I have an list<tuple<byte[], int>>. How can I discard the int and end up with a list of byte[]?
The intellisense info looks like this.
Task.Result : (byte[] * int) list
input |> List.map fst will do the trick:
> let input = [ ([| 1uy |], 1); ([| 20uy |], 20) ];;
val input : (byte [] * int) list = [([|1uy|], 1); ([|20uy|], 20)]
> input |> List.map fst;;
val it : byte [] list = [[|1uy|]; [|20uy|]]
It will map each element in the input collection using fst function, which takes the first element of a tuple and discards all the other elements.
Related
I have been doing a CodeWars exercise which can also be seen at dev.to.
The essence of it is:
There is a line for the self-checkout machines at the supermarket. Your challenge is to write a function that calculates the total amount of time required for the rest of the customers to check out!
INPUT
customers : an array of positive integers representing the line. Each integer represents a customer, and its value is the amount of time they require to check out.
n : a positive integer, the number of checkout tills.
RULES
There is only one line serving many machines, and
The order of the line never changes, and
The front person in the line (i.e. the first element in the array/list) proceeds to a machine as soon as it becomes free.
OUTPUT
The function should return an integer, the total time required.
The answer I came up with works - but it is highly imperative.
open System.Collections.Generic
open System.Linq
let getQueueTime (customerArray: int list) n =
let mutable d = new Dictionary<string,int>()
for i in 1..n do
d.Add(sprintf "Line%d" <| i, 0)
let getNextAvailableSupermarketLineName(d:Dictionary<string,int>) =
let mutable lowestValue = -1
let mutable lineName = ""
for myLineName in d.Keys do
let myValue = d.Item(myLineName)
if lowestValue = -1 || myValue <= lowestValue then
lowestValue <- myValue
lineName <- myLineName
lineName
for x in customerArray do
let lineName = getNextAvailableSupermarketLineName d
let lineTotal = d.Item(lineName)
d.Item(lineName) <- lineTotal + x
d.Values.Max()
So my question is ... is this OK F# code or should it be written in a functional way? And if the latter, how? (I started off trying to do it functionally but didn't get anywhere).
is this OK F# code or should it be written in a functional way?
That's a subjective question, so can't be answered. I'm assuming, however, that since you're doing an exercise, it's in order to learn. Learning functional programming takes years for most people (it did for me), but F# is a great language because it enables you learn gradually.
You can, however, simplify the algorithm. Think of a till as a number. The number represents the instant it's ready. At the beginning, you initialise them all to 0:
let tills = List.replicate n 0
where n is the number of tills. At the beginning, they're all ready at time 0. If, for example, n is 3, the tills are:
> List.replicate 3 0;;
val it : int list = [0; 0; 0]
Now you consider the next customer in the line. For each customer, you have to pick a till. You pick the one that is available first, i.e. with the lowest number. Then you need to 'update' the list of counters.
In order to do that, you'll need a function to 'update' a list at a particular index, which isn't part of the base library. You can define it yourself, however:
module List =
let set idx v = List.mapi (fun i x -> if i = idx then v else x)
For example, if you want to 'update' the second element to 3, you can do it like this:
> List.replicate 3 0 |> List.set 1 3;;
val it : int list = [0; 3; 0]
Now you can write a function that updates the set of tills given their current state and a customer (represented by a duration, which is also a number).
let next tills customer =
let earliestTime = List.min tills
let idx = List.findIndex (fun c -> earliestTime = c) tills
List.set idx (earliestTime + customer) tills
First, the next function finds the earliestTime in tills by using List.min. Then it finds the index of that value. Finally, it 'updates' that till by adding its current state to the customer duration.
Imagine that you have two tills and the customers [2;3;10]:
> List.replicate 2 0;;
val it : int list = [0; 0]
> List.replicate 2 0 |> fun tills -> next tills 2;;
val it : int list = [2; 0]
> List.replicate 2 0 |> fun tills -> next tills 2 |> fun tills -> next tills 3;;
val it : int list = [2; 3]
> List.replicate 2 0 |> fun tills -> next tills 2 |> fun tills -> next tills 3
|> fun tills -> next tills 10;;
val it : int list = [12; 3]
You'll notice that you can keep calling the next function for all the customers in the line. That's called a fold. This gives you the final state of the tills. The final step is to return the value of the till with the highest value, because that represents the time it finished. The overall function, then, is:
let queueTime line n =
let next tills customer =
let earliestTime = List.min tills
let idx = List.findIndex (fun c -> earliestTime = c) tills
List.set idx (earliestTime + customer) tills
let tills = List.replicate n 0
let finalState = List.fold next tills line
List.max finalState
Here's some examples, taken from the original exercise:
> queueTime [5;3;4] 1;;
val it : int = 12
> queueTime [10;2;3;3] 2;;
val it : int = 10
> queueTime [2;3;10] 2;;
val it : int = 12
This solution is based entirely on immutable data, and all functions are pure, so that's a functional solution.
