Encountering a problem whereby I am specifying Private Constants at the start of a scala step definiton file which relies on a List Buffer element to be populated, however when compiling I get a 'IndexOutOfBoundsException' because the list is empty initially and only gets populated later in a for loop.
For Example I have the following 2 constants:
private val ConstantVal1= globalExampleList(2)
private val ConstantVal2= globalExampleList(3)
globalExampleList is populated further down in the file using a for loop:
for (i <- 1 to numberOfW) {
globalExampleList += x.xy }
This List Buffer adds as many values as required to a global mutable ListBuffer.
Is there a better way to declare these constants? I've tried to declare them after the for loop but then other methods are not able to access these. I have around 4 different methods within the same file which use these values and instead of accessing it via index each time i thought it would be better to declare them as a constant to keep it neat and efficient for whenever they require changing.
Thanks
You can create list buffer of necessary size with default value and populate it later:
val globalExampleList: ListBuffer[Int] = ListBuffer.fill(numberOfW)(0)
for (i <- 0 until numberOfW) {
globalExampleList(i) = x.xy
}
But ConstantVal1, ConstantVal2 will still have original default value. So you can make them vars and re-assign them after you populate the buffer.
Your code seems to have a lot of mutations and side effects.
You have 2 ways to go.
First you can use lazy modifier
private lazy val ConstantVal1= globalExampleList(2)
private lazy val ConstantVal2= globalExampleList(3)
Or you can write the two lines after the for loop.
val globalExampleList = XXXX
for (i <- 1 to numberOfW) { globalExampleList += x.xy }
private val ConstantVal1= globalExampleList(2)
private val ConstantVal2= globalExampleList(3)
Related
How do I implement the following method in Scala.js?
import scala.scalajs.js
def toScalaArray(input: js.typedarray.Uint8Array): Array[Byte] =
// code in question
edited per request: tl;dr
input.view.map(_.toByte).toArray
Original answer
I'm not intimately familiar with Scala-js, but I can elaborate on some of the questions that came up in the comments, and improve upon your self-answer.
Also I don't quite get why I need toByte calls
class Uint8Array extends Object with TypedArray[Short, Uint8Array]
Scala treats a Uint8Array as a collection of Short, whereas you are expecting it to be a collection of Byte
Uint8Array's toArray method notes:
This member is added by an implicit conversion from Uint8Array to
IterableOps[Short] performed by method iterableOps in scala.scalajs.js.LowestPrioAnyImplicits.
So the method is returning an Array[Short] which you then .map to convert the Shorts to Bytes.
In your answer you posted
input.toArray.map(_.toByte)
which is technically correct, but it has the downside of allocating an intermediate array of the Shorts. To avoid this allocation, you can perform the .map operation on a .view of the array, then call .toArray on the view.
Views in Scala (and by extension Scala.js) are lightweight objects that reference an original collection plus some kind of transformation/filtering function, which can be iterated like any other collection. You can compose many transformation/filters on a view without having to allocate intermediate collections to represent the results. See the docs page (linked) for more.
input.view.map(_.toByte).toArray
Depending on how you intend to pass the resulting value around, you may not even need to call .toArray. For example if all you need to do is iterate the elements later on, you could just pass the view around as an Iterable[Byte] without ever having to allocate a separate array.
All the current answers require iterating over the array in user space.
Scala.js has optimizer supported conversions for typed arrays (in fact, Array[Byte] are typed arrays in modern configs). You'll likely get better performance by doing this:
import scala.scalajs.js.typedarray._
def toScalaArray(input: Uint8Array): Array[Byte] = {
// Create a view as Int8 on the same underlying data.
new Int8Array(input.buffer, input.byteOffset, input.length).toArray
}
The additional new Int8Array is necessary to re-interpret the underlying buffer as signed (the Byte type is signed). Only then, Scala.js will provide the built in conversion to Array[Byte].
When looking at the generated code, you'll see no user space loop is necessary: The built-in slice method is used to copy the TypedArray. This will almost certainly not be beatable in terms of performance by any user-space loop.
$c_Lhelloworld_HelloWorld$.prototype.toScalaArray__sjs_js_typedarray_Uint8Array__AB = (function(input) {
var array = new Int8Array(input.buffer, $uI(input.byteOffset), $uI(input.length));
return new $ac_B(array.slice())
});
If we compare this with the currently accepted answer (input.view.map(_.toByte).toArray) we see quite a difference (comments mine):
$c_Lhelloworld_HelloWorld$.prototype.toScalaArray__sjs_js_typedarray_Uint8Array__AB = (function(input) {
var this$2 = new $c_sjs_js_IterableOps(input);
var this$5 = new $c_sc_IterableLike$$anon$1(this$2);
// We need a function
var f = new $c_sjsr_AnonFunction1(((x$1$2) => {
var x$1 = $uS(x$1$2);
return ((x$1 << 24) >> 24)
}));
new $c_sc_IterableView$$anon$1();
// Here's the view: So indeed no intermediate allocations.
var this$8 = new $c_sc_IterableViewLike$$anon$6(this$5, f);
var len = $f_sc_TraversableOnce__size__I(this$8);
var result = new $ac_B(len);
// This function actually will traverse.
