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What would be the best way to split a fixed length string?
f#parsingfunctional

Hi,
I am having trouble writing a F# equivalent to this C# method. I use this method for parsing fixed length text files. Some of the records in these data files span across multiple rows.

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Name      Dob      Mi
     ThirdColumn   Fourth
_________________
John Doe  1/1/2008 c
     more info     even more
John Deer 1/2/2008 f
     more info     even more

I create a list of lists. The outer list is the number of lines per record. The inner list is the column numbers for the data.

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List< List<int>> rows 

rows.Add( new List( 11, 20 ) )
rows.Add( new List( 20 ) )

On the first line in C#, Split( line, rows[0] ) and get ["John Doe","1/1/2008","c"] as a result.
Then I move to the second item, Split ( line, rows[1] ) and get ["more info","even more"] as the result.

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public static List<string> Split(string s, List<int> i)
{
	int j = 0;
	List<string> ret = new List<string>();
	i.ForEach(delegate(int k) { ret.Add(s.Substring(j, k).Trim()); j += k; });
	if (j < s.Length)
		ret.Add(s.Substring(j).Trim());
	return ret;
}

What would be the functional approach to solving this problem?

My though would be to create a map function with an accumulator. I am not sure how to do this. This may not be correct way to solve this from a function approach. Any suggestions would be greatly appreciated.

Thanks,
jonh

.

A possible translation is the following:

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let s = "this is a string very long"
let splits = new ResizeArray<int>()
splits.Add(10)
splits.Add(15)

let Split (str:string) (bounds:ResizeArray<int>) =
let ch = str.ToCharArray()
let ret = 
    ResizeArray.mapi 
        (fun idx v -> 
            let prev = if idx = 0 then 0 else bounds.[idx - 1]
            new System.String(ch, prev, v - prev)
        ) 
        bounds
    ret.Add(str.Substring(bounds.[bounds.Count - 1]))
    ret

Split s splits;;

Cheers,

Antonio

By on 6/18/2008 5:03 AM ()

Thank you very much. This is a clever solution.

I was not thinking of mapi. I was trying to come up with a map accumulator. Your solution is much easier. ;)

By on 6/19/2008 10:11 AM ()

Hello, jonh.

There is yet simpler solution:

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open System.IO

let reader parser fname =
  seq {
    use reader = StreamReader(File.OpenRead(fname)) in
      while not reader.EndOfStream do
        let s = reader.ReadLine() 
        yield parser s
  }

let bounds = [(0,5);(7,10);(12,18)] //fixed bounds of records in string
let parser = List.map (fun (l,r) -> s.[l..r]) bounds


reader parser "f.txt"
By on 6/24/2008 9:16 PM ()

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