Here is a version that resembles your version, with all the mutability removed:
let getQueueTime (customerArray: int list) n =
let updateWith f key map =
let v = Map.find key map
map |> Map.add key (f v)
let initialLines = [1..n] |> List.map (fun i -> sprintf "Line%d" i, 0) |> Map.ofList
let getNextAvailableSupermarketLineName(d:Map<string,int>) =
let lowestLine = d |> Seq.minBy (fun l -> l.Value)
lowestLine.Key
let lines =
customerArray
|> List.fold (fun linesState x ->
let lineName = getNextAvailableSupermarketLineName linesState
linesState |> updateWith (fun l -> l + x) lineName) initialLines
lines |> Seq.map (fun l -> l.Value) |> Seq.max
getQueueTime [5;3;4] 1 |> printfn "%i"
Those loops with mutable "outer state" can be swapped for either recursive functions or folds/reduce, here I suspect recursive functions would be nicer.
I've swapped out Dictionary for the immutable Map, but it feels like more trouble than it's worth here.
Update - here is a compromise solution I think reads well:
let getQueueTime (customerArray: int list) n =
let d = [1..n] |> List.map (fun i -> sprintf "Line%d" i, 0) |> dict
let getNextAvailableSupermarketLineName(d:IDictionary<string,int>) =
let lowestLine = d |> Seq.minBy (fun l -> l.Value)
lowestLine.Key
customerArray
|> List.iter (fun x ->
let lineName = getNextAvailableSupermarketLineName d
d.Item(lineName) <- d.Item(lineName) + 1)
d.Values |> Seq.max
getQueueTime [5;3;4] 1 |> printfn "%i"
I believe there is a more natural functional solution if you approach it freshly, but I wanted to evolve your current solution.
This is less an attempt at answering than an extended comment on Mark Seemann's otherwise excellent answer. If we do not restrict ourselves to standard library functions, the slightly cumbersome determination of the index with List.findIndex can be avoided. Instead, we may devise a function that replaces the first occurrence of a value in a list with a new value.
The implementation of our bespoke List.replace involves recursion, with an accumulator to hold the values before we encounter the first occurrence. When found, the accumulator needs to be reversed and also to have the new value and the tail of the original list appended. Both of this can be done in one operation: List.fold being fed the new value and tail of the original list as initial state while the elements of the accumulator are prepended in the loop, thereby restoring their order.
module List =
// Replace the first occurrence of a specific object in a list
let replace oldValue newValue source =
let rec aux acc = function
| [] -> List.rev acc
| x::xs when x = oldValue ->
(newValue::xs, acc)
||> List.fold (fun xs x -> x::xs)
| x::xs -> aux (x::acc) xs
aux [] source
let queueTime customers n =
(List.init n (fun _ -> 0), customers)
||> List.fold (fun xs customer ->
let x = List.min xs
List.replace x (x + customer) xs )
|> List.max
queueTime [5;3;4] 1 // val it : int = 12
queueTime [10;2;3;3] 2 // val it : int = 10
queueTime [2;3;10] 2 // val it : int = 12
I'm new to F# and I want to find largest element form list of structures:
type Element = struct
val X: int
val Y: int
val RES: int
new (x, y, res) =
{X = x; Y = y; RES = res;}
override this.ToString() = sprintf "%i = %i * %i" this.RES this.X this.Y
end
X is larger than Y when X.RES > Y.RES. I wrote some code:
let max2 x y = if x.RES < y.RES then y else x //BAD LINE
let max_list list =
let rec loop hi list =
match list with
| h::t -> loop (max2 h hi) t
| [] -> hi
match list with
| h::t -> loop h t
| [] -> invalidArg "list" "Empty list"
and call:
let list = findPalindromes 1 1 List.empty //this call populates the "list"
printfn "%A" (max_list list)
This call generates 2 errors (pointing x.RES and y.RES) in line //BAD LINE:
error FS0072: Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved.