$f_sc_TraversableOnce__copyToArray__O__I__V(this$8, result, 0);
return result
});
import scala.scalajs.js
def toScalaArray(input: js.typedarray.Uint8Array): Array[Byte] =
input.toArray.map(_.toByte)
I get an error when I put the type and size of an array of classes
I have tried:
fun main(args :Array<String>) {
class modul() {
var nommodul: String? = null
var coeff: Int? = null
var note: Int? = null
}
var releve
class notes() {
var releve: array<modul>(10){""} here the erreur
}
}
First of all, your code has several errors. This might be an MCVE and/or copy-paste issue, but I need to address these before I get started on the arrays.
var releve before the notes class isn't allowed. You don't assign it, you don't declare a type, and the compiler will complain if you copy-paste the code from your question.
Second, the array var itself: Array is upper-case, and initialization is separate. This would be more valid (note that this still does not work - the solution for that comes later in this answer):
var releve: Array<modul> = Array(10) {...}
// or
var releve = Array<modul>(10) {...}
And the last thing before I start on the array itself: please read the language conventions, especially the naming ones. Your classes should all start with an upper-case letter.
Kotlin arrays are quite different from Java arrays in many ways, but the most notable one being that direct array initialization also requires an initializer.
The brackets are expected to create a new instance, which you don't. You create a String, which isn't, in your case, a modul.
There are several ways to fix this depending on how you want to do this.
If you have instances you want to add to the array, you can use arrayOf:
arrayOf(modulInstance, modulInstance2, ...)
If you want to create them directly, you can use your approach:
var releve = Array(10) { modul() }
A note about both of these: because of the initialization, you get automatic type inference and don't need to explicitly declare <modul>
If you want Java-style arrays, you need an array of nulls.
There's two ways to do this:
var releve = arrayOfNulls<modul>(10)
// or
var releve = Array<modul?>(10) { null }
I highly recommend the first one, because it's cleaner. I'm not sure if there's a difference performance-wise though.
Note that this does infer a nullable type to the array, but it lets you work with arrays in a similar way to Java. Initialization from this point is just like Java: releve[i] = modul(). This approach is mostly useful if you have arguments you want to add to each of the classes and you need to do so manually. Using the manual initializers also provides you with an index (see the documentation) which you can use while initializing.
Note that if you're using a for loop to initialize, you can use Array(10) { YourClass() } as well, and use the supplied index if you need any index-sensitive information, such as function arguments. There's of course nothing wrong with using a for loop, but it can be cleaner.
Further reading
Array
Lambdas
here some example of kotlin array initialization:
array of Library Method
val strings = arrayOf("January", "February", "March")
Primitive Arrays
val numbers: IntArray = intArrayOf(10, 20, 30, 40, 50)
Late Initialization with Indices
val array = arrayOfNulls<Number>(5)
for (i in array.indices) {
array[i] = i * i
}
See Kotlin - Basic Types for details
Here is my code.
var link = scala.collection.mutable.LinkedHashMap[String, String]()
var fieldTypeMapRDD = fixedRDD.mapPartitionsWithIndex((idx, itr) => itr.map(s => (s(8), s(9))))
fieldTypeMapRDD.foreach { i =>
println(i)
link.put(i._1, i._2)
}
println(link.size)// here size is zero
I want to access link out side loop .Please help.
Why your code is not supposed to work:
Before your foreach task is started, whole your function's closure inside foreach block is serialized and sent first to master, then to each of workers. This means each of them will have its own instance of mutable.LinkedHashMap as copy of link.
During foreach block each worker will put each of its items inside its own link copy
After your task is done you have still empty local link and several non-empty former copies on each of worker nodes.
Moral is clear: don't use local mutable collections with RDD. It's just not going to work.
One way to get whole collection to local machine is collect method.
You can use it as:
val link = fieldTypeMapRDD.collect.toMap
or in case of need to preserve the order:
import scala.collection.immutable.ListMap
val link = ListMap(fieldTypeMapRDD.collect:_*)
But if you are really into mutable collections, you can modify your code a bit. Just change
fieldTypeMapRDD.foreach {
to
fieldTypeMapRDD.toLocalIterator.foreach {
See also this question.
I'm completely new to Lift. My goal is to make a small application which will let me enter data about scientific articles (studies) into a database.
