On Wed, Apr 6, 2011 at 9:36 PM, Christopher Barker <Chris.Barker@noaa.gov> wrote:

On 4/6/11 6:24 AM, Alan G Isaac wrote:
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.round_.html

simple enough, of course, but just to be clear:

In [108]: np.round(1.23456789, 3)
Out[108]: 1.2350000000000001

so the number is rounded to the requested number of decimal places, but
then stored in a binary floating point format, which may not be able to
exactly represent that rounded number -- hence the "1" at the end. This
is simply how floating point works.

and that slight difference _probably_ doesn't matter, but it's worth
being aware of, because is does make a difference occasionally.

python has a decimal type that can work with exact decimal numbers --
numpy doesn't support that, as there is no hardware support for it (at
least on most platforms).

If you want to display it differently, you can use the string formatters:

In [110]: "%.3f"%np.round(1.23456789, 3)
Out[110]: '1.235'

HTH, Chris

--
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R (206) 526-6959 voice
7600 Sand Point Way NE (206) 526-6329 fax
Seattle, WA 98115 (206) 526-6317 main reception

Chris.Barker@noaa.gov
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