Question

我在OCaml中编写了一个程序，它应该计算前100个Bell数。这是我的代码（我使用Num模块）：

open Num
let one = num_of_int 1;;
let zero = num_of_int 0;;

计算阶乘：

let rec factorial n = 
    if n < 2 
    then one
    else (num_of_int n) */ factorial(n-1)

计算牛顿：

let rec newton n k =
    factorial n // (factorial k */ factorial (n-k))

let bell = Array.make 101 zero;;
bell.(0) <- one;;
bell.(1) <- one;;

let i = ref 2
let k = ref 0
let x = ref zero
let suma = ref zero
let n = ref 100

if !n != 0 || !n != 1 then
    while !i <= !n do   
        while !k <= (!i-1) do
            x := newton (!i-1) !k;
            suma := !suma +/ (!x */ bell.(!k));
            k := !k + 1 
        done;
        bell.(int_of_num !k) <- (!suma);   
        suma:= zero;
        k:=0;
        i:= !i + 1;
    done;;

bell.(int_of_num 20)

这是我用这种语言编写的第一个程序。编译时我遇到了一些问题。

Answer 1

您错过了;;的最后一个let n = ref 100。但是，将全局变量用作算法的辅助变量，无论如何都被认为是不好的风格。下面的代码是对问题代码的最小修复，不认可坏样式的其他方面，例如不使用评论中提到的for循环。

(* #load "nums.cma";; if in toplevel *)
open Num
let one = num_of_int 1;;
let zero = num_of_int 0;;

let rec factorial n = 
  if n < 2 
  then one
  else (num_of_int n) */ factorial(n-1) 

let rec newton n k =
  factorial n // (factorial k */ factorial (n-k))

let bell input =
  let bell = Array.make (input+1) zero in
  bell.(0) <- one;
  bell.(1) <- one;

  let i = ref 2 in
  let k = ref 0 in
  let x = ref zero in
  let suma = ref zero in
  let n = ref input in

  if !n <> 0 || !n <> 1 then
    while !i <= !n do   
      while !k <= (!i-1) do
        x := newton (!i-1) !k;
        suma := !suma +/ (!x */ bell.(!k));
        k := !k + 1 
      done;
      bell.(!k) <- (!suma);   
      suma:= zero;
      k:=0;
      i:= !i + 1;
    done;

  bell.(input)

OCaml中的贝尔数字

1 个答案: