How do i use the split method in f# correctly?
The code:
let a = "abc def"
let b = a.Split [|' '|]
Console.WriteLine("{0}", b)
does return
"System.String[]"
i tryed many variations of:
.Split([|' '|])
The language doc & other questions don't really help.
i want b to be:
["abc","def"]
thanks for helping.
Your Console.WriteLine is the actual problem here, not the fact you get an array vs a list. It's just that the list overrides ToString, so that happens to make your code work.
You can use either of these to get the same effect with an array:
Console.WriteLine(sprintf "%A" b)
printfn "%A" b
I think you want a list as result but .Split returns an array of strings, not a list of strings.
If you want a list just call Array.toList after calling .Split:
let b = a.Split [|' '|] |> Array.toList
UPDATE
Based on your comments, if you are happy with the result as an Array then all you need to change is the way you print it.
One way is to use sprintf to pretty-print the array, as stated in my comments and in the other answer. Here's another way:
open System
let a = "abc def"
let b = a.Split [|' '|]
Console.WriteLine(String.Join(";", b))
Related
I am very new to F# and Kattis. I have tried this simple problem "Which is greater" at Kattis. Link is here: https://open.kattis.com/problems/whichisgreater
I have tried with this code:
open System
let a = Console.Read()
let b = Console.Read()
if a > b then Console.WriteLine "1" else Console.WriteLine "0"
But I still get wrong answer. Anybody who can help on how to handle inputs and outputs in Kattis for F#? Maybe some simple examples can be made available?
The following is accepted by Kattis:
open System
let line = Console.ReadLine().Split ' '
let a = int64 line.[0]
let b = int64 line.[1]
Console.WriteLine(if a > b then 1 else 0)
Here, we read the line, split it on a space character into two numbers, compare them and print the required result.
It looks like they're using an old version of the F# compiler, so you have to specify an explicit entry point. Here's their sample F# solution for a different problem:
open System
[<EntryPoint>]
let main argv =
(fun _ -> Console.ReadLine()) |>
Seq.initInfinite |>
Seq.takeWhile ((<>) null) |>
Seq.iter
(fun (s : string) ->
let arr = s.Split([|' '|])
let a = int64 arr.[0]
let b = int64 arr.[1]
/// solve test case and output answer
printfn "%d" (abs (a - b))
)
0
I think that should give you enough info to solve the "which is greater" problem you're looking at. (Note that Console.Read only reads a single character, so it's not what you want for this problem. Instead, you probably want to read in the entire line, then split it into two strings at the blank space, then convert each of those strings into an integer. Coincidentally, the sample code I pasted above does something similar.)
I have a List of String declared like this:
var re1_emoticons=""::Nil
Which I have then filled in with emoticons (Strings). I am trying to add a \\ at the beginning of each String in re1_emoticons, modifying the original var.
First attempt:
re1_emoticons.foreach(t=>"""\\""" + t)
Second attempt:
re1_emoticons.foreach(t=>re1_emoticons.indexOf(t)="""\\"""+ t)
Third attempt:
re1_emoticons.foreach(t=>re1_emoticons.indexOf(t):="""\\"""+ t)
However still don't find the way. Is there a correct way to do this?
Use the map-function. It creates a new list with a function applied to each of the items. When you get the new list, you can assign it to the var you already have.
re1_emoticons = re1_emoticons.map(t => """\\""" + t)
The problem with foreach is it does not return anything. Use map instead of foreach or use for-comprehension syntax.
re1_emoticons = for(emoticon <- re1_emoticons) yield ("""\\""" + emotican)
I'd greatly appreciate if you could tell me how to make a single string from a range between two ints. Like [5..10] i would need to get a "5678910". And then I'd have to calculate how many (zeroes, ones ... nines) there are in a string.
For example: if i have a range from [1..10] i'd need to print out
1 2 1 1 1 1 1 1 1 1
For now i only have a function to search for a element in string.
`countOfElem elem list = length $ filter (\x -> x == elem) list`
But the part how to construct such a string is bugging me out, or maybe there is an easier way? Thank you.
