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Date: Sun, 30 Mar 2003 12:20:48 +0200
From: Xavier Leroy <xavier.leroy@inria.fr>
To: Christian Lindig <lindig@eecs.harvard.edu>, oliver@first.in-berlin.de,
        Caml Mailing List <caml-list@inria.fr>
Subject: Re: [Caml-list] Performance-cost of ^
Message-ID: <20030330122048.C22539@pauillac.inria.fr>
References: <20030326095209.GA579@first.in-berlin.de> <6E2CBEC7-5F86-11D7-B838-0003930FCE12@inria.fr> <20030328162110.GB548@eecs.harvard.edu>
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In-Reply-To: <20030328162110.GB548@eecs.harvard.edu>; from lindig@eecs.harvard.edu on Fri, Mar 28, 2003 at 05:21:10PM +0100
X-Spam: no; 0.00; caml-list:01 oliver:01 bandel:01 quadratic:01 damien:01 buffer:01 bin:01 inefficient:01 rec:01 doligez:01 skin:97 binary:02 module:03 string:03 wrote:03 
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> > Oliver Bandel wrote:
> > >I'm reading in a file linewise.  For some operations I need it as one
> > >long string.
> > >
> > >How to acchieve this performant?  Is it ok to use ^ for a list of
> > >lines (with  List.fold_right? or List.fold_left?)

It's inefficient for large files (quadratic time).

Christian Linding wrote:
> Damien Doligez wrote:
> > You should use String.concat.
> 
> What about using the Buffer module? It sounds like it was especially
> designed to build up long strings.

Yes, Buffer will work fine here, with about the same efficiency as
String.concat.

If the file is a regular file and isn't expected to change during
reading, the simplest and most efficient solution is:

   let ic = open_in_bin filename in
   let len = in_channel_length ic in
   let s = String.create len in
   really_input ic s 0 len;
   close_in ic;
   s

In other circumstances (i.e. reading from a pipe or socket; desire to
read the file as a text file and not a binary file), consider the
following solution:

  let b = Buffer.create 1024 in
  let s = Buffer.create 1024 in
  let rec read_channel ic =
    let n = input ic s 0 1024 in
    if n > 0 then begin Buffer.add_substring b s 0 n; read_channel ic end 
  in
    read_channel ic; Buffer.contents b

Many ways to skin a cat, I'm afraid.

- Xavier Leroy

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