I know that I should cast x and y to Element, I've tried to do it but every time I failed.
How can I fix this code or implement this functionality other way?
The F# standard library has this built-in – List.maxBy:
findPalindromes 1 1 List.empty
|> List.maxBy (fun e -> e.RES)
|> printfn "%A"
As to the error you're getting with max2, type annotations solve it:
let max2 (x:Element) (y:Element) = if x.RES < y.RES then y else x
I have a scenario where the standard List.groupBy function isn't what I want, but I don't know the right name for this function so it's making it hard to search for.
I have a list of items of type 'T, and a 'T -> 'k key-producing function. The items are already somewhat "grouped" together in the list, so that when you map the list through the key function, its result will tend have the same key in a row several times, e.g. [1; 1; 1; 2; 2; 1; 1; 3; 3; 3; 1; 1]. What I want is to get a list of lists, where the inner list contains all the items for which the key-producing function returned the same value -- but it should NOT group the different sequences of 1's together.
In other words, say my data was a list of strings, and the key-producing function was String.length. So the input is:
["a"; "e"; "i"; "to"; "of"; "o"; "u"; "and"; "for"; "the"; "I"; "O"]
The output I'm looking for would be:
[["a"; "e"; "i"]; ["to"; "of"]; ["o"; "u"]; ["and"; "for"; "the"]; ["I"; "O"]]
To think of it another way: this is like taking the first item of the list and storing the result of calling the key function. Then you'd use takeWhile (fun x -> keyFun x = origKeyFunResult) to generate the first segment. Then when that takeWhile stops returning values, you record when it stopped, and the value of keyFun x on the first value that didn't return the original result -- and go on from there. (Except that that would be O(N*M) where M is the number of sequences, and would devolve into O(N^2) in many cases -- whereas it should be possible to implement this function in O(N) time).
Now, I can write that function pretty easily. That's not the question. What I want to know is whether there's a standard name for this function. Because I thought it would be called groupBy, but that's something else. (List.groupBy String.length would return [(1, ["a"; "e"; "i"; "o"; "u"; "I"; "O"]); (2, ["to"; "of"]), (3, ["and"; "for"; "the"])], but what I want in this case is for the "a/e/i", "o/u", and "I/O" lists to remain separated, and I don't want the value that the key-generating returns to be in the output data).
Maybe there isn't a standard name for this function. But if there is, what is it?
I'm a little late and it seems that you have found a solution, and it seems that there doesn't exists a single function i F# that can handle the problem.
Just for the challenge I tried to find some usable solutions and came up with the following (whether they are efficient or not is up the reader to deside):
open System
module List =
/// <summary>
/// Generic List Extension:
/// Given a comparer function the list will be chunked into sub lists
/// starting when ever comparer finds a difference.
/// </summary>
let chunkByPredicate (comparer : 'T -> 'T -> bool) list =
let rec func (i : int, lst : 'T list) : 'T list list =
if i >= lst.Length then
List.empty
else
let first = lst.[i]
let chunk = lst |> List.skip(i) |> List.takeWhile (fun s -> comparer first s)
List.append [chunk] (func((i + chunk.Length), lst))
func (0, list) |> List.where (fun lst -> not (List.isEmpty lst))
// 1. Using List.fold to chunk by string length
let usingListFold (data : string list) =
printfn "1. Using List.fold: "
data
|> List.fold (fun (acc : string list list) s ->
if acc.Length > 0 then
let last = acc.[acc.Length - 1]
let lastLength = last.[0].Length
if lastLength = s.Length then
List.append (acc |> List.take (acc.Length - 1)) [(last |> List.append [s])]
else
List.append acc [[s]]
else
[[s]]) ([])
|> List.iter (printfn "%A")
printfn ""
// 2. Using List.chunkByPredicate
let usingListChunkByPredicate<'a> (predicate : 'a -> 'a -> bool, data : 'a list) =
printfn "2. Using List.chunkByPredicate: "
data
|> List.chunkByPredicate predicate
|> List.iter (printfn "%A")
printfn ""
[<EntryPoint>]
let main argv =
let data = ["a"; "e"; "i"; "to"; "of"; "o"; "u"; "and"; "for"; "the"; "I"; "O"]
usingListFold data
usingListChunkByPredicate<string>((fun first s -> first.Length = s.Length), data)
let intData = [0..50]
usingListChunkByPredicate<int>((fun first n -> first / 10 = n / 10), intData)
Console.ReadLine() |> ignore
0
I am new to programming and F# is my first language.
