Update:
I had misunderstood an error message, so this question is invalid.
val inputIds = Array[Any](7, 1, 3)
is sufficient to create an Array[Any] in Scala.
===========
I'm trying to create an array that needs to be passed to a method that takes an Array[Any], but the values are numeric.
At first I tried just the normal way of creating an array:
val inputIds = Array(7, 1, 4)
and got the error:
Type mismatch, expected: Array[Any], actual: Array[Int]
(from the IntelliJ editor, I guess I haven't checked if it's correct)
Then I tried creating Integer values directly, e.g. by:
Integer.getInteger("7")
and passing those into the Array constructor, but I kept getting the same error.
Now I tried:
val inputIds: Array[Any] = Array[Any](7, 1, 4)
and it gave me:
Type mismatch, expected: Array[Any], actual: Array[Array[Int]]
As you can tell I'm going a bit spare, all I want is wrapper types and not primitives! I guess Scala is trying to optimize the array for iteration, but all I need is a tiny array I can pass to a var args taking arrays of mixed type.. Or maybe a better way of creating that vararg method?? Right now its type is:
Array[Any]*
========
Okay, so I got my original question resolved (though there's still a dispute in the comments over whether I correctly understood why the error was being caused). However, it didn't solve my problem, which is the following:
I am trying to transpose an array of arrays with different types (some are nested, but ultimately either Double or Ints) and can't figure out how to get the scala type system to do it. Here's the basic example:
val integerArray = Array(7, 1, 4)
val nestedIntegerArray = Array(
Array(6, 10, 60),
Array(5, 89, 57),
Array(15, 3, 5)
)
val nestedDoubleArray = Array(
Array(.13, .9, .8),
Array(.2, .777, .57),
Array(.15, .3, .5)
)
val brokenComputation = typeErrorExample(integerArray, nestedIntegerArray, nestedDoubleArray)
where the method being called is:
private def typeErrorExample(arrays: Array[_ <: Any]*)={
val arrayDataFrame = Array(arrays).transpose
}
This gives the error:
No implicit view available from Seq[Array[_]] => Array[U].
[INFO] val arrayDataFrame = Array(arrays).transpose
What's the right way to do this? Should I use a non-array data structure instead?
Hopefully having more of the code will help the experts understand what was causing my original error too. (inputIds was renamed to integerArray for consistency).
Explicitly casting is not ideal, but should do the trick.
Array(1, 2, 3).asInstanceOf[Array[Any]]
#Adair, this appears to arrange the input arrays into the Array[Array[Any]] type that you're looking for.
def transpose(arrays: Array[_]*) :Array[Array[_]] =
arrays.indices.map(x => arrays.map(_(x)).toArray).toArray
Now here's a word of advice: DON'T DO IT.
It's unsafe. This will throw an exception if the number of arrays passed exceeds the dimensions of each array.
Type Any is not what you want. When a data element becomes type Any then you've lost type information and it's a royal pain trying to get it back.
Jason's answer works, so does doing what the compiler error suggests when I ran it outside of intelliJ and changing the datatype in the method signature:
arrays: Array[_ <: Any]*
scala> Array(1, 2, 3).map(_.asInstanceOf[Any])
res6: Array[Any] = Array(1, 2, 3)
There are many answers which are correct. But why even need to do asInstanceOf. You can do it easily.
scala> val arr: Array[Any] = Array(1,2,3)
arr: Array[Any] = Array(1, 2, 3)
Simple, Work is done.
Related
Scala: Detect and extract something more specific from a collection of Any values.
(Motivation: The Saddle library -- the only Scala library I have found that provides a Frame type, which is critical for data science -- leads me to this puzzle. See last section for details.)
The problem
Imagine a collection c of type C[Any]. Suppose that some of the elements of c are of a type T which is strictly more specific than Any.
I would like a way to find all the elements of type T, and to then create an object d of type C[T], rather than C[Any].
Some code demonstrating the problem
scala> val c = List(0,1,"2","3")
<console>:11: warning: a type was inferred to be `Any`;
this may indicate a programming error.
val c = List(0,1,"2","3")
^
c: List[Any] = List(0, 1, 2, 3)
scala> :t c(0)
Any // I wish this were more specific
// Scala can convert some elements to Int.
scala> val c0 = c(0) . asInstanceOf[Int]
c0: Int = 0
// But how would I detect which?
scala> val d = c.slice(0,2)
d: List[Any] = List(0, 1) // I wish this were List[Int]
Motivation: Why the Saddle library leads me to this problem
Saddle lets you manipulate "Frames" (tables). Frames can have columns of various types. Some systems (e.g. Pandas) assign a separate type to each column. Every Frame in Saddle, however, has exactly three type parameters: The type of row labels, the type of column labels, and the type of cells.
