Anyone can tell how to print a Stack data structure in OCaml?
The build-in Stack type definition is like :
type 'a t = { mutable c : 'a list }
exception Empty
let create () = { c = [] }
let clear s = s.c <- []
let push x s = s.c <- x :: s.c
let pop s = match s.c with hd::tl -> s.c <- tl; hd | [] -> raise Empty
let length s = List.length s.c
let iter f s = List.iter f s.c
Want to print and keep its elements in place, which means do not use pop and push.
Better to use the pattern matching to complete the problem.
Code should be like:
let print_stack stack =???
This looks like it could be homework. You should show something you've tried that didn't work, and explain why you think it didn't work. This is a lot more valuable that just having somebody give you the answer.
If it's not homework: if you think about it, you can find a good implementation at another place in the standard library. The implementation of Stack.iter tells you where to look.
It seems like the function Stack.iter does exactly what you want:
let print_stack print_elem stack = Stack.iter print_elem
Where. print_elem prints one element of the stack.
e.g. let print_elem_int n = (print_int n; print_newline ())
Finally get the answer:
let rec print_s {c=l}=
match l with
| [] -> raise Empty
| [x] -> print_int x; print_string " "
| h :: ts -> print_int h; print_string " "; print_s {c=ts}
;;
Improved version:
let print_s2 {c=l}=
let rec print_l list =
match list with
| [] -> raise Empty
| [x] -> print_int x; print_string " "
| h :: ts -> print_int h; print_string " "; print_l ts
in
print_l l
;;
Related
I'm trying to implement a queue in F#. One way I thought about going about it is using a mutable list. I believe the logic is all right, however when I run the code I get the error
/home/runner/main.fs(12,5): error FS0027: This value is not mutable. Consider using the mutable keyword, e.g. 'let mutable queue = expression'.
compiler exit status 1
^[[3;1R
This is my implementation code
let create_job_list jobCount resultLis =
let resultLis =
[for i in 1..jobCount do yield System.Random().Next(10)]
resultLis
let job_list = create_job_list 10 []
let mutable queue = []
let push x queue =
printfn "%A" x
queue <- (x::queue)
let pop resQueue =
let resQ = resQueue |> List.rev
match resQ with
| head::tail -> printfn "%A" head; queue <- (tail |> List.rev)
| [] -> failwith "No more jobs to do right now."
let rec jobQueue job_list =
match job_list with
| head::tail when head%2=1 -> (push head queue) |> ignore; jobQueue tail
| head::tail when head%2=0 -> (pop queue) |> ignore; jobQueue tail
| _ -> failwith "Empty list"
The error tells me to use let mutable queue = expression but I'm using let mutable queue = []
Where am I going wrong?
Thanks
You have declared queue as mutable, but you are passing it as an argument that isn't mutable anymore. Since your queue variable is visible inside the push and pop you don't need to have it as parameters.
This should compile:
let create_job_list jobCount resultLis =
let resultLis =
[for i in 1..jobCount do yield System.Random().Next(10)]
resultLis
let job_list = create_job_list 10 []
let mutable queue = []
let push x=
printfn "%A" x
queue <- (x::queue)
let pop resQueue =
let resQ = resQueue |> List.rev
match resQ with
| head::tail -> printfn "%A" head; queue <- (tail |> List.rev)
| [] -> failwith "No more jobs to do right now."
let rec jobQueue job_list =
match job_list with
| head::tail when head%2=1 -> (push head) |> ignore; jobQueue tail
| head::tail when head%2=0 -> (pop) |> ignore; jobQueue tail
| _ -> failwith "Empty list"
What you need is to pass queue by reference. Your push function should then look something like this:
let push x (queue: byref<_>) =
printfn "%A" x
queue <- (x::queue)
and the jobQueue like this
let rec jobQueue job_list =
match job_list with
| head::tail when head%2=1 -> (push head &queue) |> ignore; jobQueue tail
| head::tail when head%2=0 -> (pop queue) |> ignore; jobQueue tail
| _ -> failwith "Empty list"
I haven't tried this code, but it should at least get you started.
