I have a question about setting the result to Empty (i.e. nothing), I've researched in past questions and did not find a good solution.
The question is quite simple, say I have a List of Int and List of Bool
val a = List(1,2,3,4,5)
val b = List(F,T,T,F,F)
and I want to zip them and do some mapping:
val result = (a,b).zipped.map((x,y)=>(if(b) a else ())
I assume I am doing the right thing above which takes each element of a and b and does the operation where if b is true, return a, else return nothing. I expect the result to only have the numbers (2,3). However, my Eclipse seems to indicate that the result generated is List[AnyVal] instead of List[Int].
I have tested the same setting, but using Lists, and when I set b to List(), the process works and Eclipse is understanding that I want to set an empty List, so I am lost where I when wrong..
Thanks in advance
You want val result = (a zip b).filter(_._2).map(_._1). map never filters, so don't attempt to return () from its argument and hope it will filter.
val a = List(1, 2, 3, 4, 5)
val b = List(false, true, true, false, false)
a.zip(b) // zip two lists
.filter(_._2) // filter if second element is true
.map(_._1) // grab first element of tuple
// List[Int] = List(2, 3)
You could also use collect, which will keep only the elements that match a pattern. You can think of it like a map that discards anything it's not defined for.
val a = List(1, 2, 3, 4, 5)
val b = List(false, true, true, false, false)
(a zip b) collect { case (x, true) => x }
Related
I am new to Scala and new OOP too. How can I update a particular element in a list while creating a new list.
val numbers= List(1,2,3,4,5)
val result = numbers.map(_*2)
I need to update third element only -> multiply by 2. How can I do that by using map?
You can use zipWithIndex to map the list into a list of tuples, where each element is accompanied by its index. Then, using map with pattern matching - you single out the third element (index = 2):
val numbers = List(1,2,3,4,5)
val result = numbers.zipWithIndex.map {
case (v, i) if i == 2 => v * 2
case (v, _) => v
}
// result: List[Int] = List(1, 2, 6, 4, 5)
Alternatively - you can use patch, which replaces a sub-sequence with a provided one:
numbers.patch(from = 2, patch = Seq(numbers(2) * 2), replaced = 1)
I think the clearest way of achieving this is by using updated(index: Int, elem: Int). For your example, it could be applied as follows:
val result = numbers.updated(2, numbers(2) * 2)
list.zipWithIndex creates a list of pairs with original element on the left, and index in the list on the right (indices are 0-based, so "third element" is at index 2).
val result = number.zipWithIndex.map {
case (n, 2) => n*2
case n => n
}
This creates an intermediate list holding the pairs, and then maps through it to do your transformation. A bit more efficient approach is to use iterator. Iterators a 'lazy', so, rather than creating an intermediate container, it will generate the pairs one-by-one, and send them straight to the .map:
val result = number.iterator.zipWithIndex.map {
case (n, 2) => n*2
case n => n
}.toList
1st and the foremost scala is FOP and not OOP. You can update any element of a list through the keyword "updated", see the following example for details:
Signature :- updated(index,value)
val numbers= List(1,2,3,4,5)
print(numbers.updated(2,10))
Now here the 1st argument is the index and the 2nd argument is the value. The result of this code will modify the list to:
List(1, 2, 10, 4, 5).
Allright so i don't know if this is possible, but let's say we have the following list:
List(1, 2, 3, 1)
If i want to apply a map over this, is there a way for me to check if i've already had a value before, e.g. on the 4th value (the 2nd 1) it'll say that it already came across the 1 and then throw an error or something.
This would be the role of a foldLeft stage:
List(1, 2, 3, 1).foldLeft(List[Int]()) {
// The item has already been encountered:
case (uniqueItems, b) if uniqueItems.contains(b) => {
// If as stated, you want to throw an exception, that's where you could do it
uniqueItems
}
// New item not seen yet:
case (uniqueItems, b) => uniqueItems :+ b
}
foldLeft traverses a sequence while working (at each new element), with a result based on the previous ones.
For each element, the pattern matching (uniqueItems, b) should be understood this way: uniqueItems is the "accumulator" (it's initialized as List[Int]()) and will be updated (or not) for each item of the list. And b if the new item of the list which is currently being processed.
By the way, this example is a (non-efficient) distinct over a list.
List(1, 2, 3, 1).distinct.map (n => n*n)
// res163: List[Int] = List(1, 4, 9)
This code removes duplicates, then performs the mappings in a self documenting, brief manner.
fold is probably the way to go. The problem is that each iteration has to carry both the memory of previous elements as well as the map() results as it is being built.
List(1, 2, 3, 11).foldRight((Set[Int](),List[String]())) {case (i, (st, lst)) =>
if (st(i)) throw new Error //duplicate encountered
else (st + i, i.toString :: lst) //add to memory and map result
}._2 //pull the map result from the tuple
Suppose I have an imperative algorithm that keeps two indices left and right and moves them from left to right and from right to left
var left = 0
var right = array.length - 1
while (left < right) { .... } // move left and right inside the loop
Now I would like to write this algorithm without mutable indices.
How can I do that ? Do you have any examples of such algorithms ? I would prefer a non-recursive approach.
You can map pairs of elements between your list and its reverse, then go from left to right through that list of pairs and keep taking as long as your condition is satisfied:
val list = List(1, 2, 3, 4, 5)
val zipped = list zip list.reverse
val filtered = zipped takeWhile { case (a, b) => (a < b) }
Value of filtered is List((1, 5), (2, 4)).
Now you can do whatever you need with those elements:
val result = filtered map {
case (a, b) =>
// do something with each left-right pair, e.g. sum them
a + b
}
println(result) // List(6, 6)
If you need some kind of context dependant operation (that is, each
iteration depends on the result of the previous one) then you have to
use a more powerful abstraction (monad), but let's not go there if
this is enough for you. Even better would be to simply use recursion, as pointed out by others, but you said that's not an option.
EDIT:
Version without extra pass for reversing, only constant-time access for elem(length - index):
val list = List(1, 2, 3, 4, 5)
val zipped = list.view.zipWithIndex
val filtered = zipped takeWhile { case (a, index) => (a < list(list.length - 1 - index)) }
println(filtered.toList) // List((1, 0), (2, 1))
val result = filtered map {
case (elem, index) => // do something with each left-right pair, e.g. sum them
val (a, b) = (elem, list(list.length - 1 - index))
a + b
}
println(result.toList) // List(6, 6)
Use reverseIterator:
scala> val arr = Array(1,2,3,4,5)
arr: Array[Int] = Array(1, 2, 3, 4, 5)
scala> arr.iterator.zip(arr.reverseIterator).foreach(println)
(1,5)
(2,4)
(3,3)
(4,2)
(5,1)
This function is efficient on IndexedSeq collections, which Array is implicitly convertible to.
It really depends on what needs to be done at each iteration, but here's something to think about.
array.foldRight(0){case (elem, index) =>
if (index < array.length/2) {
/* array(index) and elem are opposite elements in the array */
/* do whatever (note: requires side effects) */
index+1
} else index // do nothing
} // ignore result
Upside: Traverse the array only once and no mutable variables.
Downside: Requires side effects (but that was implied in your example). Also, it'd be better if it traversed only half the array, but that would require early breakout and Scala doesn't offer an easy/elegant solution for that.
myarray = [1,2,3,4,5,6]
rmyarray = myarray[::-1]
Final_Result = []
for i in range(len(myarray)//2):
Final_Result.append(myarray[i])
Final_Result.append(rmyarray[i])
print(Final_Result)
# This is the simple approach I think 😉.
I was wondering what is the most elegant way of getting the increasing prefix of a given sequence. My idea is as follows, but it is not purely functional or any elegant:
val sequence = Seq(1,2,3,1,2,3,4,5,6)
var currentElement = sequence.head - 1
val increasingPrefix = sequence.takeWhile(e =>
if (e > currentElement) {
currentElement = e
true
} else
false)
The result of the above is:
List(1,2,3)
You can take your solution, #Samlik, and effectively zip in the currentElement variable, but then map it out when you're done with it.
sequence.take(1) ++ sequence.zip(sequence.drop(1)).
takeWhile({case (a, b) => a < b}).map({case (a, b) => b})
Also works with infinite sequences:
val sequence = Seq(1, 2, 3).toStream ++ Stream.from(1)
sequence is now an infinite Stream, but we can peek at the first 10 items:
scala> sequence.take(10).toList
res: List[Int] = List(1, 2, 3, 1, 2, 3, 4, 5, 6, 7)
Now, using the above snippet:
val prefix = sequence.take(1) ++ sequence.zip(sequence.drop(1)).
takeWhile({case (a, b) => a < b}).map({case (a, b) => b})
Again, prefix is a Stream, but not infinite.
scala> prefix.toList
res: List[Int] = List(1, 2, 3)
N.b.: This does not handle the cases when sequence is empty, or when the prefix is also infinite.
