Math cube root

[Zangief Ief]

Hi,

Is there a cube root (as http://en.wikipedia.org/wiki/Cube_root)
function in Ruby? Math module seems not offer it... except a
Math.sqrt().

Thanks.

 

[Sergio Arbeo]

def cube_root(x)
Math.exp(Math.log(x.to_f)/3.to_f)
end

Cheers,

Serabe

P.D. Proof:
Cube root of x is equal to x^(1/3)

x^(1/3) = y
ln(x^(1/3)) = ln y
(1/3)ln x = ln y

Ergo

y = e^((ln x)/3)

 

[Sergio Arbeo]

So, so sorry.

Even easier.

def cube_root(x)
x**(1/3.0)
end

Cheers,

Serabe

 

[Fabian Streitel]

[Note: parts of this message were removed to make it a legal post.]

So, so sorry.

Even easier.

but nice math anyways
Greetz

 

[Sergio Arbeo]

but nice math anyways

Four years studying maths at the university should be useful for
something, should'nt it?

Cheers,

Serabe

 

[Fabian Streitel]

[Note: parts of this message were removed to make it a legal post.]

one might argue that way, I guess *g*
gives me the satisfaction that what I'm getting into my head
right now for the next exam is not entirely useless *g*

 

[Thomas Preymesser]

[Note: parts of this message were removed to make it a legal post.]

Hi,

Is there a cube root (as http://en.wikipedia.org/wiki/Cube_root)

class Numeric
def root(arg)
arg**(1.0/self)
end
end

p 3.root 27
p 2.root 9

--
Thomas Preymesser
(e-mail address removed)
http://thopre.googlepages.com/
http://thopre.wordpress.com/
Jack Benny <http://www.brainyquote.com/quotes/authors/j/jack_benny.html> -
"I don't deserve this award, but I have arthritis and I don't deserve that
either."

 

[Bertram Scharpf]

Hi,

Am Samstag, 11. Jul 2009, 19:48:50 +0900 schrieb Zangief Ief:

Is there a cube root (as http://en.wikipedia.org/wiki/Cube_root)
function in Ruby? Math module seems not offer it... except a
Math.sqrt().

I suppose you mean the method mentioned under "Cube root on standard
calculator". Here's an implementation:

class Numeric
def sqrt
Math.sqrt self
end
def cbrt
neg = self < 0
c = abs.sqrt.sqrt
i = 2
loop do
w = c
i.times { w = w.sqrt }
i *= 2
w *= c
break if c == w
c = w
end
neg ? -c : c
end
end

If you like it in C (only Ruby 1.8):

http://bertram-scharpf.homelinux.com:8808/doc_root/bs-ruby-2.3/rdoc/classes/Numeric.src/M000033.html

Bertram

 

[jzakiya]

[Note:  parts of this message were removed to make it a legal post.]

Hi,

Is there a cube root (ashttp://en.wikipedia.org/wiki/Cube_root)

class Numeric
  def root(arg)
    arg**(1.0/self)
  end
end

p 3.root 27
p 2.root 9

--
Thomas Preymesser
[email protected]://thopre.googlepages.com/http://thopre.wordpress.com/
Jack Benny <http://www.brainyquote.com/quotes/authors/j/jack_benny.html>  -
"I don't deserve this award, but I have arthritis and I don't deserve that
either."

Correction:
class Numeric
def root(arg); self**(1.0/arg) end
end

 

[Zangief Ief]

Thanks you! Thomas Preymesser's method is perfect.
There is no correction to do jzakiya.

 

[Sergey Avseyev]

How can you explain this:

$ irb
1.9.2p180 (main):001:0> 1000 ** (1.0/3)
9.999999999999998
1.9.2p180 (main):002:0> Math.sqrt(100)
10.0

 

[Jeremy Bopp]

How can you explain this:

$ irb
1.9.2p180 (main):001:0> 1000 ** (1.0/3)
9.999999999999998
1.9.2p180 (main):002:0> Math.sqrt(100)
10.0

You're using floating point arithmetic which is always inexact. The
1.0/3 part cannot be represented with infinite precision, so it's
basically rounded at a certain point. The result is then used for the
rest of the operation, which may compound the inaccuracy introduced by
the initial rounding.

If you must use floating point operations, be prepared to accept results
that are only *close* to what you expect, where close is largely
dependent on the operations being performed.

http://en.wikipedia.org/wiki/Floating_point

-Jeremy

 

[Sergey Avseyev]

Thanks a lot for your explanations. They really help me a lot. You are
great