I need a very precise number, but ipython is rounding it wrong

Asked Jul 11 '16 at 17:00

Active Jul 11 '16 at 17:24

Viewed 41 times

Basically, I have a constant called F

F=-5.9341737921479211e-05

and I need to get the cubic root of this constant. But when I try to do either

F**(1/3)

np.power(F,(1/3))

I always get the result F=1.0, which is completely wrong. The correct result is approximately -0.039. How can I get a precise value instead of this completely wrong result?

It seems to be something basic, but I have no idea why it's happening and I couldn't find anything related.

edited Jul 11 '16 at 17:24

asked Jul 11 '16 at 17:00

Daniel Alvarenga

1

Do you know what `1/3` is in the version of Python you're using? – jonrsharpe Jul 11 '16 at 17:01
In Python 2.7, '/' does Integer division. Probably this helps you: http://stackoverflow.com/questions/21316968/division-in-python-2-7-and-3-3 – Supahupe Jul 11 '16 at 17:04
0.333... ? Isn't it? – Daniel Alvarenga Jul 11 '16 at 17:04
It works as expected in Python3 – Arun Ghosh Jul 11 '16 at 17:04
3

Don't guess - type `1/3` into a Python shell and see what you get. In Python 2, you'll get `0`. Then try, e.g., `1.0/3` instead. – Tim Peters Jul 11 '16 at 17:04
In python 2.x, you could also write F**(1.0/3.0) – Supahupe Jul 11 '16 at 17:05
2

You are using python 2 do this: `-abs(F)**(1.0/3)` – kasravnd Jul 11 '16 at 17:05
You have a minus sign in your post. I don't believe you want it there. Without it, your equations works with python 3 an python 2.7 (using @Supahupe's formula – joel goldstick Jul 11 '16 at 17:05
@jonrsharpe This is a negative number and the answer would be slightly different than the duplicate. – kasravnd Jul 11 '16 at 17:07
@Kasramvd if they're getting `1.0`, it's an integer division problem. Solving that they may see other problems, but they can solve those too! – jonrsharpe Jul 11 '16 at 17:11
I am using ipython, and when I type, for example: 1/3 it returns to me 0.333333333. I tried all the methods you guys said on the comments, but nothing is working – Daniel Alvarenga Jul 11 '16 at 17:24
Usingthe library from __future__ import division, I could make the constant F work. But now another constant G=-0.043983285322792547 when I try to get the cubic root, I just receive the result "nan" – Daniel Alvarenga Jul 11 '16 at 17:32
I could make it working following this response: http://stackoverflow.com/a/1361750/6519480 – Daniel Alvarenga Jul 11 '16 at 17:51

I need a very precise number, but ipython is rounding it wrong

0 Answers0