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)
Related
I am little bit confused about +: and :: operators that are available.
It looks like both of them gives the same results.
scala> List(1,2,3)
res0: List[Int] = List(1, 2, 3)
scala> 0 +: res0
res1: List[Int] = List(0, 1, 2, 3)
scala> 0 :: res0
res2: List[Int] = List(0, 1, 2, 3)
For my novice eye source code for both methods looks similar (plus-colon method has additional condition on generics with use of builder factories).
Which one of these methods should be used and when?
+: works with any kind of collection, while :: is specific implementation for List.
If you look at the source for +: closely, you will notice that it actually calls :: when the expected return type is List. That is because :: is implemented more efficiently for the List case: it simply connects the new head to the existing list and returns the result, which is a constant-time operation, as opposed to linear copying the entire collection in the generic case of +:.
+: on the other hand, takes CanBuildFrom, so you can do fancy (albeit, not looking as nicely in this case) things like:
val foo: Array[String] = List("foo").+:("bar")(breakOut)
(It's pretty useless in this particular case, as you could start with the needed type to begin with, but the idea is you can prepend and element to a collection, and change its type in one "go", avoiding an additional copy).
Has anyone got an example of how to use andThen with Lists? I notice that andThen is defined for List but the documentations hasn't got an example to show how to use it.
My understanding is that f andThen g means that execute function f and then execute function g. The input of function g is output of function f. Is this correct?
Question 1 - I have written the following code but I do not see why I should use andThen because I can achieve the same result with map.
scala> val l = List(1,2,3,4,5)
l: List[Int] = List(1, 2, 3, 4, 5)
//simple function that increments value of element of list
scala> def f(l:List[Int]):List[Int] = {l.map(x=>x-1)}
f: (l: List[Int])List[Int]
//function which decrements value of elements of list
scala> def g(l:List[Int]):List[Int] = {l.map(x=>x+1)}
g: (l: List[Int])List[Int]
scala> val p = f _ andThen g _
p: List[Int] => List[Int] = <function1>
//printing original list
scala> l
res75: List[Int] = List(1, 2, 3, 4, 5)
//p works as expected.
scala> p(l)
res74: List[Int] = List(1, 2, 3, 4, 5)
//but I can achieve the same with two maps. What is the point of andThen?
scala> l.map(x=>x+1).map(x=>x-1)
res76: List[Int] = List(1, 2, 3, 4, 5)
Could someone share practical examples where andThen is more useful than methods like filter, map etc. One use I could see above is that with andThen, I could create a new function,p, which is a combination of other functions. But this use brings out usefulness of andThen, not List and andThen
andThen is inherited from PartialFunction a few parents up the inheritance tree for List. You use List as a PartialFunction when you access its elements by index. That is, you can think of a List as a function from an index (from zero) to the element that occupies that index within the list itself.
If we have a list:
val list = List(1, 2, 3, 4)
We can call list like a function (because it is one):
scala> list(0)
res5: Int = 1
andThen allows us to compose one PartialFunction with another. For example, perhaps I want to create a List where I can access its elements by index, and then multiply the element by 2.
val list2 = list.andThen(_ * 2)
scala> list2(0)
res7: Int = 2
scala> list2(1)
res8: Int = 4
This is essentially the same as using map on the list, except the computation is lazy. Of course, you could accomplish the same thing with a view, but there might be some generic case where you'd want to treat the List as just a PartialFunction, instead (I can't think of any off the top of my head).
In your code, you aren't actually using andThen on the List itself. Rather, you're using it for functions that you're passing to map, etc. There is no difference in the results between mapping a List twice over f and g and mapping once over f andThen g. However, using the composition is preferred when mapping multiple times becomes expensive. In the case of Lists, traversing multiple times can become a tad computationally expensive when the list is large.
With the solution l.map(x=>x+1).map(x=>x-1) you are traversing the list twice.
When composing 2 functions using the andThen combinator and then applying it to the list, you only traverse the list once.
val h = ((x:Int) => x+1).andThen((x:Int) => x-1)
l.map(h) //traverses it only once
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 })
I am currently facing the following issue.
I have code that essentially has the following cases:
val toList = this.toString.match {
case "" => List[MyType]()
case _ => this.val :: this.prev.toList
}
Obviously not exact but its the general gist. It works fine but I want the values appended to the list in the reverse order. Is there any good way to do this? Intellij throws errors if I try to reverse the order and do
this.prev.toList :: this.val
and also if I try to use operations like ++. Is what I'm trying to do impossible based on the structure of my class?
The specific errors I get involve "cannot resolve ::" or whatever symbol I use when I try to put this.prev.toList before this.val.
And yes the "this" aren't necessary- I included it to hopefully make my problem easier to understand.
:: adds an element at the beginning of this list
scala> 1 :: List(2,3)
List(1, 2, 3)
+: is the equivalent of ::
scala> 1 +: List(2,3)
List(1, 2, 3)
:+ append element at the end of the list
scala> List(1,2) :+ 3
List(1, 2, 3)
However the cost of prepending on List is O(1) but the appending one is O(n)!
For "numerous" collections you could consider other datastructure like Vector:
Vector provides very fast append and prepend
http://www.scala-lang.org/api/2.11.7/index.html#scala.collection.immutable.Vector
You can append with this method :+:
this.prev.toList :+ this.val // is `val` really then name?
But keep in mind that appending to a List can be very inefficient for long lists.
I have two lists that I zip and go through the zipped result and call a function. This function returns a List of Strings as response. I now want to collect all the responses that I get and I do not want to have some sort of buffer that would collect the responses for each iteration.
seq1.zip(seq2).foreach((x: (Obj1, Obj1)) => {
callMethod(x._1, x._2) // This method returns a Seq of String when called
}
What I want to avoid is to create a ListBuffer and keep collecting it. Any clues to do it functionally?
Why not use map() to transform each input into a corresponding output ? Here's map() operating in a simple scenario:
scala> val l = List(1,2,3,4,5)
scala> l.map( x => x*2 )
res60: List[Int] = List(2, 4, 6, 8, 10)
so in your case it would look something like:
seq1.zip(seq2).map((x: (Obj1, Obj1)) => callMethod(x._1, x._2))
Given that your function returns a Seq of Strings, you could use flatMap() to flatten the results into one sequence.