A common algorithm in functional languages that applies an operation to each member of a sequence, from left to right.
Questions tagged [foldleft]
103 questions
1
vote
1 answer
SortedMap manipulation using foldleft
I have a code that converts the following datatypes:
from: SortedMap[Long, SortedMap[String, Double]]
to: SortedMap[String, Array[Double]]
Can anyone please explain how this code does the above manipulation?
val convertedDataWindow =…
user3370773
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1
vote
1 answer
write reverse in scala, using foldleft
here's an implementation:
def reverse[A](l: List[A]): List[A] = foldLeft(l, List[A]())((acc,h) => Cons(h,acc))
I don't understand what is inderstood by the compiler with (acc,h); initially, the f function meets (ListA,l), which are 2 lists, so is…
lolveley
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1
vote
2 answers
Scala - Execute arbitrary number of Futures sequentially but dependently
I'm trying to figure out the neatest way to execute a series of Futures in sequence, where one Future's execution depends on the previous. I'm trying to do this for an arbitrary number of futures.
User case:
I have retrieved a number of Ids from my…
healsjnr
- 405
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1
vote
2 answers
Type mismatch error with foldLeft method in Scala
I have a method that uses foldLeft in Scala.
def bitSetToByte(b:collection.BitSet, sh:Int=0) =
((0 /: b) {(acc, input) => acc + (1 << (input - sh))}).toByte
The method has two parameters for the anonymous function, so I replaced it with _ by…
prosseek
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ocaml fold_left all elements with even index on left and elements with uneven index on right
I have a problem with the following assignment:
The function
fold_left can be used to implement many operations that scan through a list from left to right. It takes three arguments: a function
f, an initial accumulator, and a list. For each…
MAMR
- 1
0
votes
2 answers
reduceLeft (or, foldLeft) vs reduceRight (or, foldRight) in scala
I am learning scala from coursera. In the reduceLeft and reduceRight description this comes along:
and then on the very next slide the teacher says this code pattern is abstracted out as reduceLeft
My questions:
I think the pattern in the first…
figs_and_nuts
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2 answers
What does e:B, f:(B,A)=>B) : B
I am confused about what this means.
I understand currying but I can't seem to read the code altogether.
def foldLeft [A,B](xs:List[A], e:B, f:(B,A)=>B): B
Mr.mr
- 11
0
votes
0 answers
scala spark foldLeft on map with array
I have a configuration in a form of a map:
val config = Map[String, Array[String]] = Map("column1" -> Array("field1"), "column2" -> Array("field1","field2"))
I want to use foldLeft to apply this to a dataframe and dynamically filter nested fields…
Kylo
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0
votes
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The problem of foldl (with multi level foldl) in haskell
The foldr version was fast than the foldl version:
the foldr version:
cartProdN9 :: [[a]] -> [[a]]
cartProdN9 xss =
foldr h1 [[]] xss where
h1 xs yss = foldr g [] xs where
g x zss = foldr f zss yss where
f xs yss = (x:xs):yss…
jiamo
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0
votes
2 answers
Haskell Functions (map,foldr, foldl)
I am struggling to think of a way to utilize these functions for this beginner level coding class that I am taking to learn functional programming in Haskell. The functions I have to write are shown below, asum is supposed to turn a list of integers…
TrynaLearnCode
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0
votes
2 answers
How to use a foldLeft using a regex
I am trying to use a foldLeft to replace user names with a hidden value and print them out
private def removeNamesFromErrorMessage(errorMessage: String): Unit = {
val userNames = List("mary", "john")
val errorMessage = "users mary and john…
SimonL
- 3
- 1
0
votes
1 answer
Scala Argonaut folding across a list?
I'm very new to Argonaut (which I'm forced to use since the codebase is older, using an old version of scalaz, and we have no intentions to update it since we're rewriting this outdated code from scratch), and I'm trying to find a way to dump some…
Sebastian
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0
votes
1 answer
FoldR using FoldL on finite lists
I have read here that on finite lists is possible to write foldR in term of foldL in the following way (using coq):
Fixpoint fold {A : Type} {B : Type} (f : A -> B -> B) (v : B) (l : list A) :=
match l with
| nil => v
| x :: xs => f x (fold f…
Marco Mantovani
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0
votes
0 answers
Prettier way in scala to foldLeft a column?
I have an SQL which I am trying to code in spark scala using dataframes
SELECT country,
Substr(substring_col, 1, 3) AS Code,
CASE
WHEN Substr(substring_col, 1, 9) = '238208700' THEN
'columnName1'
WHEN…
Gladiator
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0
votes
1 answer
Foldleft in Racket
I have a question regarding trying to define a recursively defined foldl function in Racket.
Here is my approach:
(define foldl
(lambda (z c xs)
(match xs
(empty z)
((make-pair x xs) (foldl c (c z x) xs)))))
Unfortunately, when I…
TI-30X Plus
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