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#language en
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= Not a Number =

Not a Number (NaN) are a special state that indicates an undefined value. NaNs are only defined for floating point variables.

The following python functions exists and can be used to test if a value is NaN:

 * {{{isnan()}}}

 * {{{isfinite()}}}

The following mathematical functions exists that can be used with arrays that contain NaNs:

 * {{{nansum()}}}
 * {{{nanmax()}}}
 * {{{nanmin()}}}
 * {{{scipy.stats.nanmean()}}}
 * {{{scipy.stats.nanstd()}}}
 * {{{scipy.stats.nanmedian()}}}
 
== Example ==

{{{
In [23]: a=arange(4,dtype=float)

In [24]: a[1]=nan

In [25]: i=isfinite(a)

In [26]: a
Out[26]: array([  0.,  NaN,   2.,   3.])

In [27]: i
Out[27]: array([ True, False,  True,  True], dtype=bool)

In [28]: a[i]
Out[28]: array([ 0.,  2.,  3.])

In [29]: mean(a[i])
Out[29]: 1.6666666666666667

In [30]: mean(a)
Out[30]: nan

In [31]: import scipy.stats

In [32]: scipy.stats.nanmean(a)
Out[32]: 1.6666666666666667
}}}