Understanding takeWhile termination condition during Iterator.iterate in Scala
I am having some trouble understanding why this code seemingly terminates early:
object intersectionDemo {
val arr = Array(2, 4, 7, 8, 12, 20)
val arr2 = Array(3, 4, 8, 9, 11, 20)
Iterator.iterate((arr, arr2, List[Int]())) {
case (one, two, res) =>
if (one.head == two.head)
(one.tail, two.tail, one.head :: res)
else if (one.head > two.head)
(one, two.tail, res)
else
(one.tail, two, res)
}.takeWhile { case (a, b, _) => a.nonEmpty && b.nonEmpty }
.toList.last._3.reverse
}
It returns List(4, 8) though it should return List(4, 8, 20) and if I inspect the first or second arrays here: _._1 or _._2 I get Array(20). I do know there are other ways to solve the intersection of two arrays problem, I am just a little confused as to why the last element doesn't get inserted.
My guess is that it sees that one.tail and two.tail are empty and terminates the iteration, but shouldn't one.head get inserted still?
scala
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
add a comment |
I am having some trouble understanding why this code seemingly terminates early:
object intersectionDemo {
val arr = Array(2, 4, 7, 8, 12, 20)
val arr2 = Array(3, 4, 8, 9, 11, 20)
Iterator.iterate((arr, arr2, List[Int]())) {
case (one, two, res) =>
if (one.head == two.head)
(one.tail, two.tail, one.head :: res)
else if (one.head > two.head)
(one, two.tail, res)
else
(one.tail, two, res)
}.takeWhile { case (a, b, _) => a.nonEmpty && b.nonEmpty }
.toList.last._3.reverse
}
It returns List(4, 8) though it should return List(4, 8, 20) and if I inspect the first or second arrays here: _._1 or _._2 I get Array(20). I do know there are other ways to solve the intersection of two arrays problem, I am just a little confused as to why the last element doesn't get inserted.
My guess is that it sees that one.tail and two.tail are empty and terminates the iteration, but shouldn't one.head get inserted still?
scala
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
add a comment |
I am having some trouble understanding why this code seemingly terminates early:
object intersectionDemo {
val arr = Array(2, 4, 7, 8, 12, 20)
val arr2 = Array(3, 4, 8, 9, 11, 20)
Iterator.iterate((arr, arr2, List[Int]())) {
case (one, two, res) =>
if (one.head == two.head)
(one.tail, two.tail, one.head :: res)
else if (one.head > two.head)
(one, two.tail, res)
else
(one.tail, two, res)
}.takeWhile { case (a, b, _) => a.nonEmpty && b.nonEmpty }
.toList.last._3.reverse
}
It returns List(4, 8) though it should return List(4, 8, 20) and if I inspect the first or second arrays here: _._1 or _._2 I get Array(20). I do know there are other ways to solve the intersection of two arrays problem, I am just a little confused as to why the last element doesn't get inserted.
My guess is that it sees that one.tail and two.tail are empty and terminates the iteration, but shouldn't one.head get inserted still?
scala
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
I am having some trouble understanding why this code seemingly terminates early:
object intersectionDemo {
val arr = Array(2, 4, 7, 8, 12, 20)
val arr2 = Array(3, 4, 8, 9, 11, 20)
Iterator.iterate((arr, arr2, List[Int]())) {
case (one, two, res) =>
if (one.head == two.head)
(one.tail, two.tail, one.head :: res)
else if (one.head > two.head)
(one, two.tail, res)
else
(one.tail, two, res)
}.takeWhile { case (a, b, _) => a.nonEmpty && b.nonEmpty }
.toList.last._3.reverse
}
It returns List(4, 8) though it should return List(4, 8, 20) and if I inspect the first or second arrays here: _._1 or _._2 I get Array(20). I do know there are other ways to solve the intersection of two arrays problem, I am just a little confused as to why the last element doesn't get inserted.
My guess is that it sees that one.tail and two.tail are empty and terminates the iteration, but shouldn't one.head get inserted still?
scala
scala
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
asked Dec 28 '18 at 5:35
Ahmad RagabAhmad Ragab
4731722
4731722
add a comment |
add a comment |
1 Answer
1
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oldest
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Think about it. (I'm using List syntax to demonstrate the Array status, but you get the idea.)
// 20::Nil 20::Nil (Nil, Nil, 20 :: 8 :: 4 :: Nil)
if (one.head == two.head) (one.tail, two.tail, one.head :: res)
The 20 gets inserted when one and two are emptied so if you takeWhile neither is empty then it hasn't been inserted yet.
If you change it to .dropWhile{ ... }.next._3.reverse then you get the results you're looking for.
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
add a comment |
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1 Answer
1
active
oldest
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1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Think about it. (I'm using List syntax to demonstrate the Array status, but you get the idea.)
// 20::Nil 20::Nil (Nil, Nil, 20 :: 8 :: 4 :: Nil)
if (one.head == two.head) (one.tail, two.tail, one.head :: res)
The 20 gets inserted when one and two are emptied so if you takeWhile neither is empty then it hasn't been inserted yet.
If you change it to .dropWhile{ ... }.next._3.reverse then you get the results you're looking for.
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
add a comment |
Think about it. (I'm using List syntax to demonstrate the Array status, but you get the idea.)
// 20::Nil 20::Nil (Nil, Nil, 20 :: 8 :: 4 :: Nil)
if (one.head == two.head) (one.tail, two.tail, one.head :: res)
The 20 gets inserted when one and two are emptied so if you takeWhile neither is empty then it hasn't been inserted yet.
If you change it to .dropWhile{ ... }.next._3.reverse then you get the results you're looking for.
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
add a comment |
Think about it. (I'm using List syntax to demonstrate the Array status, but you get the idea.)
// 20::Nil 20::Nil (Nil, Nil, 20 :: 8 :: 4 :: Nil)
if (one.head == two.head) (one.tail, two.tail, one.head :: res)
The 20 gets inserted when one and two are emptied so if you takeWhile neither is empty then it hasn't been inserted yet.
If you change it to .dropWhile{ ... }.next._3.reverse then you get the results you're looking for.
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
Think about it. (I'm using List syntax to demonstrate the Array status, but you get the idea.)
// 20::Nil 20::Nil (Nil, Nil, 20 :: 8 :: 4 :: Nil)
if (one.head == two.head) (one.tail, two.tail, one.head :: res)
The 20 gets inserted when one and two are emptied so if you takeWhile neither is empty then it hasn't been inserted yet.
If you change it to .dropWhile{ ... }.next._3.reverse then you get the results you're looking for.
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
answered Dec 28 '18 at 6:01
jwvhjwvh
25.8k52038
25.8k52038
add a comment |
add a comment |
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