By the way, I guess you are aware that .Net already has a Queue implementation?
Let's says I have a string of a length N that contains only 0 or 1. I want to split that string in multiples strings and each string should contains only one digit.
Example:
00011010111
Should be split into:
000
11
0
1
0
111
The only solution I can think of if using a for loop with a string builder (Written in pseudo code below, more c# like sorry):
result = new list<string>
tmpChar = ""
tmpString = ""
for each character c in MyString
if tmpchar != c
if tmpsString != ""
result.add tmpString
tmpString = ""
endIf
tmpchar = c
endIf
tmpString += tmpChar
endFor
Do you have any other solution and maybe a clever solution that use a more functional approach?
I think Seq.scan would be a good fit for this, this is a very procedural problem in nature, preserving the order like that. But here is code that I believe does what you are asking.
"00011010111"
|> Seq.scan (fun (s, i) x ->
match s with
| Some p when p = x -> Some x, i
| _ -> Some x, i + 1 ) (None, 0)
|> Seq.countBy id
|> Seq.choose (function
| (Some t, _), n -> Some(t, n)
| _ -> None )
|> Seq.toList
Perhaps something along the lines of:
let result =
let rec groupWhileSame xs result =
match xs with
| a when a |> Seq.isEmpty -> result
| _ ->
let head = xs |> Seq.head
let str = xs |> Seq.takeWhile ((=) head)
let rest = xs |> Seq.skipWhile ((=) head)
groupWhileSame rest (Seq.append result [str])
groupWhileSame (myStr) []
Seq.fold (fun (acc:(string list)) x ->
match acc with
| y::rst when y.StartsWith(string x) -> (string x) + y::rst
| _ -> (string x)::acc)
[]
"00011010111"
Consider this function (which is generic):
let chunk s =
if Seq.isEmpty s then []
else
let rec chunk items chunks =
if Seq.isEmpty items then chunks
else
let chunks' =
match chunks with
| [] -> [(Seq.head items, 1)]
| x::xs ->
let c,n = x in let c' = Seq.head items in
if c = c' then (c, n + 1) :: xs else (c', 1) :: x :: xs
chunk (Seq.tail items) chunks'
chunk s [] |> List.rev
It returns a list of tuples, where each tuple represents an item and its repetitions.
So
"00011010111" |> Seq.toList |> chunk
actually returns
[('0', 3); ('1', 2); ('0', 1); ('1', 1); ('0', 1); ('1', 3)]
Basically, we're doing run length encoding (which is admittedly a bit wasteful in the case of your example string).
To get the list of strings that you want, we use code like following:
"00011010111"
|> Seq.toList
|> chunk
|> List.map (fun x -> let c,n = x in new string(c, n))
Here's a working version of OP's proposal with light syntax:
let chunk (s: string) =
let result = System.Collections.Generic.List<string>()
let mutable tmpChar = ""
let mutable tmpString = ""
for c in s do
if tmpChar <> string c then
if tmpString <> "" then
result.Add tmpString
tmpString <- ""
tmpChar <- string c
tmpString <- tmpString + tmpChar
result.Add tmpString
result
No attempt was made to follow a functional style.
On the odd chance, that someone has a brilliant idea...
I am not sure if there is a good way to generalize that.
EDIT: I think it might be nice to explain exactly what the inputs and outputs are. The code below is only how I approached the solution.
Inputs: data, recipe
data: set of string, string list here also called "set of named lists"
recipe: list of commands
Command Print (literal|list reference)
Adds the literal to the output or if it is a list reference, it adds the head of the referenced list to the output.
Command While (list reference)
when referenced list not empty --> next command
when referenced list empty --> skip entries in recipe list past the matching Wend.