Real world data is typically a mix of strings and numbers. The way such tables are represented in Saddle is as a Frame with a cell type of Any. I'd like to downcast (upcast? polymorphism is hard) a column to something more specific than a Series of Any values. I'd also like to be able to test a column, to be sure that the cast is appropriate.
I posted an issue on Saddle's Github site about the puzzle.
You could do something like this
scala> val c = List(0,1,"2","3")
c: List[Any] = List(0, 1, 2, 3)
scala> c.collect { case x: Int => x; case s: String => s.toInt }
res0: List[Int] = List(0, 1, 2, 3)
If you just want the Int types you can simply drop the second case.
I'm use SQLActionBuilder, such as seq"""select ...""", to create a common/wide sql query and I not care about the result column count it is.
Document example use as[TupleX] to decided result type,in my stage, I want use List[String] replace TupleX type.
I have attempted with sQLActionBuilder.as[List[String]] but a compile error encounter:
Error:(8, 187) could not find implicit value for parameter rconv: slick.jdbc.GetResult[List[String]]
val x = reportUtilRepository.find(List())("td_report_region")(1469635200000L, 1475251200000L)("region" :: Nil, "clicks" :: "CPC" :: Nil)(List("1", "2"), List("regionType" -> "1"))(_.as[List[String]]).map(x => println(x.toString))
and sQLActionBuilder.as[List[(String, String, String)]] works well. So how can I use List[String] to match a common result.
I think a straight way is implement a GetResult[List[String]] as compiler tips but I don't know how to do it.Other ways also welcome.
Thanks.
First of all querying database always returns a list of tuples, so the result type will be List[TupleX] because each row is represented as a list record and then columns in each row are respectively tuple elements.
Therefore, your data will look like List((1,2,3),(3,4,5)) where data type is List[(Int, Int, Int)]. To produce List[Int] you might do following:
val a = List((1,2,3),(3,4,5))
a map {x => List(x._1, x._2, x._3)} flatten
res0: List[Int] = List(1, 2, 3, 3, 4, 5)
I am trying to compare an item from a List of type Strings to an integer. I tried doing this but I get an error saying that:
'value < is not a member of List[Int]'
The line of code that compares is something similar to this:
if(csvList.map(x => x(0).toInt) < someInteger)
Besides the point of why this happens, I wondered why I didn't get an error
when I used a different type of comparison, such as ' == '.
So if I run the line:
if( csvList.map(x => x(0).toInt) == someInteger)
I don't get an error. Why is that?
Let's start with some introductions before answering the questions
Using the REPL you can understand a bit more what you are doing
scala> List("1", "2", "3", "33").map(x => x(0).toInt)
res1: List[Int] = List(49, 50, 51, 51)
The map function is used to transform every element, so x inside the map will be "1" the first time, "2" the second, and so on.
When you are using x(0) you are accessing the first character in the String.
scala> "Hello"(0)
res2: Char = H
As you see the type after you have mapped your strings is a List of Int. And you can compare that with an Int, but it will never be equals.
scala> List(1, 2, 3) == 5
res0: Boolean = false
This is very much like in Java when you try
"Hello".equals(new Integer(1));
If you want to know more about the reasons behind the equality problem you can check out Why has Scala no type-safe equals method?
Last but not least, you get an error when using less than because there is no less than in the List class.
Extra:
If you want to know if the second element in the list is smaller than 2 you can do
scala> val data = List("1", "10", "20")
data: List[String] = List(1, 10, 20)
scala> 5 < data(1).toInt
res2: Boolean = true
Although it is a bit strange, maybe you should transform the list of string is something a bit more typed like a case class and then do your business logic with a more clear data model.
You can refer to
Why == operator and equals() behave differently for values of AnyVal in Scala
Every class support operator ==, but may not support <,> these operators.
in your code
csvList.map(x => x(0).toInt)
it returns a List<int>, and application use it to compare with a int,
so it may process a implicit type conversion. Even the compiler doesn't report it as a error. Generally, it's not good to compare value with different types.
csvList.map(x => x(0).toInt) converts the entire csvList to a List[Int] and then tries to apply the operator < to List[Int] and someInteger which does not exist. This is essentially what the error message is saying.
There is no error for == since this operator exists for List though List[T] == Int will always return false.
