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question to file parsing
f#parsing

Hi all,

i am just learning F# (coming from a C/C++/C# world) and have a hard time to understand this starnge new & fascinating world.

For that purpose i try to write some kind of logfile-parser (IIS-Logs, actually) & now i hit the following wall:

i read in the IIS logfile and mapped it to a string-array. (Thanks to Don Syme's Expert F# book...)

Now I want to parse that array for occurences of a certain string pattern and write out the the index of the array-elements into a list.

Then i want to split that array at these elements into new arrays, so that i have several arrays with the first element always the one with the string-pattern.

I would like to do this in a _as_least_imperative_as_possible_ & _as_pure_as_possible_

Is it possible to help me with a code snippet for that purpose? Would be really great!

Thank you in advance for your *very* appreciated help,

- Karl

.

Hi KarlTi,

Here is a purely functional piece of code to do what I think you are looking for. I think your second post wasn't doing the "Then i want to split that array..." part. The code below does just that, in fact it skips the intermediate step of getting the list of indexes altogether. Sometimes thinking what you would do imperatively is counter-productive when programming functionally. In this case trying to create an intermediate list of indexes makes the functional solution harder and less efficient.

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let f p xs = 
  let f xs = 
    match xs with
    | [] -> None
    | x::xs' -> let ys, zs = break p xs'
                Some (x::ys, zs)
  unfold f xs

let result = f (fun (x : string) -> x.StartsWith("#Date:")) logfile

This code uses lists and the following general-purpose helper functions. You could do the same thing with sequences, but it is harder and less educational about (pure) functional programming.

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let unfold f x =
  let rec unfold' x acc =
    match f x with
    | None -> List.rev acc
    | Some (a, x') -> unfold' x' (a::acc)
  unfold' x []
    
let break p xs = 
  let rec break' xs acc =
    match xs with
    | [] -> List.rev acc, []
    | x::_ when p x -> List.rev acc, xs
    | x::xs' -> break' xs' (x::acc)
  break' xs []
By on 6/3/2008 7:16 AM ()

OK, I tried several things & what i can come up with is the code sequence below.
I use a ref that i thread through Seq.choose. Looks not too elegant, but nevertheless...

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#light

open System
open System.IO
open System.Text

let logfile =
    seq { use reader = new StreamReader(File.OpenRead(@"C:\Karl\YALP\ex080508.log"))
          while not reader.EndOfStream do             yield reader.ReadLine() }

let i = ref 0
let r =
    logfile |>
    Seq.choose
        (fun x ->
             incr i
             let y = String.split [' '] x 
             let z = List.hd y
             if z = "#Date:" then 
                Some ((sprintf "%d" !i)::y)
             else
                None)

Looks not quite right for me...
Especially the sprintf & the incr.

Maybe someone can point me to the right direction....

Thank you & enjoy your weekend!

- Karl

By on 5/31/2008 12:00 PM ()

I haven't looked at your problem very closely, but i would probably annotate each line in the sequence with the index of the line:

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let logfile' = Seq.mapi (fun idx line -> (idx, line)) logfile

and then filter the tuple sequence based on the contents of the lines.

By on 5/31/2008 5:02 PM ()

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