Command Wend (list reference)
replace referenced list with tail (reference list)
when referenced list is empty, next command
when referenced list is not empty, next command is the matching while above.
Outputs: string list
The best answer is the implementation of that which is shortest and which allows to re-use that algorithm in new contexts.
This is not just a programming problem for the fun of it, btw. It is basically what happens if you try to implement data driven text templating.
The code below is my attempt to solve this problem.
The first code snippet is a non-generalized solution.
The second code snippet is an attempt to isolate the algorithm.
If you play with the code, simply paste the second snippet below the first snippet and both versions are working.
The whole topic is about understanding better how to separate the iteration algorithm from the rest of the code and then to simply apply it, in contrast of having all the other code within.
Would it not be great, if there was a way to abstract the way the statements are being processed and the looping of the while/wend, such,
that it can be reused in my main code, just as I keep re-using other "iteration schemes", such as List.map?
The commonalities between my main code and this study are:
An evolving "environment" which is threaded through all steps of the computation.
Collections, which need to be iterated in a well-formed nested manner. (Malformed would be: while x while y wend x wend y)
A series of "execution steps" form the body of each of those "while wend" loops.
Done in a "pure" manner. As you will note, nothing is mutable in the study. Want to keep it like that.
Each "While" introduces a new scope (as for binding values), which is discarded again, once the while loop is done.
So, I am looking for something like:
run: CommandClassifier -> commandExecutor -> Command list -> EnvType -> EnvType
where
CommandClassifier could be a function of the form Command -> NORMAL|LOOP_START|LOOP_END
and commandexecutor: Command -> EnvType -> EnvType
Of course, nesting of those while-blocks would not be limited to 2 (just tried to keep the testProgram() small).
SideNote: the "commands list" is an AST from a preceding parser run, but that should not really matter.
type MiniLanguage =
| Print of string
| While of string
| Wend of string
let testProgram =
[ Print("Hello, I am your Mini Language program")
While("names")
Print("<names>")
While("pets")
Print("<pets>")
Wend("pets")
Print("Done with pets.")
Wend("names")
Print("Done with names.")
]
type MiniEnvironment = { Bindings : Map<string,string>; Collections : Map<string, string list> }
let init collections =
{ Bindings = Map.empty; Collections = Map.ofList collections}
let bind n v env =
let newBindings =
env.Bindings
|> Map.remove n
|> Map.add n v
{ env with Bindings = newBindings; }
let unbind n env =
{ env with Bindings = Map.remove n env.Bindings; }
let bindingValue n env =
if env.Bindings.ContainsKey n then
Some(env.Bindings.Item n)
else
None
let consumeFirstFromCollection n env =
if env.Collections.ContainsKey n then
let coll = env.Collections.Item n
match coll with
| [] -> env |> unbind n
| _ ->
let first = coll.Head
let newCollections =
env.Collections
|> Map.remove n
|> Map.add n coll.Tail
{ env with Collections = newCollections }
|> bind n first
else failwith ("Unknown collection: " + n)
// All do functions take env - the execution environment - as last argument.
// All do functions return (a new) env as single return parameter.
let rec doPrint (s : string) env =
if s.StartsWith("<") && s.EndsWith(">") then
match bindingValue (s.Substring (1, s.Length - 2 )) env with
| Some(v) -> v
| _ -> s
else s
|> printfn "%s"
env
let rec skipPastWend name code =
match code with
| (Wend(cl) :: rest) when cl = name -> rest
| [] -> failwith "No Wend found!"
| (_ :: rest) -> skipPastWend name rest
let rec doWhileX name code env =
match code with
| (Print(s) :: rest) -> env |> (doPrint s) |> doWhileX name rest
| (While(cn) :: rest) -> env |> doWhile cn rest |> ignore; env |> doWhileX name (skipPastWend cn rest)
| (Wend(cn) :: rest) when cn = name -> env
| [] -> failwith ("While without Wend for: " + name)
| _ -> failwith ("nested while refering to same collection!")