Perhaps what you are trying to do is compare each item of the List to an Int. If that is the case, something like this would do:
scala> List("1","2","3").map(x => x.toInt < 2)
res18: List[Boolean] = List(true, false, false)
The piece of code csvList.map(x => x(0).toInt) actually returns a List[Int], that is not comparable with a integer (not sure what it would mean to say that List(1,2) < 3).
If you want to compare each element of the list to your number, making sure they are all inferior to it, you would actually write if(csvList.map(x => x.toInt).forall { _ < someInteger })
For example, if I do
scala> val a = Set(1,2,3)
a: scala.collection.immutable.Set[Int] = Set(1, 2, 3)
in the REPL, I want to see the most refined type of "a" in order to know whether it's really a HashSet. In C++, typeid(a).name() would do it. What is the Scala equivalent?
scala> val a = Set(1,2,3)
a: scala.collection.immutable.Set[Int] = Set(1, 2, 3)
scala> a.getClass.getName
res0: java.lang.String = scala.collection.immutable.Set$Set3
(Yes, it really is an instance of an inner class called Set3--it's a set specialized for 3 elements. If you make it a little larger, it'll be a HashTrieSet.)
Edit: #pst also pointed out that the type information [Int] was erased; this is how JVM generics work. However, the REPL keeps track since the compiler still knows the type. If you want to get the type that the compiler knows, you can
def manifester[A: ClassManifest](a: A) = implicitly[ClassManifest[A]]
and then you'll get something whose toString is the same as what the REPL reports. Between the two of these, you'll get as much type information as there is to be had. Of course, since the REPL already does this for you, you normally don't need to use this. But if for some reason you want to, the erased types are available from .typeArguments from the manifest.
How in the world do you get just an element at index i from the List in scala?
I tried get(i), and [i] - nothing works. Googling only returns how to "find" an element in the list. But I already know the index of the element!
Here is the code that does not compile:
def buildTree(data: List[Data2D]):Node ={
if(data.length == 1){
var point:Data2D = data[0] //Nope - does not work
}
return null
}
Looking at the List api does not help, as my eyes just cross.
Use parentheses:
data(2)
But you don't really want to do that with lists very often, since linked lists take time to traverse. If you want to index into a collection, use Vector (immutable) or ArrayBuffer (mutable) or possibly Array (which is just a Java array, except again you index into it with (i) instead of [i]).
Safer is to use lift so you can extract the value if it exists and fail gracefully if it does not.
data.lift(2)
This will return None if the list isn't long enough to provide that element, and Some(value) if it is.
scala> val l = List("a", "b", "c")
scala> l.lift(1)
Some("b")
scala> l.lift(5)
None
Whenever you're performing an operation that may fail in this way it's great to use an Option and get the type system to help make sure you are handling the case where the element doesn't exist.
Explanation:
This works because List's apply (which sugars to just parentheses, e.g. l(index)) is like a partial function that is defined wherever the list has an element. The List.lift method turns the partial apply function (a function that is only defined for some inputs) into a normal function (defined for any input) by basically wrapping the result in an Option.
Why parentheses?
Here is the quote from the book programming in scala.
Another important idea illustrated by this example will give you insight into why arrays are accessed with parentheses in Scala. Scala has fewer special cases than Java. Arrays are simply instances of classes like any other class in Scala. When you apply parentheses surrounding one or more values to a variable, Scala will transform the code into an invocation of a method named apply on that variable. So greetStrings(i) gets transformed into greetStrings.apply(i). Thus accessing an element of an array in Scala is simply a method call like any other. This principle is not restricted to arrays: any application of an object to some arguments in parentheses will be transformed to an apply method call. Of course this will compile only if that type of object actually defines an apply method. So it's not a special case; it's a general rule.
Here are a few examples how to pull certain element (first elem in this case) using functional programming style.
// Create a multdimension Array
scala> val a = Array.ofDim[String](2, 3)
a: Array[Array[String]] = Array(Array(null, null, null), Array(null, null, null))
scala> a(0) = Array("1","2","3")
scala> a(1) = Array("4", "5", "6")
scala> a
Array[Array[String]] = Array(Array(1, 2, 3), Array(4, 5, 6))
// 1. paratheses
scala> a.map(_(0))
Array[String] = Array(1, 4)
// 2. apply
scala> a.map(_.apply(0))
Array[String] = Array(1, 4)
// 3. function literal
scala> a.map(a => a(0))
Array[String] = Array(1, 4)
// 4. lift
scala> a.map(_.lift(0))
Array[Option[String]] = Array(Some(1), Some(4))
// 5. head or last
scala> a.map(_.head)
Array[String] = Array(1, 4)
Please use parentheses () to access the list of elements, as shown below.
list_name(index)