I'm trying to create a class that has a map of keys -> function calls, and the following code is not behaving as I would like it to.
class MyClass {
val rnd = scala.util.Random
def method1():Double = {
rnd.nextDouble
}
def method2():Double = {
rnd.nextDouble
}
def method3():Double = {
rnd.nextDouble
}
def method4():Double = {
rnd.nextDouble
}
def method5():Double = {
rnd.nextDouble
}
var m = Map[String,Double]()
m += {"key1"-> method1}
m += {"key2"-> method2}
m += {"key3"-> method3}
m += {"key4"-> method4}
m += {"key5"-> method5}
def computeValues(keyList:List[String]):Map[String,Double] = {
var map = Map[String,Double]()
keyList.foreach(factor => {
val value = m(factor)
map += {factor -> value}
})
map
}
}
object Test {
def main(args : Array[String]) {
val b = new MyClass
for(i<-0 until 3) {
val computedValues = b.computeValues(List("key1","key4"))
computedValues.foreach(element => println(element._2))
}
}
}
The following output
0.022303440910331762
0.8557634244639081
0.022303440910331762
0.8557634244639081
0.022303440910331762
0.8557634244639081
indicates that once the function is placed in the map, it's a frozen instance (each key producing the same value for each pass). The behavior I would like to see is that the key would refer to a function call, generating a new random value rather than just returning the instance held in the map.
The problem is with the signature of your map m. You described that you wanted to put functions in the map; however you've declared it as Map[String, Double] which is just a map of strings to doubles. The correct type would be Map[String, () => Double].
Because brackets are optional on no-arg method invocations, the difference in types here is very important. When the map is being populated, the methods are invoked at insertion time in order to match the type signature (I believe this is done by an implicit conversion but I'm not 100% sure).
By simply changing the declared signature of your map, the functions are inserted as you desire, and can be evaluated during computeValues (requires a change on line 35 to map += {factor -> value()}), resulting in the following output (tested on Scala 2.8):
0.662682479130198
0.5106611727782306
0.6939805749938253
0.763581022199048
0.8785861039613938
0.9310533868752249
You need to map the keys to functions, not to the answer that the function would give you. Try this:
var m = Map[String,() => Double]()
m += /* etc. */
m.values.foreach(println(x => x()))
m.values.foreach(println(x => x()))
I would use scala's type inference to define the map.
There's no need to define the methods separately, if they're only used via the map.
Also you can use an immutable val, instead of a mutable var.
val m = Map( "key1" -> {() => rnd.nextDouble},
"key2" -> {() => rnd.nextDouble},
"key3" -> {() => rnd.nextDouble},
"key4" -> {() => rnd.nextDouble},
"key5" -> {() => rnd.nextDouble})
You also need to change line 35 to value()
Related
I have following HOCON config:
a {
b.c.d = "val1"
d.f.g = "val2"
}
HOCON represents paths "b.c.d" and "d.f.g" as objects. So, I would like to have a reader, which reads these configs as Map[String, String], ex:
Map("b.c.d" -> "val1", "d.f.g" -> "val2")
I've created a reader and trying to do it recursively:
import scala.collection.mutable.{Map => MutableMap}
private implicit val mapReader: ConfigReader[Map[String, String]] = ConfigReader.fromCursor(cur => {
def concat(prefix: String, key: String): String = if (prefix.nonEmpty) s"$prefix.$key" else key
def toMap(): Map[String, String] = {
val acc = MutableMap[String, String]()
def go(
cur: ConfigCursor,
prefix: String = EMPTY,
acc: MutableMap[String, String]
): Result[Map[String, Object]] = {
cur.fluent.mapObject { obj =>
obj.value.valueType() match {
case ConfigValueType.OBJECT => go(obj, concat(prefix, obj.pathElems.head), acc)
case ConfigValueType.STRING =>
acc += (concat(prefix, obj.pathElems.head) -> obj.asString.right.getOrElse(EMPTY))
}
obj.asRight
}
}
go(cur, acc = acc)
acc.toMap
}
toMap().asRight
})
It gives me the correct result but is there a way to avoid MutableMap here?
P.S. Also, I would like to keep implementation by "pureconfig" reader.
The solution given by Ivan Stanislavciuc isn't ideal. If the parsed config object contains values other than strings or objects, you don't get an error message (as you would expect) but instead some very strange output. For instance, if you parse a typesafe config document like this
"a":[1]
The resulting value will look like this:
Map(a -> [
# String: 1
1
])
And even if the input only contains objects and strings, it doesn't work correctly, because it erroneously adds double quotes around all the string values.