and doWhile name code env =
let e0 = env |> consumeFirstFromCollection name
match bindingValue name e0 with
| Some(s) ->
e0 |> doWhileX name code |> doWhile name code
| None -> env
let rec run (program : MiniLanguage list) env =
match program with
| (Print(s) :: rest) -> env |> (doPrint s) |> run rest
| (While(cn) :: rest) ->
env
|> doWhile cn rest |> ignore
env |> run (skipPastWend cn program)
| (Wend(cn) :: rest) -> failwith "wend found in run()"
| [] -> env
let test() =
init [ "names", ["name1"; "name2"; "name3"; ]; "pets", ["pet1"; "pet2"] ]
|> run testProgram
|> printfn "%A"
(*
Running test() yields:
Hello, I am your Mini Language program
name1
pet1
pet2
Done with pets.
name2
pet1
pet2
Done with pets.
name3
pet1
pet2
Done with pets.
Done with names.
{Bindings = map [];
Collections =
map [("names", ["name1"; "name2"; "name3"]); ("pets", ["pet1"; "pet2"])];}
*)
Here my first version of isolating the algorithm. The number of callbacks is not entirely pretty. Can anyone come up with something simpler?
// The only function I had to "modify" to work with new "generalized" algorithm.
let consumeFirstFromCollection1 n env =
if env.Collections.ContainsKey n then
let coll = env.Collections.Item n
match coll with
| [] -> (env |> unbind n , false)
| _ ->
let first = coll.Head
let newCollections =
env.Collections
|> Map.remove n
|> Map.add n coll.Tail
({ env with Collections = newCollections }
|> bind n first , true)
else failwith ("Unknown collection: " + n)
type NamedList<'n,'t when 'n : comparison> = 'n * List<'t>
type Action<'a,'c> = 'c -> 'a -> 'a
type LoopPreparer<'a,'c> = 'c -> 'a -> 'a * bool
type CommandType = | RUN | BEGIN | END
type CommandClassifier<'c> = 'c -> CommandType
type Skipper<'c> = 'c -> List<'c> -> List<'c>
type InterpreterContext<'a,'c> =
{ classifier : CommandClassifier<'c>
executor : Action<'a,'c>
skipper : Skipper<'c>
prepareLoop : LoopPreparer<'a,'c>
isMatchingEnd : 'c -> 'c -> bool
}
let interpret (context : InterpreterContext<'a,'c>) (program : 'c list) (env : 'a) : 'a =
let rec loop front (code : 'c list) e =
let e0,hasData = e |> context.prepareLoop front
if hasData
then
e0
|> loop1 front (code)
|> loop front (code)
else e
and loop1 front code e =
match code with
| x :: more when (context.classifier x) = RUN ->
//printfn "RUN %A" x
e |> context.executor x |> loop1 front more
| x :: more when (context.classifier x) = BEGIN ->
//printfn "BEGIN %A" x
e |> loop x more |> ignore
e |> loop1 front (context.skipper x more)
| x :: more when (((context.classifier x) = END) && (context.isMatchingEnd front x)) -> /// && (context.isMatchingEnd front x)
//printfn "END %A" x
e
| [] -> failwith "No END."
| _ -> failwith "TODO: Not sure which case this is. But it is not a legal one!"
let rec interpr code e =
match code with
| [] -> e
| (first :: rest) ->
match context.classifier first with
| RUN -> env |> context.executor first |> interpr rest
| BEGIN ->
e |> loop first rest |> ignore
e |> interpr (context.skipper first rest)
| END -> failwith "END without BEGIN."
