I'm using Scala, and have a ResultSet with an unknown number of elements. I'd like to loop through the set, processing each row, and end up with an array of the processed elements. The function rs.next() moves the pointer to the next element and returns true if that element is a meaningful row and false if that element is not (either after the last row, or the return was empty to begin with). So even though the following won't work, I'd like something structured like:
while (rs.next()) yield new foo(rs)
I tried the answer for this question, like so:
if (rs.next()) Iterator.continually(new foo(rs)).takeWhile(rs.next())
But this doesn't work because the element is created before the condition is checked, behaving as a do-while which has to process the first bad element. As written, it will create a foo using the last element of the resultset but not return it and requires the initial rs.next() to get started.
I have multiple other ways to approach this problem: another query to count the number of elements and then using take, using a boolean var that's set equal to rs.next() and then used to return a null element if false and also used for the takeWhile, a mutable collection that is added to in a while loop, and so on.
But I feel like I must be missing something, because some part of each of those solutions feels inelegant. Is there a simple functional construction that will repeatedly check a condition, creating an element to add to an iterator so long as the condition is true?
Related
I was solving the problem Filter Positions in a List:(For a given list with integers, return a new list removing the elements at odd positions)
I came up with
arr.zipWithIndex.filter(_._2 %2 == 1).map(_._1)
But someone has suggested that below code would be faster
arr.view.zipWithIndex.filter{ _._2 % 2 != 0 }.map { _._1}.force.toList
I know, View creates unconditional collection(Lazies evaluation), But
at which step (Method call) it will help us.
Meaning:
arr.view, creates view on which zipWithIndex will work, and zipWithIndex will process each element to create Map of value and Index. I guess till now, no optimization.
filter method has to work on every element then it only it can skip or select it.
I am not sure, how adding view in this case would help.
Using view means that all the operations can happen at once, instead of one at a time.
arr.zipWithIndex.filter(_._2 %2 == 1).map(_._1)
This works, but it creates 3 new lists in the process, first it runs the zipWithIndex producing a new list with the result. Then passes than new list to the filter, creating another list, and finally calls map on that list producing the final list.
So in that we creates two intermediate lists that we don't really need.
arr.view.zipWithIndex.filter{ _._2 % 2 != 0 }.map { _._1}.force.toList
This version uses view so if can perform all those operations in one shot, without needing to create those intermediate collections at each step.
I've made this recursive metod in Scala that returns a list made of all distinct elements of another list.
object es20 extends App{
def filledList:List[Int]=List()
#scala.annotation.tailrec
def distinct(l:List[Int]):List[Int] ={
if (l.isEmpty) filledList
if (filledList.forall(_!=l.head)) l.head::filledList
distinct(l.tail)
}
println(distinct(List(1,1,5,6,6,3,8,3))) //Should print List(1,5,6,3,8)
}
However, when I compile the code and then I run it, there's this exception:
java.util.NoSuchElementException: head of empty list
I thought that this exception was handle by the condition if (l.isEmpty).
How can I fix the code?
In Scala method returns last expression of the block. In your case you have three expressions: two if-expressions which result in unit and call to distinct, so checks will be executed every time you call distinct, no matter if the list is empty or not.
To fix it you can use if / else construct, or pattern match on input list, or make operation on headOption.
Anyway I doubt if this code correct: you trying to check something on 'filledList' which is always empty
You can fix this particular error by inserting else before the second if. However, as mentioned in the other answer, your code isn't correct, and won't work anyway, you need to rewrite it.
Also, I understand, that you are just trying to write this function as an exercise (if not, just do list.distinct), but I submit, that implementing quadratic solutions to trivially linear problems is never a good exercise to begin with.
I'm learning Scala now, and I have a scenario where I have to compare an element (say num) with all the elements in a list.
Assume,
val MyList = List(1, 2, 3, 4)
If num is equal to anyone the elements in the list, I need to return true. I know to do it recursively using the head and tail functions, but is there a simpler way to it (I think I'll be able to do it using foreach, but I'm not sure how to implement it exactly)?
There is number of possibilities:
val x = 3
MyList.contains(x)
!MyList.forall(y => y != x) // early exit, basically the same as .contains
If you plan to do it frequently, you may consider to convert your list to Set, cause every .contains lookup on list in worst case is proportional to number of elements, whereas on Set it is effectively constant
val mySet = MyList.toSet
mySet.contains(x)
or simply:
mySet(x)
A contains method is pretty standard for lists in any language. Scala's List has it too:
http://www.scala-lang.org/api/current/scala/collection/immutable/List.html
As others have answered, the contains method on the list will do exactly this, and it's the most understandable/performant way.
Looking at your closing comments though, you wouldn't be able to do it (in an elegant fashion) with foreach, since that returns Unit. Foreach "does" something for each element, but you don't get any result back. It's useful for logging/println statements, but it doesn't act as a transformation.
If you want to run a function on every element individually, you would use map, which returns a List of the results of applying the function. So assuming num = 3, then MyList.map(_ == num) would return List(false, false, true, false). Since you're looking for a single result, and not a list of results, then this is not what you're after.
In order to collapse a sequence of things into a single result, you would use a fold over the data. Folding involves a function that takes two arguments (the result so far, and the current thing in the list) and returns the new running result. So that this can work on the very first element, you also need to provide the initial value to use for the ongoing result (usually some sort of zero).
In your particular case, then, you want a Boolean answer at the end - "was an element found that was equal to num". So the running result would be "have I seen an element so far that was equal to num". Which means the initial value is false. And the function itself should return true if an element has already been seen, or if the current element is equal to num.
