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
Apply function (scanLeft) to partition to create new column in dataframe
I want to do a scanLeft type of operation on one column of a dataframe. Scanleft is not pararellizable, but in my case I only want to apply this function to the elements that are already in the same partition. Therefore operation can be exectued in…
Aneta
- 31
- 3
1
vote
1 answer
foldLeft on List of Tuples: strange unexpected results
I am having a list of tuples which elements I want to be summed:
val t = Seq(1,2,3,4,5).map(el => (el,1))
t: Seq[(Int, Int)] = List((1,1), (2,1), (3,1), (4,1), (5,1))
t.foldLeft(0,0){ case ((a,b),(c,d)) => (a+b,c+d)}
res3: (Int, Int) = (14,6)
The…
Sergey Bushmanov
- 23,310
- 7
- 53
- 72
1
vote
2 answers
Haskell foldl and foldl1 produce "No instance for (Num [Char]) arising from the literal"
I think these two fold functions are the same, but only the second one works. The first one produce No instance for (Num [Char]) arising from the literal ‘12’ error. Why does the first one produce this error?
foldl1 (\x y -> (show x) ++ (show y))…
MaxSha
- 129
- 1
- 7
1
vote
1 answer
'diverging implicit expansion'-error on foldleft HList
I am trying to do a type-calculation to build some nested structure of collections but I get a 'diverging implicit expansion for type shapeless.ops.hlist.LeftFolder'-error when I try to use a Map-like collection with a List[T] as key or value type.…
user3508638
- 195
- 1
- 7
1
vote
1 answer
Haskell: foldl Function applied as infix Function?
Is the function I give to foldl applied in an infix way?
Example
foldl (-) 0 [1,2,3]
= 0-1-2-3
= -6
so more generally:
foldl f x [a,b,c]
is applied as:
(((x `f` a) `f` b) `f` c)
I know it's recursive, but can I think about it that way?
simplesystems
- 839
- 2
- 14
- 28
1
vote
3 answers
group by with foldleft scala
I have the following list in input:
val listInput1 =
List(
"itemA,CATs,2,4",
"itemA,CATS,3,1",
"itemB,CATQ,4,5",
"itemB,CATQ,4,6",
"itemC,CARC,5,10")
and I want to write a function in scala using groupBy and foldleft ( just…
scalacode
- 1,096
- 1
- 16
- 38
1
vote
3 answers
Scala - Remove all elements in a list/map of strings from a single String
Working on an internal website where the URL contains the source reference from other systems. This is a business requirement and cannot be changed.
i.e.…
crowgers
- 232
- 3
- 19
1
vote
3 answers
How to implement this foldl0 function without helper method?
I have following code:
function foldr0(list, func) {
if (list.length == 0) {
return 0;
} else {
return func(list[0], foldr0(list.slice(1), func));
}
}
function foldl0(list, func) {
if (list.length == 0) {
return 0;
} else {
…
Cowsay
- 71
- 4
1
vote
1 answer
Understanding FoldLeft in ScalaZ
I'm reading some articles about ScalaZ and have a question about understanding it. In this article, we generalize the sum function, to abstract away the type to be summed.
def sum[T](xs: List[T])(implicit m: Monoid[T]) = //...
Where
trait…
St.Antario
- 26,175
- 41
- 130
- 318
1
vote
2 answers
Haskell-- Trying to make my foldl function work
I have a function that converts [a, b, c, d, . . . ] to [(a,b), (c, d), . . .]. Now this works and here is the code for that:
makeTuple :: [a] -> [(a,a)]
makeTuple [] = []
makeTuple [a] = []
makeTuple (x:y:xs) = (x,y): (makeTuple xs)
Now the next…
lou tej
- 51
- 2
- 4
1
vote
2 answers
Elm: How to log inside a foldl
I have the following code:
findPerson name peeps = List.foldl
(\a b -> case b of
Just _ -> b
Nothing -> if a.name == name then
Just a
else Nothing
) Nothing peeps
I would like to log the values of a…
Rich
- 5,603
- 9
- 39
- 61
1
vote
2 answers
Scala map with dependent variables
In scala I have a list of functions that return a value. The order in which the functions are executed are important since the argument of function n is the output of function n-1.
This hints to use foldLeft, something like:
val base: A
val funcs:…
Tim
- 2,000
- 4
- 27
- 45
1
vote
2 answers
Better way for aggregation on list of case classes
I have list of case classes. Output requires aggregation on different parameters of case class. Looking for more optimized way to do it.
Example:
case class Students(city: String, college: String, group: String,
name: String,…
jony
- 119
- 3
- 13
1
vote
3 answers
Scala: How do I use foldLeft with a generic array?
I've this method:
def isSorted[A](as: Array[A], ordered: (A, A) => Boolean): Boolean = {
val sorted = for (it <- as.sliding(2))
yield {
if (it.length == 2) ordered.apply(it(0), it(1))
else true
}
…
Abhijit Sarkar
- 21,927
- 20
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- 219
1
vote
2 answers
folding a list of classes to a single, comma separated string (in Scala)
I have a need to take a list of case classes and convert them to a to a single, comma separated string (with no heading or tailing comma).
case class State(name: String)
def toLine(states: State*): String = {
}
so, toLine(State("one"),…
Ehud Kaldor
- 753
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- 20