So I gave this a shot myself and came up with a recursive solution that reports an error for things like lists or null and doesn't add quotes that shouldn't be there.
implicit val reader: ConfigReader[Map[String, String]] = {
implicit val r: ConfigReader[String => Map[String, String]] =
ConfigReader[String]
.map(v => (prefix: String) => Map(prefix -> v))
.orElse { reader.map { v =>
(prefix: String) => v.map { case (k, v2) => s"$prefix.$k" -> v2 }
}}
ConfigReader[Map[String, String => Map[String, String]]].map {
_.flatMap { case (prefix, v) => v(prefix) }
}
}
Note that my solution doesn't mention ConfigValue or ConfigReader.Result at all. It only takes existing ConfigReader objects and combines them with combinators like map and orElse. This is, generally speaking, the best way to write ConfigReaders: don't start from scratch with methods like ConfigReader.fromFunction, use existing readers and combine them.
It seems a bit surprising at first that the above code works at all, because I'm using reader within its own definition. But it works because the orElse method takes its argument by name and not by value.
You can do the same without using recursion. Use method entrySet as following
import scala.jdk.CollectionConverters._
val hocon =
"""
|a {
| b.c.d = "val1"
| d.f.g = val2
|}""".stripMargin
val config = ConfigFactory.load(ConfigFactory.parseString(hocon))
val innerConfig = config.getConfig("a")
val map = innerConfig
.entrySet()
.asScala
.map { entry =>
entry.getKey -> entry.getValue.render()
}
.toMap
println(map)
Produces
Map(b.c.d -> "val1", d.f.g -> "val2")
With given knowledge, it's possible to define a pureconfig.ConfigReader that reads Map[String, String] as following
implicit val reader: ConfigReader[Map[String, String]] = ConfigReader.fromFunction {
case co: ConfigObject =>
Right(
co.toConfig
.entrySet()
.asScala
.map { entry =>
entry.getKey -> entry.getValue.render()
}
.toMap
)
case value =>
//Handle error case
Left(
ConfigReaderFailures(
ThrowableFailure(
new RuntimeException("cannot be mapped to map of string -> string"),
Option(value.origin())
)
)
)
}
I did not want to write custom readers to get a mapping of key value pairs. I instead changed my internal data type from a map to list of pairs (I am using kotlin), and then I can easily change that to a map at some later internal stage if I need to. My HOCON was then able to look like this.
additionalProperties = [
{first = "sasl.mechanism", second = "PLAIN"},
{first = "security.protocol", second = "SASL_SSL"},
]
additionalProducerProperties = [
{first = "acks", second = "all"},
]
Not the best for humans... but I prefer it to having to build custom parsing components.
I have created my scala map as :
val A:Map[String, String] = Map()
Then I am trying to add entries as :
val B = AttributeCodes.map { s =>
val attributeVal:String = <someString>
if (!attributeVal.isEmpty)
{
A + (s -> attributeVal)
}
else
()
}
And after this part of the code, I am seeing A is still empty. And, B is of type :
Pattern: B: IndexedSeq[Any]
I need a map to add entries and the same or different map in return to be used later in the code. However, I can not use "var" for that. Any insight on this problem and how to resolve this?
Scala uses immutability in many cases and encourages you to do the same.
Do not create an empty map, create a Map[String, String] with .map and .filter
val A = AttributeCodes.map { s =>
val attributeVal:String = <someString>
s -> attributeVal
}.toMap.filter(e => !e._1.isEmpty && !e._2.isEmpty)
In Scala, the default Map type is immutable. <Map> + <Tuple> creates a new map instance with the additional entry added.
There are 2 ways round this:
Use scala.collection.mutable.Map instead:
val A:immutable.Map[String, String] = immutable.Map()
AttributeCodes.forEach { s =>
val attributeVal:String = <someString>
if (!attributeVal.isEmpty){
A.put(s, attributeVal)
}
}
Create in immutable map using a fold:
val A: Map[String,String] = AttributeCodes.foldLeft(Map(), { m, s =>
val attributeVal:String = <someString>
if (!attributeVal.isEmpty){
m + (s -> attributeVal)
} else {
m
}
}
Following is a simple map entry assignment:
scala> var myl = mutable.Map[String,String]()
myl: scala.collection.mutable.Map[String,String] = Map()
myl("abc") = "123"
I would like to mimic that assignment structure in my own class that works with mutable Tuple's. Now, "getting" a value from a Map is achieved via the "apply" method:
e.g mutable.HashMap:
override def apply(key: A): B = {
val result = findEntry(key)
if (result eq null) default(key)
else result.value
}
I was not however able to find how the map entry is "set" via myMap("myKey") = "myval". A pointer to the Scala source code to do that would be appreciated. Thanks.
