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[Caml-list] How to read three integers from a text-file... ?
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Date: -- (:)
From: Tomasz Zielonka <zielony@c...>
Subject: Re: [Caml-list] How to read three integers from a text-file... ?
On Tue, Apr 23, 2002 at 12:41:49PM +0200, Jacek Chrzaszcz wrote:
> Hello list,

[Hello]    ;)

> Is there a clean way (a one-liner) to read a constant number of
> integers separated by whitespace from a text-file (or stdin) ?

For 'constant numbers' greater than 1 there are at least five
solutions:

1) Write function which will read 'n' ints from input and return a list
   of them. But that would be unsafe, inefficient and would cause 'this
   pattern-matching is not exhaustive' warnings in destructuring 'let'
   bindings.

2) Write several functions: read_1_int, read_2_ints, ..., read_5_ints,
   which will return tuples with appropriate number of ints. Safer but
   still inefficient (tuple allocation, copying) and a bit clumsy.

3) Create CamlP4 macro/(syntax extension) which will expand to
   'let's binding subsequent ints to given names. Something like:

   READINTS(a, b, c, d)

   (sorry, I know it looks like a CPP macro)

   expanded to:

   let a = get_int () in
   let b = get_int () in
   let c = get_int () in
   let d = get_int () in

4) Create mechanism dual to printf, but as far as I understand
   OCaml's printf, this would require extending typechecker.

5) Just 'n' times use a function which reads 1 integer from input.

Personally I prefer option 5.

I write

  let read_int = ()

at top of my program, to avoid using this standard OCaml function.
Then I define get_int function, which just reads characters one by one.

  get_int: unit -> int

When using such imperative function, one should be careful. OCaml's
evaluation order is not left to right, so following constructs may have
effects other than desired:

  let pair = (get_int (), get_int ())

  f (get_int ()) (get_int ())

> I know I can use String.index, Str.split or read char by char (this
> sucks), but you have to admit the Pascal or even C versions are more
> appealing.

Reading char by char sucks? Why? 

Scanf probably does it, and it isn't exactly _reading_ char by char
when you have buffered I/O.

I participated in online contest recently and I made some "research" in
integer reading :) 

I've even made benchmarks - the task was to read first integer from input
as 'n', sum following 'n' unsigned integers and print the result.  Here
are the times for n=200000 on PIII 850 (gcc ocamlopt 3.04).

program         time    description
-----------------------------------------------------------------------
c_scanf        0.201s - C scanf ("%d", ...
c_scanf4       0.173s - C scanf ("%d%d%d%d", ...
c_macro        0.095s - my C macro (using getchar())
c_macro2       0.037s - my C macro with input pre-loaded into array

ml_genlex      5.032s - Genlex, Genlex.npeek, List.map & List.fold_left
ml_genlex2     3.583s - Genlex with other sum function
ml_ocamllex    0.517s - ocamllex + int_of_string
ml_simple      0.100s - char by char tail-recursive reading function
ml_string2     0.061s - similar function, but with input pre-loading

Note that C programs were compiled without optimisation. With -O3
c_macro2 peforms even faster - 0m0.028s.

I think that ml_simple solution is satisfactory for such contests. It's
fast, simple and quite safe to use. I can send the source if someone's
interested.

> I am asking this question, because our students want to use Ocaml for
> competing in various programming contests, where the comfort and speed
> (of programming) are essential.

My simple solution is comfortable enough for me. 

Speed of execution is also important in contests - you rather don't
want to spend 5 seconds to just read the data.

> Jacek Chrzaszcz

tomek

-- 
Tomek Zielonka <t.zielonka@students.mimuw.edu.pl>
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