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(***********************************************************************)
(* *)
(* Objective Caml *)
(* *)
(* Pierre Weis, projet Cristal, INRIA Rocquencourt *)
(* *)
(* Copyright 2001 Institut National de Recherche en Informatique et *)
(* en Automatique. All rights reserved. This file is distributed *)
(* only by permission. *)
(* *)
(***********************************************************************)
(*
Manipulation of lists.
- [] is the empty list
- :: is the infix operator that builds list cells.
Hence 1 :: (2 :: []) is the list that contains 1 and 2 in this order.
*)
(* interval min max = [min; min+1; ...; max-1; max] *)
let rec interval min max =
if min > max then [] else min :: interval (succ min) max
;;
(*
Case analysis on list l is written
match l with
| [] -> ``nil case''
| x :: tail -> ``non nil case,
with x (the head of l) and tail (the tail of l) available''
Function can perform direct case analysis on their argument,
if introduced by the function keyword. Hence,
let f (x) =
match x with
| [] -> ...
| ...
can be abreviated as
let f = function
| [] -> ...
| ...
*)
(* filter p L returns the list of the elements in list L
that satisfy predicate p *)
let rec filter p = function
| [] -> []
| a :: r -> if p a then a :: filter p r else filter p r
;;
(* Application: removing all numbers multiple of n from a list of integers *)
let remove_multiples_of n =
filter (fun m -> m mod n <> 0)
;;
(* The sieve itself *)
let sieve max =
let rec filter_again = function
| [] -> []
| n :: r as l ->
if n * n > max then l else n :: filter_again (remove_multiples_of n r)
in
filter_again (interval 2 max)
;;
let usage() =
print_string "Usage: sieve <n>\n";
exit 2
;;
(* The entry point *)
let main () =
if Array.length Sys.argv <> 2 then usage () else
begin
try
let n = int_of_string Sys.argv.(1) in
List.iter (fun n -> print_int n; print_string " ") (sieve n);
print_newline ()
with Failure "int_of_string" ->
print_string "Bad integer constant";
print_newline ()
end
;;
if !Sys.interactive then () else main ();;
|