The method you want to implement is called update() and takes two parameters, one for the input value passed in parentheses and the other for the assigned value.
class QueryParams {
var params = ""
def update(name: String, value: String) { params += s"$name=$value&" }
}
For example:
val p = new QueryParams()
p("q") = "SFO"
p("start") = "10"
p("rows") = "10"
p.params
Consider the following code that counts the frequency of each string in the list and stores the results in the mutable map. This works great, but I don't understand where the += method is defined?! Is this some weird implicit conversion thing or what? I saw this code somewhere but it didn't include an explanation for the +=.
val list = List("a", "b", "a")
val counts = new scala.collection.mutable.HashMap[String, Int]().withDefaultValue(0)
list.foreach(counts(_) += 1)
counts
//> res7: scala.collection.mutable.Map[String,Int] = Map(a -> 2, b -> 1)
The apply of map returns an Int, but Int doesn't have a += and this method updates the map with a new value, so it looks as if the apply returns a mutable integer that has a += method...
This is not an implicit conversion - it is a desugaring.
Writing:
x += 1
desugars to:
x = x + 1
if the class of x does not have a += method defined on it.
In the same way:
counts("a") += 1
desugars to:
counts("a") = counts("a") + 1
because counts("a") is an Int, and Int does not have a += method defined.
On the other hand, writing:
x(expression1) = expression2
desugars to a call to the update method in Scala:
x.update(expression1, expression2)
Every mutable Map has an update method defined - it allows setting keys in the map.
So the entire expression is desugared to:
list.foreach(x => counts.update(x, counts(x) + 1))
This += is not to be confused with the += method on mutable.Maps in Scala. That method updates the entry in the map if that key already existed, or adds a new key-value pair. It returns the this reference, that is, the same map, so you can chain += calls. See ScalaDoc or the source code.
For these moments where you wonder what compiler magic is happening in a part of your code, scalac -print is your best friend (see this question).
If you do a scalac -print C.scala where C.scala is
package test
class C {
def myMethod() {
val counts = new scala.collection.mutable.HashMap[String, Int]().withDefaultValue(0)
counts("a") += 1
}
}
you get
package test {
class C extends Object {
def myMethod(): Unit = {
val counts: collection.mutable.Map = new collection.mutable.HashMap().withDefaultValue(scala.Int.box(0));
counts.update("a", scala.Int.box(scala.Int.unbox(counts.apply("a")).+(1)))
};
def <init>(): test.C = {
C.super.<init>();
()
}
}
It came as a surprise for me also, but apparently scalac will transform
map(key) =<op> rhs
to
map.update(key, map.apply(key) <op> rhs)
I'm trying to implement a simple web application server as a personal project to improve my Scala, but I've hit upon a problem.
I'd like to be able to set up routes using code like the following:
def routes()
{
get("/wobble")
{
...many lines of code here...
}
get("/wibble")
{
...many lines of code here...
}
post("/wibble")
{
...many lines of code here...
}
post("/wobble")
{
...many lines of code here...
}
}
routes is called by the server when it starts and get and post are functions defined by me like this:
get(url:String)(func:()=>String)=addroute("GET",url,func)
post(url:String(func:()=>String)=addroute("POST",url,func)
addroute(method:String,url:String,f:()=>String)
{
routesmap+=(method->Map[String,()=>String](url,func))
}
Unfortunately, I've had nothing but problems with this. Could anyone tell me the correct way in Scala to add an anonymous function (as passed in as a parameter in the defined routes function above) to a Map (or any other Scala collection for that matter)?
Here is a working example:
scala> var funcs = Map[String,(Int)=>Int]()
funcs: scala.collection.immutable.Map[String,Int => Int] = Map()
scala> funcs += ("time10", i => i * 10 )
scala> funcs += ("add2", i => i + 2 )
scala> funcs("add2")(3)
res3: Int = 5
scala> funcs("time10")(10)
res4: Int = 100
You can also add a declared function:
val minus5 = (i:Int) => i - 5
funcs += ( "minus5", minus5)
Or a method:
def square(i: Int) = i*i
funcs += ("square", square)
In your case, you can have two maps, one for GET and one for POST. It should simplify the design (and at most, you will end with four maps if you include DEL and PUT).
May be, this one ? :
type Fonc = ( (=> String) => Unit)
var routesmap = Map[String,Map[String,()=>String]]()
def addRoute(method:String,url:String,f:()=>String) = {
routesmap+=(method-> (routesmap.getOrElse(method,Map[String,()=>String]()) + (url->f)))
}
def get(url:String):Fonc = (x => addRoute("GET",url,() => x))
def post(url:String):Fonc = (x => addRoute("POST",url,() => x))
def routes()
{
post("/wobble")
{
"toto"
}
get("/wibble")
{
"titi"
}
}
you can try this code :
def addRoute(method:String,url:String,f:()=>String) = {
routesmap+=(method-> (routesmap.getOrElse(method,Map[String,()=>String]()) + (url->f)))
}
def get(url:String,func:()=>String)= addRoute("GET",url,func)
def post(url:String,func:()=>String)= addRoute("POST",url,func)
def routes()
{
get("/wobble",()=>{"toto"})
get("/wibble",()=>{println("test")
"titi"})
}
and execute these commands
scala> routes
scala> routesmap.get("GET").get("/wibble")()