interpr program env
let test1() =
let context : InterpreterContext<MiniEnvironment,MiniLanguage> =
{ classifier = fun c-> match c with | MiniLanguage.Print(_) -> RUN | MiniLanguage.While(_) -> BEGIN | MiniLanguage.Wend(_) -> END;
executor = fun c env -> match c with | Print(s) -> doPrint s env | _ -> failwith "Not a known command.";
skipper = fun c cl -> match c with | While(n) -> skipPastWend n cl | _ -> failwith "first arg of skipper SHALL be While!"
prepareLoop = fun c env -> match c with | While(n) -> (consumeFirstFromCollection1 n env) | _ -> failwith "first arg of skipper SHALL be While!"
isMatchingEnd = fun cwhile cx -> match cwhile,cx with | (While(n),Wend(p)) when n = p -> true | _ -> false
}
init [ "names", ["name1"; "name2"; "name3"; ]; "pets", ["pet1"; "pet2"] ]
|> interpret context testProgram
|> printfn "%A"
How to modify below code to Return "string" so that returned output displayed on my MVC page and also would like to accept enteredChar from user.
Is there better way to do create this pyramid?
Current code:
let enteredChar = 'F' // As Interactive window doesn't support to Read Input
let mylist = ['A'..enteredChar]
let mylistlength = mylist |> List.length
let myfunc i x tlist1 =
(for j = 0 to mylistlength-i-2 do printf "%c" ' ')
let a1 = [for p in tlist1 do if p < x then yield p]
for p in a1 do printf "%c" p
printf "%c" x
let a2 = List.rev a1
for p in a2 do printf "%c" p
printfn "%s" " "
mylist |> List.iteri(fun i x -> myfunc i x mylist)
Output:
A
ABA
ABCBA
ABCDCBA
ABCDEDCBA
ABCDEFEDCBA
A few small optimizations could be:
Use StringBuilder instead of printf which is quite slow with long strings.
Use Array instead of List since Array works better with String.
Here is a version producing a pyramid string, which is kept closely to your code:
open System
open System.Text
let generateString c =
let sb = StringBuilder()
let generate i x arr =
String.replicate (Array.length arr-i-1) " " |> sb.Append |> ignore
let a1 = Array.filter (fun p -> p < x) arr
String(a1) |> sb.Append |> ignore
sb.Append x |> ignore
String(Array.rev a1) |> sb.Append |> ignore
sb.AppendLine " " |> ignore
let arr = [|'A'..c|]
arr |> Array.iteri(fun i x -> generate i x arr)
sb.ToString()
generateString 'F' |> printfn "%s"
As an alternative to Daniel's solution, you can achieve what you want with minimal changes to the code logic. Instead of using printf that writes the output to the console, you can use Printf.bprintf which writes the output to a specified StringBuilder. Then you can simply get the resulting string from the StringBuilder.
The modified function will look like this. I added parameter str and replaced printf with Printf.bprintf str (and printfn with bprintf together with additional \n char):
let myfunc i x tlist1 str =
(for j = 0 to mylistlength-i-2 do Printf.bprintf str "%c" ' ')
let a1 = [for p in tlist1 do if p < x then yield p]
for p in a1 do Printf.bprintf str "%c" p
Printf.bprintf str "%c" x
let a2 = List.rev a1
for p in a2 do Printf.bprintf str "%c" p
Printf.bprintf str "%s\n" " "
To call the function, you first create StringBuilder and then pass it to myfunc in every call. At the end, you can get the result using ToString method:
let str = StringBuilder()
mylist |> List.iteri(fun i x -> myfunc i x mylist str)
str.ToString()
I think Daniel's solution looks nicer, but this is the most direct way to tunr your printing code into a string-building code (and it can be done, pretty much, using Search & Replace).
