rum"])
dict["bar"]?.update({$0.append("gin")})
print(dict["bar"]!.rawValue)
// > ["rum", "gin"]
}
For what it's worth, I do not see a way to keep caller and wrapper in sync. Even if we declare init(_ val: inout T) we will end up with a copy in rawValue.
Performance is not necessarily an issue since the compiler optimizes structs heavily. I'd benchmark any mutable solution against what looks like lots of copy-updates in the code.
Since Swift 4.1 you can provide a default value to the subscript which allows you to solve this quite naturally now:
dictArray["test", default: []].append("hello")
Can I retrieve a Method via reflection, somehow combine it with a target object, and return it as something that looks like a function in Scala (i.e. you can call it using parenthesis)? The argument list is variable. It doesn't have to be a "first-class" function (I've updated the question), just a syntactic-looking function call, e.g. f(args).
My attempt so far looks something like this (which technically is pseudo-code, just to avoid cluttering up the post with additional definitions):
class method_ref(o: AnyRef, m: java.lang.reflect.Method) {
def apply(args: Any*): some_return_type = {
var oa: Array[Object] = args.toArray.map { _.asInstanceOf[Object] }
println("calling: " + m.toString + " with: " + oa.length)
m.invoke(o, oa: _*) match {
case x: some_return_type => x;
case u => throw new Exception("unknown result" + u);
}
}
}
With the above I was able to get past the compiler errors, but now I have a run-time exception:
Caused by: java.lang.IllegalArgumentException: argument type mismatch
The example usage is something like:
var f = ... some expression returning method_ref ...;
...
var y = f(x) // looks like a function, doesn't it?
UPDATE
Changing the args:Any* to args:AnyRef* actually fixed my run-time problem, so the above approach (with the fix) works fine for what I was trying to accomplish. I think I ran into a more general issue with varargs here.
Sure. Here's some code I wrote that add an interface to a function. It's not exactly what you want, but I think it can be adapted with few changes. The most difficult change is on invoke, where you'll need to change the invoked method by the one obtained through reflection. Also, you'll have to take care that the received method you are processing is apply. Also, instead of f, you'd use the target object. It should probably look something like this:
def invoke(proxy: AnyRef, method: Method, args: Array[AnyRef]) = method match {
case m if /* m is apply */ => target.getClass().getMethod("name", /* parameter type */).invoke(target, args: _*)
case _ => /* ??? */
}
Anyway, here's the code:
import java.lang.reflect.{Proxy, InvocationHandler, Method}
class Handler[T, R](f: Function1[T, R])(implicit fm: Manifest[Function1[T, R]]) extends InvocationHandler {
def invoke(proxy: AnyRef, method: Method, args: Array[AnyRef]) = method.invoke(f, args: _*)
def withInterface[I](implicit m: Manifest[I]) = {
require(m <:< manifest[Function1[T, R]] && m.erasure.isInterface)
Proxy.newProxyInstance(m.erasure.getClassLoader(), Array(m.erasure), this).asInstanceOf[I]
}
}
object Handler {
def apply[T, R](f: Function1[T, R])(implicit fm: Manifest[Function1[T, R]]) = new Handler(f)
}
And use it like this:
trait CostFunction extends Function1[String, Int]
Handler { x: String => x.length } withInterface manifest[CostFunction]
The use of "manifest" there helps with syntax. You could write it like this:
Handler({ x: String => x.length }).withInterface[CostFunction] // or
Handler((_: String).length).withInterface[CostFunction]
One could also drop the manifest and use classOf instead with a few changes.
If you're not looking for a generic invoke that takes the method name--but rather, you want to capture a particular method on a particular object--and you don't want to get too deeply into manifests and such, I think the following is a decent solution:
class MethodFunc[T <: AnyRef](o: Object, m: reflect.Method, tc: Class[T]) {
def apply(oa: Any*): T = {
val result = m.invoke(o, oa.map(_.asInstanceOf[AnyRef]): _*)
if (result.getClass == tc) result.asInstanceOf[T]
else throw new IllegalArgumentException("Unexpected result " + result)
}
}
Let's see it in action:
val s = "Hi there, friend"
val m = s.getClass.getMethods.find(m => {
m.getName == "substring" && m.getParameterTypes.length == 2
}).get
val mf = new MethodFunc(s,m,classOf[String])
scala> mf(3,8)
res10: String = there
The tricky part is getting the correct type for the return value. Here it's left up to you to supply it. For example,if you supply classOf[CharSequence] it will fail because it's not the right class. (Manifests are better for this, but you did ask for simple...though I think "simple to use" is generally better than "simple to code the functionality".)