I need to get the single attribute uuid and not Seq(UUID) from below class
case class Country(uuid: UUID, name: String, code:String)
val countries = Seq(
Country(20354d7a-e4fe-47af-8ff6-187bca92f3f9, "Afghanistan", "AFG"),
Country(caa8b54a-eb5e-4134-8ae2-a3946a428ec7,"Albania", "ALB"),
Country(bd2cbad1-6ccf-48e3-bb92-bc9961bc011e, "Algeria", "DZA")
)
val xyz: UUID = Country_uuid_from_countries
I tried val UUIDs = countries.map(_.uuid) but it returs Seq[UUID]
UUIDs: Seq[UUID] = List(20354d7a-e4fe-47af-8ff6-187bca92f3f9,
caa8b54a-eb5e-4134-8ae2-a3946a428ec7,
bd2cbad1-6ccf-48e3-bb92-bc9961bc011e
)
How do I just get UUID?
So you have a List of Countries, and a function (logic) for transforming one Country into AnotherCountry. And what you really want at the end is another List of AnotherCountries.
That is a well know problem. Every time you have a value A inside a context F[_] (a List is a context of multiplicity), and a function A => B. And you want to apply this transformation preserving the context to get an F[B] as a result.
Then you can use def map[F[_], A, B](fa: F[A])(f: A => B): F[B].
In the case of Scala, is common that the context themselves provide these functions as methods.
So, the only thing you need to do is this:
final case class Country(uuid: UUID, name: String, code: String)
final case class AnotherCountry(uuid: UUID)
val countries = List(
Country(20354d7a-e4fe-47af-8ff6-187bca92f3f9, "Afghanistan", "AFG"),
Country(caa8b54a-eb5e-4134-8ae2-a3946a428ec7,"Albania", "ALB"),
Country(bd2cbad1-6ccf-48e3-bb92-bc9961bc011e, "Algeria", "DZA")
)
val anotherCountires = countries.map { country =>
AnotherCountry(uuid = country.uuid)
}
Related
I'm new to Scala and attempting to do some data analysis.
I have a CSV files with a few headers - lets say item no., item type, month, items sold.
I have made an Item class with the fields of the headers.
I split the CSV into a list with each iteration of the list being a row of the CSV file being represented by the Item class.
I am attempting to make a method that will create maps based off of the parameter I send in. For example if I want to group the items sold by month, or by item type. However I am struggling to send the Item.field into a method.
F.e what I am attempting is something like:
makemaps(Item.month);
makemaps(Item.itemtype);
def makemaps(Item.field):
if (item.field==Item.month){}
else (if item.field==Item.itemType){}
However my logic for this appears to be wrong. Any ideas?
def makeMap[T](items: Iterable[Item])(extractKey: Item => T): Map[T, Iterable[Item]] =
items.groupBy(extractKey)
So given this example Item class:
case class Item(month: String, itemType: String, quantity: Int, description: String)
You could have (I believe the type ascriptions are mandatory):
val byMonth = makeMap[String](items)(_.month)
val byType = makeMap[String](items)(_.itemType)
val byQuantity = makeMap[Int](items)(_.quantity)
val byDescription = makeMap[String](items)(_.description)
Note that _.month, for instance, creates a function taking an Item which results in the String contained in the month field (simplifying a little).
You could, if so inclined, save the functions used for extracting keys in the companion object:
object Item {
val month: Item => String = _.month
val itemType: Item => String = _.itemType
val quantity: Item => Int = _.quantity
val description: Item => String = _.description
// Allows us to determine if using a predefined extractor or using an ad hoc one
val extractors: Set[Item => Any] = Set(month, itemType, quantity, description)
}
Then you can pass those around like so:
val byMonth = makeMap[String](items)(Item.month)
The only real change semantically is that you explicitly avoid possible extra construction of lambdas at runtime, at the cost of having the lambdas stick around in memory the whole time. A fringe benefit is that you might be able to cache the maps by extractor if you're sure that the source Items never change: for lambdas, equality is reference equality. This might be particularly useful if you have some class representing the collection of Items as opposed to just using a standard collection, like so:
object Items {
def makeMap[T](items: Iterable[Item])(extractKey: Item => T): Map[T,
Iterable[Item]] =
items.groupBy(extractKey)
}
class Items(val underlying: immutable.Seq[Item]) {
def makeMap[T](extractKey: Item => T): Map[T, Iterable[Item]] =
if (Item.extractors.contains(extractKey)) {
if (extractKey == Item.month) groupedByMonth.asInstanceOf[Map[T, Iterable[Item]]]
else if (extractKey == Item.itemType) groupedByItemType.asInstanceOf[Map[T, Iterable[Item]]]
else if (extractKey == Item.quantity) groupedByQuantity.asInstanceOf[Map[T, Iterable[Item]]]
else if (extractKey == Item.description) groupedByDescription.asInstanceOf[Map[T, Iterable[Item]]]
else throw new AssertionError("Shouldn't happen!")