If I understand your question (this likely belongs on Code Review) here's one way to rewrite your function:
let showPyramid (output: TextWriter) lastChar =
let chars = [|'A' .. lastChar|]
let getRowChars n =
let rec loop acc i =
[|
if i < n then let c = chars.[i] in yield c; yield! loop (c::acc) (i+1)
else yield! List.tail acc
|]
loop [] 0
let n = chars.Length
for r = 1 to n do
output.WriteLine("{0}{1}{0}", String(' ', n - r), String(getRowChars r))
Example
showPyramid Console.Out 'F'
or, to output to a string
use output = new StringWriter()
showPyramid output 'F'
let pyramid = output.ToString()
EDIT
After seeing Tomas' answer I realized I skipped over "return a string" in your question. I updated the code and added examples to show how you could do that.
let pyramid (ch:char) =
let ar = [| 'A'..ch |]
let len = ar.Length
Array.mapi
(fun i ch ->
let ar = ar.[0..i]
String.replicate (len - i - 1) " " + new string(ar) + new string((Array.rev ar).[1..]))
ar
|> String.concat "\n"
pyramid 'F' |> printfn "%s"
Here's another approach that seems to be a good demonstration of functional composition. I bet it's the shortest solution among the answers here. :)
let charsToString = Seq.map string >> String.concat String.Empty
let pyramid lastChar =
let src = '-'::['A'..lastChar] |> List.toArray
let len = Array.length src - 1
fun row col -> row-abs(col-len+1)+1 |> max 0 |> Array.get src // (1)
>> Seq.init (len*2-1) >> charsToString // (2)
|> Seq.init len // (3)
pyramid 'X' |> Seq.iter (printfn "%s")
First, we generate an unusual array of initial data. Its element [0] contains a space or whatever separator you want to have; I preferred dash (-) for debugging purposes.
The (1) line makes a function that calculates what character to be placed. The result of row-abs(col-len+1)+1 can be either positive (and there is a char to be placed) or zeronegative, and there should be a space. Note that there is no if statement: it is hidden within the max function;
The (2) line composes a function int -> string for generating an individual row;
The (3) line passes the function above as argument for sequence initializer.
The three lines can be written in a more verbose way:
let genCell row col = row-abs(col-len+1)+1 |> max 0 |> Array.get src
let genRow = genCell >> Seq.init (len*2-1) >> charsToString
Seq.init len genRow
Note genRow needs no formal argument due to functional composition: the argument is being bound into genCell, returning a function of a single argument, exactly what Seq.init needs.
This is probably trivial, and I do have a solution but I'm not happy with it. Somehow, (much) simpler forms don't seem to work and it gets messy around the corner cases (either first, or last matching pairs in a row).
To keep it simple, let's define the matching rule as any two or more numbers that have a difference of two. Example:
> filterTwins [1; 2; 4; 6; 8; 10; 15; 17]
val it : int list = [2; 4; 6; 8; 10; 15; 17]
The code I currently use is this, which just feels sloppy and overweight:
let filterTwins list =
let func item acc =
let prevItem, resultList = acc
match prevItem, resultList with
| 0, []
-> item, []
| var, [] when var - 2 = item
-> item, item::var::resultList
| var, hd::tl when var - 2 = item && hd <> var
-> item, item::var::resultList
| var, _ when var - 2 = item
-> item, item::resultList
| _
-> item, resultList
List.foldBack func list (0, [])
|> snd
I intended my own original exercise to experiment with List.foldBack, large lists and parallel programming (which went well) but ended up messing with the "easy" part...
Guide through the answers
Daniel's last, 113 characters*, easy to follow, slow
Kvb's 2nd, 106 characters* (if I include the function), easy, but return value requires work
Stephen's 2nd, 397 characters*, long winded and comparably complex, but fastest
Abel's, 155 characters*, based on Daniel's, allows duplicates (this wasn't a necessity, btw) and is relatively fast.
There were more answers, but the above were the most distinct, I believe. Hope I didn't hurt anybody's feelings by accepting Daniel's answer as solution: each and every one solution deserves to be the selected answer(!).