I tried something like this, but it wouldn't work.
let intList = map (read::Int->String) [15..22]
I tried something like this, but it wouldn't work. let intList = map (read::Int->String) [15..22]
Well... the purpose of read is to parse strings to read-able values. Hence it has a type signature String -> a, which obviously doesn't unify with Int -> String. What you want here is the inverse1 of read, it's called show.
Indeed map show [15..22] gives almost the result you asked for – the numbers as decimal-encoded strings – but still each number as a seperate list element, i.e. type [String] while you want only String. Well, how about asking Hoogle? It gives the function you need as the fifth hit: concat.
If you want to get fancy you can then combine the map and concat stages: both the concatMap function and the >>= operator do that. The most compact way to achieve the result: [15..22]>>=show.
1show is only the right inverse of read, to be precise.
I need to create a function that basically works like this :
insert_char("string" 'x') outputs "sxtxrxixnxg".
So here is my reasoning :
Create a list with every single character in the string :
let inserer_car(s, c) =
let l = ref [] in
for i = 0 to string.length(s) - 1 do
l := s.[i] :: !l
done;
Then, I want to use List.map to turn it into a list like ['s', 'x', 't', 'x' etc.].
However, I don't really know how to create my function to use with map. Any help would be appreciated!
I'm a beginner in programming and especially in ocaml! so feel free to assume I'm absolutely ignorant.
If you were using Core, you could write it like this:
open Core.Std
let insert_char s c =
String.to_list s
|> (fun l -> List.intersperse l c)
|> String.of_char_list
Or, equivalently:
let insert_char s c =
let chars = String.to_list s in
let interspersed_chars = List.intersperse chars c in
String.of_char_list interspersed_chars
This is just straightforward use of existing librariies. If you want the implementation of List.intersperse, you can find it here. It's quite simple.
A map function creates a copy of a structure with different contents. For lists, this means that List.map f list has the same length as list. So, this won't work for you. Your problem requires the full power of a fold.
(You could also solve the problem imperatively, but in my opinion the reason to study OCaml is to learn about functional programming.)
Let's say you're going to use List.fold_left. Then the call looks like this:
let result = List.fold_left myfun [] !l
Your function myfun has the type char list -> char -> char list. In essence, its first parameter is the result you've built so far and its second parameter is the next character of the input list !l. The result should be what you get when you add the new character to the list you have so far.
At the end you'll need to convert a list of characters back to a string.
I have a question I am rather unsure about.
My questions is as follows
let myFunc (text:string) (times:int) = ....
What I want this function to do is put the string together as many times as specified by the times parameter.
if input = "check " 3 I want the output string = "check check check"
I have tried with a loop, but couldn't seem to make it work.
Anyone?
Actually the function is already in String module:
let multiply text times = String.replicate times text
To write your own function, an efficient way is using StringBuilder:
open System.Text
let multiply (text: string) times =
let sb = new StringBuilder()
for i in 1..times do
sb.Append(text) |> ignore
sb.ToString()
If you want to remove trailing whitespaces as in your example, you can use Trim() member in String class to do so.
A variation on pad's solution, given that it's just a fold:
let multiply n (text: string) =
(StringBuilder(), {1..n})
||> Seq.fold(fun b _ -> b.Append(text))
|> sprintf "%O"
If you want a pure functional "do-it-yourself" version for F# learning purposes, then something like the following snippet will do:
let myFunc times text =
let rec grow result doMore =
if doMore > 0 then
grow (result + text) (doMore- 1)
else
result
grow "" times
Here is the test:
> myFunc 3 "test";;
val it : string = "testtesttest"
Otherwise you should follow the pointer about the standard F# library function replicate given in pad's answer.
String.replicate already provides the functionality you're looking for.
If for some reason you want the arguments reversed, you can do it as follows:
(* A general function you should add to your utilities *)
let flip f a b = f b a
let myFunc = flip String.replicate
In a simple recursive fashion:
let rec dupn = function
|s,1 -> s
|s,n -> s ^ dupn(s, n-1)