Ruby因子函数

Question

我疯了:因子的Ruby函数在哪里？不,我不需要教程实现,我只想要库中的函数.这不是数学!

我开始怀疑,这是一个标准的库函数吗？

Answer 1

标准库中没有因子函数.

Answer 2

喜欢这样比较好

(1..n).inject(:*) || 1

Answer 3

它不在标准库中,但您可以扩展Integer类.

class Integer
  def factorial_recursive
    self <= 1 ? 1 : self * (self - 1).factorial
  end
  def factorial_iterative
    f = 1; for i in 1..self; f *= i; end; f
  end
  alias :factorial :factorial_iterative
end

NB由于明显的性能原因,迭代因子是更好的选择.

Answer 4

从http://rosettacode.org/wiki/Factorial#Ruby无耻地抄袭,我个人最喜欢的是

class Integer
  def fact
    (1..self).reduce(:*) || 1
  end
end

>> 400.fact
=> 64034522846623895262347970319503005850702583026002959458684445942802397169186831436278478647463264676294350575035856810848298162883517435228961988646802997937341654150838162426461942352307046244325015114448670890662773914918117331955996440709549671345290477020322434911210797593280795101545372667251627877890009349763765710326350331533965349868386831339352024373788157786791506311858702618270169819740062983025308591298346162272304558339520759611505302236086810433297255194852674432232438669948422404232599805551610635942376961399231917134063858996537970147827206606320217379472010321356624613809077942304597360699567595836096158715129913822286578579549361617654480453222007825818400848436415591229454275384803558374518022675900061399560145595206127211192918105032491008000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

此实现也恰好是Rosetta Code中列出的变体中最快的.

更新#1

添加|| 1处理零情况.

更新#2

感谢Mark Thomas,这是一个更有效,优雅和模糊的版本:

class Integer
  def fact
    (2..self).reduce(1,:*)
  end
end

Answer 5

您还可以使用Math.gamma归结为整数参数的阶乘的函数.

Answer 6

class Integer
  def !
    (1..self).inject(:*)
  end
end

例子

!3  # => 6
!4  # => 24

Answer 7

在数学中,factorial of n只是gamma function of n+1
(参见:http://en.wikipedia.org/wiki/Gamma_function)

如果需要,Ruby Math.gamma()只需使用Math.gamma(n+1)并将其强制转换为整数.

Answer 8

我会做

(1..n).inject(1, :*)

Answer 9

我刚写了自己的:

def fact(n)
  if n<= 1
    1
  else
    n * fact( n - 1 )
  end
end

此外,您可以定义下降阶乘:

def fall_fact(n,k)
  if k <= 0
    1
  else
    n*fall_fact(n - 1, k - 1)
  end
end

Answer 10

怀着对所有参与并花时间帮助我们的人的崇高敬意，我想分享我对此处列出的解决方案的基准。参数：

迭代次数 = 1000

n = 6

                                     user     system      total        real
Math.gamma(n+1)                   0.000383   0.000106   0.000489 (  0.000487)
(1..n).inject(:*) || 1            0.003986   0.000000   0.003986 (  0.003987)
(1..n).reduce(1, :*)              0.003926   0.000000   0.003926 (  0.004023)
1.upto(n) {|x| factorial *= x }   0.003748   0.011734   0.015482 (  0.022795)

对于 n = 10

  user     system      total        real
0.000378   0.000102   0.000480 (  0.000477)
0.004469   0.000007   0.004476 (  0.004491)
0.004532   0.000024   0.004556 (  0.005119)
0.027720   0.011211   0.038931 (  0.058309)