* counting done with function names as one character
Would this do what you want?
let filterTwins l =
let rec filter l acc flag =
match l with
| [] -> List.rev acc
| a :: b :: rest when b - 2 = a ->
filter (b::rest) (if flag then b::acc else b::a::acc) true
| _ :: t -> filter t acc false
filter l [] false
This is terribly inefficient, but here's another approach using more built-in functions:
let filterTwinsSimple l =
l
|> Seq.pairwise
|> Seq.filter (fun (a, b) -> b - 2 = a)
|> Seq.collect (fun (a, b) -> [a; b])
|> Seq.distinct
|> Seq.toList
Maybe slightly better:
let filterTwinsSimple l =
seq {
for (a, b) in Seq.pairwise l do
if b - 2 = a then
yield a
yield b
}
|> Seq.distinct
|> Seq.toList
How about this?
let filterPairs f =
let rec filter keepHead = function
| x::(y::_ as xs) when f x y -> x::(filter true xs)
| x::xs ->
let rest = filter false xs
if keepHead then x::rest else rest
| _ -> []
filter false
let test = filterPairs (fun x y -> y - x = 2) [1; 2; 4; 6; 8; 10; 15; 17]
Or if all of your list's items are unique, you could do this:
let rec filterPairs f s =
s
|> Seq.windowed 2
|> Seq.filter (fun [|a;b|] -> f a b)
|> Seq.concat
|> Seq.distinct
let test = filterPairs (fun x y -> y - x = 2) [1; 2; 4; 6; 8; 10; 15; 17]
EDIT
Or here's another alternative which I find elegant. First define a function for breaking a list into a list of groups of consecutive items satisfying a predicate:
let rec groupConsec f = function
| [] -> []
| x::(y::_ as xs) when f x y ->
let (gp::gps) = groupConsec f xs
(x::gp)::gps
| x::xs -> [x]::(groupConsec f xs)
Then, build your function by collecting all results back together, discarding any singletons:
let filterPairs f =
groupConsec f
>> List.collect (function | [_] -> [] | l -> l)
let test = filterPairs (fun x y -> y - x = 2) [1; 2; 4; 6; 8; 10; 15; 17]
The following solution is in the spirit of your own, but I use a discriminate union to encapsulate aspects of the algorithm and reign in the madness a bit:
type status =
| Keep of int
| Skip of int
| Tail
let filterTwins xl =
(Tail, [])
|> List.foldBack
(fun cur (prev, acc) ->
match prev with
| Skip(prev) when prev - cur = 2 -> (Keep(cur), cur::prev::acc)
| Keep(prev) when prev - cur = 2 -> (Keep(cur), cur::acc)
| _ -> (Skip(cur), acc))
xl
|> snd
Here's another solution which uses a similar discriminate union strategy as my other answer but it works on sequences lazily so you can watch those twin (primes?) roll in as they come:
type status =
| KeepTwo of int * int
| KeepOne of int
| SkipOne of int
| Head
let filterTwins xl =
let xl' =
Seq.scan
(fun prev cur ->
match prev with
| KeepTwo(_,prev) | KeepOne prev when cur - prev = 2 ->
KeepOne cur
| SkipOne prev when cur - prev = 2 ->
KeepTwo(prev,cur)
| _ ->
SkipOne cur)
Head
xl
seq {
for x in xl' do
match x with
| KeepTwo(a,b) -> yield a; yield b
| KeepOne b -> yield b
| _ -> ()
}
for completeness sake, I'll answer this with what I eventually came up with, based on the friendly suggestions in this thread.
The benefits of this approach are that it doesn't need Seq.distinct, which I believe is an improvement as it allows for duplicates. However, it still needs List.rev which doesn't make it the fastest. Nor is it the most succinct code (see comparison of solution in question itself).
let filterTwins l =
l
|> Seq.pairwise
|> Seq.fold (fun a (x, y) ->
if y - x = 2 then (if List.head a = x then y::a else y::x::a)
else a) [0]
|> List.rev
|> List.tail