} else {
Items.makeMap(underlying)(extractKey)
}
lazy val groupedByMonth = Items.makeMap[String](underlying)(Item.month)
lazy val groupedByItemType = Items.makeMap[String](underlying)(Item.itemType)
lazy val groupedByQuantity = Items.makeMap[Int](underlying)(Item.quantity)
lazy val groupedByDescription = Items.makeMap[String](underlying)(Item.description)
}
(that is almost certainly a personal record for asInstanceOfs in a small block of code... I'm not sure if I should be proud or ashamed of this snippet)
I have 2 Lists: lista and listb. For each element in lista, I want to check if a_type of each element is in b_type of listb. If true, get the b_name for corresponding b_type and construct an object objc. And, then I should return the list of of constructed objc.
Is there a way to do this in Scala and preferably without any mutable collections?
case class obja = (a_id: String, a_type: String)
case class objb = (b_id: String, b_type: String, b_name: String)
case class objc = (c_id: String, c_type: String, c_name: String)
val lista: List[obja] = List(...)
val listb: List[objb] = List(...)
def getNames(alist: List[obja], blist: List[objb]): List[objc] = ???
Lookup in lists requires traversal in O(n) time, this is inefficient. Therefore, the first thing you do is to create a map from b_type to b_name:
val bTypeToBname = listb.map(b => (b.b_type, b_name)).toMap
Then you iterate through lista, look up in the map whether there is a corresponding b_name for a given a.a_type, and construct the objc:
val cs = for {
a <- lista
b_name <- bTypeToBname.get(a.a_type)
} yield objc(a.a_id, a.a_type, b_name)
Notice how Scala for-comprehensions automatically filter those cases for which bTypeToBname(a.a_type) isn't defined: then the corresponding a is simply skipped. This because we use bTypeToBname.get(a.a_type) (which returns an Option), as opposed to calling bTypeToBname(a.a_type) directly (this would lead to a NoSuchElementException). As far as I understand, this filtering is exactly the behavior you wanted.
case class A(aId: String, aType: String)
case class B(bId: String, bType: String, bName: String)
case class C(cId: String, cType: String, cName: String)
def getNames(aList: List[A], bList: List[B]): List[C] = {
val bMap: Map[String, B] = bList.map(b => b.bType -> b)(collection.breakOut)
aList.flatMap(a => bMap.get(a.aType).map(b => C(a.aId, a.aType, b.bName)))
}
Same as Andrey's answer but without comprehension so you can see what's happening inside.
// make listb into a map from type to name for efficiency
val bs = listb.map(b => b.b_type -> b_name).toMap
val listc: Seq[objc] = lista
.flatMap(a => // flatmap to exclude types not in listb
bs.get(a.a_type) // get an option from blist
.map(bName => objc(a.a_id, a.a_type, bName)) // if there is a b name for that type, make an objc
)
Using Monocle I can define a Lens to read a case class member without issue,
val md5Lens = GenLens[Message](_.md5)
This can used to compare the value of md5 between two objects and fail with an error message that includes the field name when the values differ.
Is there a way to produce a user-friendly string from the Lens alone that identifies the field being read by the lens? I want to avoid providing the field name explicitly
val md5LensAndName = (GenLens[Message](_.md5), "md5")
If there is a solution that also works with lenses with more than one component then even better. For me it would be good even if the solution only worked to a depth of one.
This is fundamentally impossible. Conceptually, lens is nothing more than a pair of functions: one to get a value from object and one to obtain new object using a given value. That functions can be implemented by the means of accessing the source object's fields or not. In fact, even GenLens macro can use a chain field accessors like _.field1.field2 to generate composite lenses to the fields of nested objects. That can be confusing at first, but this feature have its uses. For example, you can decouple the format of data storage and representation:
import monocle._
case class Person private(value: String) {
import Person._
private def replace(
array: Array[String], index: Int, item: String
): Array[String] = {
val copy = Array.ofDim[String](array.length)
array.copyToArray(copy)
copy(index) = item
copy
}
def replaceItem(index: Int, item: String): Person = {
val array = value.split(delimiter)
val newArray = replace(array, index, item)
val newValue = newArray.mkString(delimiter)
Person(newValue)
}
def getItem(index: Int): String = {
val array = value.split(delimiter)
array(index)
}
}
object Person {
private val delimiter: String = ";"
val nameIndex: Int = 0
val cityIndex: Int = 1
def apply(name: String, address: String): Person =
Person(Array(name, address).mkString(delimiter))
}
val name: Lens[Person, String] =
Lens[Person, String](
_.getItem(Person.nameIndex)
)(
name => person => person.replaceItem(Person.nameIndex, name)
)
val city: Lens[Person, String] =
Lens[Person, String](
_.getItem(Person.cityIndex)
)(
city => person => person.replaceItem(Person.cityIndex, city)
)
val person = Person("John", "London")
val personAfterMove = city.set("New York")(person)
println(name.get(personAfterMove)) // John
println(city.get(personAfterMove)) // New York
While not very performant, that example illustrates the idea: Person class don't have city or address fields, but by wrapping data extractor and a string rebuild function into Lens, we can pretend it have them. For more complex objects, lens composition works as usual: inner lens just operates on extracted object, relying on outer one to pack it back.
I have two futures.
One future (idsFuture) holds the computation to get the list of ids. The type of the idsFuture is Future[List[Int]]
Another Future(dataFuture) holds an array of A where A is defined as case class A(id: Int, data: String). The type of dataFuture is Future[Array[A]]
I want to filter dataFuture's using ids present in idsFuture.
For example-
case class A(id: Int, data: String)
val dataFuture = Future(Array(A(1,"a"), A(2,"b"), A(3,"c")))
val idsFuture = Future(List(1,2))
I should get another future having Array((A(1,"a"), A(2,"b"))
I currently do
idsFuture.flatMap{
ids => dataFuture.map(datas => datas.filter(data => ids.contains(data.id)))}
Is there a better solution?
You could use for-comprehension here instead of flatMap + map like this:
for {
ds <- dataFuture
idsList <- idsFuture
ids = idsList.toSet
} yield ds filter { d => ids(d.id) }
Note that apply on Set is faster then contains on List.
Thanks to the answers to my previous question, I was able to create a function macro such that it returns a Map that maps each field name to its value of a class, e.g.
...
trait Model
case class User (name: String, age: Int, posts: List[String]) extends Model {
val numPosts: Int = posts.length
...
def foo = "bar"
...
}
So this command
val myUser = User("Foo", 25, List("Lorem", "Ipsum"))
myUser.asMap
returns
Map("name" -> "Foo", "age" -> 25, "posts" -> List("Lorem", "Ipsum"), "numPosts" -> 2)
This is where Tuples for the Map are generated (see Travis Brown's answer):
...
val pairs = weakTypeOf[T].declarations.collect {
case m: MethodSymbol if m.isAccessor =>
val name = c.literal(m.name.decoded)
val value = c.Expr(Select(model, m.name))
reify(name.splice -> value.splice).tree
}
...
Now I want to ignore fields that have #transient annotation. How would I check if a method has a #transient annotation?
I'm thinking of modifying the snippet above as
val pairs = weakTypeOf[T].declarations.collect {
case m: MethodSymbol if m.isAccessor && !m.annotations.exists(???) =>
val name = c.literal(m.name.decoded)
val value = c.Expr(Select(model, m.name))
reify(name.splice -> value.splice).tree
}
but I can't find what I need to write in exists part. How would I get #transient as an Annotation so I could pass it there?
Thanks in advance!
The annotation will be on the val itself, not on the accessor. The easiest way to access the val is through the accessed method on MethodSymbol:
def isTransient(m: MethodSymbol) = m.accessed.annotations.exists(
_.tpe =:= typeOf[scala.transient]
)
Now you can just write the following in your collect:
case m: MethodSymbol if m.isAccessor && !isTransient(m) =>
Note that the version of isTransient I've given here has to be defined in your macro, since it needs the imports from c.universe, but you could factor it out by adding a Universe argument if you're doing this kind of thing in several macros.