Putting this together, it would look like this:
MyList.foldLeft(false) { case (runningResult, listElem) =>
// return true if runningResult is true, or if listElem is the target number
runningResult || listElem == num
}
This doesn't have the nice aspect of stopping as soon as the target value has been found - and it's nowhere near as concise as calling MyList.contains. But as an instructional example, this is how you could implement this yourself from the primitive functional operations on a list.
List has a method for that:
val found = MyList.contains(num)
I am looping over the following lines from a csv file to parse them. I want to identify the first line since its the header. Whats the best way of doing this instead of making a var counter holder.
var counter = 0
for (line <- lines) {
println(CsvParser.parse(line, counter))
counter++
}
I know there is got to be a better way to do this, newbie to Scala.
Try zipWithIndex:
for (line <- lines.zipWithIndex) {
println(CsvParser.parse(line._1, line._2))
}
#tenshi suggested the following improvement with pattern matching:
for ((line, count) <- lines.zipWithIndex) {
println(CsvParser.parse(line, count))
}
I totally agree with the given answer, still that I've to point something important out and initially I planned to put in a simple comment.
But it would be quite long, so that, leave me set it as a variant answer.
It's prefectly true that zip* methods are helpful in order to create tables with lists, but they have the counterpart that they loop the lists in order to create it.
So that, a common recommendation is to sequence the actions required on the lists in a view, so that you combine all of them to be applied only producing a result will be required. Producing a result is considered when the returnable isn't an Iterable. So is foreach for instance.
Now, talking about the first answer, if you have lines to be the list of lines in a very big file (or even an enumeratee on it), zipWithIndex will go through all of 'em and produce a table (Iterable of tuples). Then the for-comprehension will go back again through the same amount of items.
Finally, you've impacted the running lenght by n, where n is the length of lines and added a memory footprint of m + n*16 (roughtly) where m is the lines' footprint.
Proposition
lines.view.zipWithIndex map Function.tupled(CsvParser.parse) foreach println
Some few words left (I promise), lines.view will create something like scala.collection.SeqView that will hold all further "mapping" function producing new Iterable, as are zipWithIndex and map.
Moreover, I think the expression is more elegant because it follows the reader and logical.
"For lines, create a view that will zip each item with its index, the result as to be mapped on the result of the parser which must be printed".
HTH.
If I have an ArrayList<Double> dblList and a Predicate<Double> IS_EVEN I am able to remove all even elements from dblList using:
Collections2.filter(dblList, IS_EVEN).clear()
if dblList however is a result of a transformation like
dblList = Lists.transform(intList, TO_DOUBLE)
this does not work any more as the transformed list is immutable :-)
Any solution?
Lists.transform() accepts a List and helpfully returns a result that is RandomAccess list. Iterables.transform() only accepts an Iterable, and the result is not RandomAccess. Finally, Iterables.removeIf (and as far as I see, this is the only one in Iterables) has an optimization in case that the given argument is RandomAccess, the point of which is to make the algorithm linear instead of quadratic, e.g. think what would happen if you had a big ArrayList (and not an ArrayDeque - that should be more popular) and kept removing elements from its start till its empty.
But the optimization depends not on iterator remove(), but on List.set(), which is cannot be possibly supported in a transformed list. If this were to be fixed, we would need another marker interface, to denote that "the optional set() actually works".
So the options you have are:
Call Iterables.removeIf() version, and run a quadratic algorithm (it won't matter if your list is small or you remove few elements)
Copy the List into another List that supports all optional operations, then call Iterables.removeIf().
The following approach should work, though I haven't tried it yet.
Collection<Double> dblCollection =
Collections.checkedCollection(dblList, Double.class);
Collections2.filter(dblCollection, IS_EVEN).clear();
The checkCollection() method generates a view of the list that doesn't implement List. [It would be cleaner, but more verbose, to create a ForwardingCollection instead.] Then Collections2.filter() won't call the unsupported set() method.
The library code could be made more robust. Iterables.removeIf() could generate a composed Predicate, as Michael D suggested, when passed a transformed list. However, we previously decided not to complicate the code by adding special-case logic of that sort.
Maybe:
Collection<Double> odds = Collections2.filter(dblList, Predicates.not(IS_EVEN));
or
dblList = Lists.newArrayList(Lists.transform(intList, TO_DOUBLE));
Collections2.filter(dblList, IS_EVEN).clear();
As long as you have no need for the intermediate collection, then you can just use Predicates.compose() to create a predicate that first transforms the item, then evaluates a predicate on the transformed item.
For example, suppose I have a List<Double> from which I want to remove all items where the Integer part is even. I already have a Function<Double,Integer> that gives me the Integer part, and a Predicate<Integer> that tells me if it is even.
I can use these to get a new predicate, INTEGER_PART_IS_EVEN
Predicate<Double> INTEGER_PART_IS_EVEN = Predicates.compose(IS_EVEN, DOUBLE_TO_INTEGER);
Collections2.filter(dblList, INTEGER_PART_IS_EVEN).clear();
After some tries, I think I've found it :)
final ArrayList<Integer> ints = Lists.newArrayList(1, 2, 3, 4, 5);
Iterables.removeIf(Iterables.transform(ints, intoDouble()), even());
System.out.println(ints);
[1,3,5]
I don't have a solution, instead I found some kind of a problem with Iterables.removeIf() in combination with Lists.TransformingRandomAccessList.
The transformed list implements RandomAccess, thus Iterables.removeIf() delegates to Iterables.removeIfFromRandomAccessList() which depends on an unsupported List.set() operation.
Calling Iterators.removeIf() however would be successful, as the remove() operation IS supported by Lists.TransformingRandomAccessList.
see: Iterables: 147
Conclusion: instanceof RandomAccess does not guarantee List.set().
Addition:
In special situations calling removeIfFromRandomAccessList() even works:
if and only if the elements to erase form a compact group at the tail of the List or all elements are covered by the Predicate.