Strange output from arange()
Wanderer
wanderer at dialup4less.com
Mon Jul 25 15:42:52 EDT 2011
On Jul 25, 3:20 pm, Christopher Barrington-Leigh
<christophe... at gmail.com> wrote:
> The following code:
>
> from pylab import arange
> nSegments=5.0
> print arange(0,1.0+1.0/nSegments,1.0/nSegments)
> nSegments=6.0
> print arange(0,1.0+1.0/nSegments,1.0/nSegments)
> nSegments=8.0
> print arange(0,1.0+1.0/nSegments,1.0/nSegments)
> nSegments=10.0
> print arange(0,1.0+1.0/nSegments,1.0/nSegments)
>
> gives an output of:
>
> [ 0. 0.2 0.4 0.6 0.8 1. ]
> [ 0. 0.16666667 0.33333333 0.5 0.66666667
> 0.83333333 1. 1.16666667]
> [ 0. 0.125 0.25 0.375 0.5 0.625 0.75 0.875 1. ]
> [ 0. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1. ]
>
> These arrays have lengths, 6, 8, 9, and 11, in stead of 6, 7, 9, and
> 11.
> What is going on for the case of n=6?
It's rounding.
See http://docs.scipy.org/doc/numpy/reference/generated/numpy.arange.html
stop : number
End of interval. The interval does not include this value,
except in some cases where step is not an integer and floating point
round-off affects the length of out.
The stops are
5 -- 1.2
6 -- 1.1666666666666666666666667
8 -- 1.125
10 -- 1.1
Only 6 has to be rounded up.
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