Here is my code:
let areAnagrams (firstString: string) (secondString: string) =
let countCharacters (someString: string) =
someString.ToLower().ToCharArray() |> Array.toSeq
|> Seq.countBy (fun eachChar -> eachChar)
|> Seq.sortBy (snd >> (~-))
countCharacters firstString = countCharacters secondString
let testString1 = "Laity"
let testString2 = "Italy"
printfn "It is %b that %s and %s are anagrams." (areAnagrams testString1 testString2) (testString1) (testString2)
This is the output:
It is false that Laity and Italy are anagrams.
What went wrong? What changes should I make?
Your implementation of countCharacters sorts the tuples just using the second element (the number of occurrences for each character), but if there are multiple characters that appear the same number of times, then the order is not defined.
If you run the countCharacters function on your two samples, you can see the problem:
> countCharacters "Laity";;
val it : seq<char * int> = seq [('l', 1); ('a', 1); ('i', 1); ('t', 1); ...]
> countCharacters "Italy";;
val it : seq<char * int> = seq [('i', 1); ('t', 1); ('a', 1); ('l', 1); ...]
One solution is to just use Seq.sort and sort the tuples using both the letter code and the number of occurrences.
The other problem is that you are comparing two seq<_> values and this does not use structural comparison, so you'll need to turn the result into a list or an array (something that is fully evaluated):
let countCharacters (someString: string) =
someString.ToLower().ToCharArray()
|> Seq.countBy (fun eachChar -> eachChar)
|> Seq.sort
|> List.ofSeq
Note that you do not actually need Seq.countBy - because if you just sort all the characters, it will work equally well (the repeated characters will just be one after another). So you could use just:
let countCharacters (someString: string) =
someString.ToLower() |> Seq.sort |> List.ofSeq
Sorting the characters of the two strings gives you an easy solution but this could be a good example of recursion.
You can immediately exclude strings of different length.
You can also filter out all the occurrences of a char per iteration, by replacing them with an empty string.
let rec areAnagram (x:string) (y:string) =
if x.Lenght <> t.Lenght
then false else
if x.Lenght = 0
then true else
let reply = x.[0].ToString ()
areAnagram
(x.Replace (reply,""))
(y.Replace (reply,""))
The above should be faster than sorting for many use cases.
Anyway we can go further and transform it into a fast Integer Sorting without recursion and string replacements
let inline charToInt c =
int c - int '0'
let singlePassAnagram (x:string) =
let hash : int array = Array.zeroCreate 100
x |> Seq.iter (fun c->
hash.[charToInt c] <- (hash.[charToInt c]+1)
)
let areAnagramsFast
(x:string) (y:string) =
if x.Length <> y.Length
then false else
(singlePassAnagram x) =
(singlePassAnagram y)
Here is a fiddle
Say I have this:
let coor = seq { ... }
// val coor : seq<int * int> = seq[(12,34); (56, 78); (90, 12); ...]
I'm trying to get the value of the first number of the second element in the sequence, in this case 56. Looking at the MSDN Collection API reference, Seq.nth 1 coor returns (56, 78), of type seq <int * int>. How do I get 56 out of it?
I suggest you go through Tuple article:
http://msdn.microsoft.com/en-us/library/dd233200.aspx
A couple of exceptions that might shed some light on the problem:
Function fst is used to access the first element of the tuple:
(1, 2) |> fst // returns 1
Function snd is used to access the second element
(1, 2) |> snd // returns 2
In order to extract element from wider tuples you can use following syntax:
let _,_,a,_ = (1, 2, 3, 4) // a = 3
To use it in various collections (well lambdas that are passed to collection's functions), let's start with following sequence:
let s = seq {
for i in 1..3 do yield i,-i
}
We end up with
seq<int * int> = seq [(1, -1); (2, -2); (3, -3)]
Let's say we want to extract only the first element (note the arguments of the lambda):
s |> Seq.map (fun (a, b) -> a)
Or even shorter:
s |> Seq.map fst
And lets finally go back to your question.
s |> Seq.nth 1 |> fst
It's a tuple, so you could use the function fst;
> let value = fst(Seq.nth 1 coor);;
val value : int = 56
...or access it via pattern matching;
> let value,_ = Seq.nth 1 coor;;
val value : int = 56