If by elegant you mean concise and self-explanatory, it's probably something like the following:
sequence.inits.dropWhile(xs => xs != xs.sorted).next
inits gives us an iterator that returns the prefixes longest-first. We drop all the ones that aren't sorted and take the next one.
If you don't want to do all that sorting, you can write something like this:
sequence.scanLeft(Some(Int.MinValue): Option[Int]) {
case (Some(last), i) if i > last => Some(i)
case _ => None
}.tail.flatten
If the performance of this operation is really important, though (it probably isn't), you'll want to use something more imperative, since this solution still traverses the entire collection (twice).
And, another way to skin the cat:
val sequence = Seq(1,2,3,1,2,3,4,5,6)
sequence.head :: sequence
.sliding(2)
.takeWhile{case List(a,b) => a <= b}
.map(_(1)).toList
// List[Int] = List(1, 2, 3)
I will interpret elegance as the solution that most closely resembles the way we humans think about the problem although an extremely efficient algorithm could also be a form of elegance.
val sequence = List(1,2,3,2,3,45,5)
val increasingPrefix = takeWhile(sequence, _ < _)
I believe this code snippet captures the way most of us probably think about the solution to this problem.
This of course requires defining takeWhile:
/**
* Takes elements from a sequence by applying a predicate over two elements at a time.
* #param xs The list to take elements from
* #param f The predicate that operates over two elements at a time
* #return This function is guaranteed to return a sequence with at least one element as
* the first element is assumed to satisfy the predicate as there is no previous
* element to provide the predicate with.
*/
def takeWhile[A](xs: Traversable[A], f: (Int, Int) => Boolean): Traversable[A] = {
// function that operates over tuples and returns true when the predicate does not hold
val not = f.tupled.andThen(!_)
// Maybe one day our languages will be better than this... (dependant types anyone?)
val twos = sequence.sliding(2).map{case List(one, two) => (one, two)}
val indexOfBreak = twos.indexWhere(not)
// Twos has one less element than xs, we need to compensate for that
// An intuition is the fact that this function should always return the first element of
// a non-empty list
xs.take(i + 1)
}
Simply, I have two lists and I need to extract the new elements added to one of them.
I have the following
val x = List(1,2,3)
val y = List(1,2,4)
val existing :List[Int]= x.map(xInstance => {
if (!y.exists(yInstance =>
yInstance == xInstance))
xInstance
})
Result :existing: List[AnyVal] = List((), (), 3)
I need to remove all other elements except the numbers with the minimum cost.
Pick a suitable data structure, and life becomes a lot easier.
scala> x.toSet -- y
res1: scala.collection.immutable.Set[Int] = Set(3)
Also beware that:
if (condition) expr1
Is shorthand for:
if (condition) expr1 else ()
Using the result of this, which will usually have the static type Any or AnyVal is almost always an error. It's only appropriate for side-effects:
if (condition) buffer += 1
if (condition) sys.error("boom!")
retronym's solution is okay IF you don't have repeated elements that and you don't care about the order. However you don't indicate that this is so.
Hence it's probably going to be most efficient to convert y to a set (not x). We'll only need to traverse the list once and will have fast O(log(n)) access to the set.
All you need is
x filterNot y.toSet
// res1: List[Int] = List(3)
edit:
also, there's a built-in method that is even easier:
x diff y
(I had a look at the implementation; it looks pretty efficient, using a HashMap to count ocurrences.)
The easy way is to use filter instead so there's nothing to remove;
val existing :List[Int] =
x.filter(xInstance => !y.exists(yInstance => yInstance == xInstance))
val existing = x.filter(d => !y.exists(_ == d))
Returns
existing: List[Int] = List(3)