Strange output from arange()

Wanderer wanderer at dialup4less.com
Mon Jul 25 21:42:52 CEST 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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