I haven't gotten around to making a database yet, still playing with getting an entry form to work. So I decided that I will hold a few studies in memory, in a list of a companion object to the Study class. I also created an accumulator variable to dispense unique IDs, as long as I don't have a DBMS managing the IDs.
As a smoke test, I visited the page showing the list of studies (seeded with two studies in code), then visited the form page, entered a new study, and navigated again to the list of studies. I was surprised to see that my new study has the ID of 1 instead of 3, so at some point, my accumulator variable must have been reset. But the ListBuffer collecting studies was not reset, because it showed all three studies. Adding more studies results in the counter incrementing by 1 every time.
The literature I have found on Lift (the two free books on Liftweb, as well as Gilberto Garcia's Lift Application Development Cookbook) are incomplete and are more like a collection of mini-tutorials, they don't explain how Lift works.
So what is the actual lifecycle of my Study object, and why did one mutable variable get reset after re-opening the page but not another?
package org.rumtscho.litsuche
package study
import scala.collection.mutable.ListBuffer
class Study private[study](
val id: Int,
val handmadeAuthorRef: String,
val humanReadableDescription: String )
{
}
object Study {
val seedStudies = List(
new Study(dispenseNextFreeId, "Brooks1975", "Tells us that throwing more programmers at a project makes it run late, not early"),
new Study(dispenseNextFreeId, "Dijkstra1968", "Recognizes that we need structured code")
)
private var studiesList = seedStudies.to[ListBuffer]
private var currentId = 0
private def dispenseNextFreeId: Int = {
currentId = currentId + 1
return currentId
}
def allStudies = studiesList.toList
def addStudy(reference: String, description: String): Unit = {
studiesList += new Study(dispenseNextFreeId, reference, description)
}
}
Here is the representation of the three studies:
update My understanding of what is happening (could be wrong, of course):
I open the page showing the list of studies. This calls allStudies. studiesList is initialized to contain Brooks1975 and Dijkstra1968. During the construction of the studies, currentId is initialized to 0 and then incremented to 2.
I open the entry form and add a third study. addStudy retrieves allStudies from memory, without initializing it again. It initializes currentId to 0. I end up with a third study with the ID 1.
I display all studies, then return to the entry form page. This time, addStudy retrieves both allStudies and currentId from memory. I get a fourth study with the ID of 2.
The comments have pointed out that this is probably Scala-specific and not related to Lift. But still, I don't understand why currentId is initialized two times (in steps 1 and 2), and not either once (as soon as the object itself is created) or every time it is read. I would have expected the first behavior, but even reinitializing every time seems more logical than randomly reinitializing one time only.
Go into the scala REPL, enter paste mode (:paste) command, and put in the following:
def increment {
currentId = currentId + 1
}
increment
increment
var currentId = 0
then try
var currentId = 0
def increment {
currentId = currentId + 1
}
increment
increment
In the first example, currentId ends up with value 0. In the second, it ends up with value 2. Why does this happen? I'm not an expert on Scala declaration, but it seems that this is the same problem you are running in to.
It seems that the solution is as #jcern suggests. In general, I'd say put all your declarations at the top of your classes or objects, and always declare before using a variable, and not the other way around.
In my function, can I have a variable that
Retains its value between function calls.
Is only visible inside that function
Is unique for each thread i.e. if I'm calling the function from two threads then there are two variables that are static with regard to each thread.
Why I want that:
I have a function in which I fill in a list and return that list. The problem is that if I declare a variable normally, then I will have to allocate memory for it every time I call the function. I want to avoid that and allocate only once then every time I call the function it would fill in that variable with the proper values then return it.
I can do the following inside a class:
class MyClass {
val __readLineTemp = mutable.IndexedSeq.fill[Int](5)(-1)
def readLine() = {
var i = 0
while (i < __readLineTemp.length)
{
__readLineTemp(i) = Random.nextInt()
i += 1
}
__readLineTemp
}
}
My problems with this approach is that it doesn't satisfy the points 2 and 3 namely visibility only inside the method and being unique for each thread. However, for point 3 I can simply make each thread initialise its own object of MyClass.
I understand there is probably no way of achieving exactly what I want, but sometimes people come up with clever ideas to overcome this, specially that Scala seems quite deep and there is a lot of tricks you can do
You can use a closure to satisfy 1 and 2:
def foo = {
var a = 5
() => {
a = a + 1
a
}
}
i.e. create a closure that will contain the static variable (in your case, this is __readLineTemp) and return a function that's the only thing with access to the variable.
Then use it like this to satisfy the thread requirement:
val t1 = new Thread(new Runnable {
def run = {
val f = new DynamicVariable(foo)
println(f.value())
println(f.value())
}
})