[Numpy-discussion] 2 greatest values, in a 3-d array, along one axis
Angus McMorland
amcmorl at gmail.com
Fri Aug 3 12:02:49 EDT 2012
On 3 August 2012 11:18, Jim Vickroy <jim.vickroy at noaa.gov> wrote:
> Hello everyone,
>
> I'm trying to determine the 2 greatest values, in a 3-d array, along one
> axis.
>
> Here is an approach:
>
> # ------------------------------------------------------
> # procedure to determine greatest 2 values for 3rd dimension of 3-d
> array ...
> import numpy, numpy.ma
> xcnt, ycnt, zcnt = 2,3,4 # actual case is (1024, 1024, 8)
> p0 = numpy.empty ((xcnt,ycnt,zcnt))
> for z in range (zcnt) : p0[:,:,z] = z*z
> zaxis = 2 # max
> values to be determined for 3rd axis
> p0max = numpy.max (p0, axis=zaxis) # max
> values for zaxis
> maxindices = numpy.argmax (p0, axis=zaxis) #
> indices of max values
> p1 = p0.copy() # work
> array to scan for 2nd highest values
> j, i = numpy.meshgrid (numpy.arange (ycnt), numpy.arange
> (xcnt))
> p1[i,j,maxindices] = numpy.NaN # flag
> all max values
> p1 = numpy.ma.masked_where (numpy.isnan (p1), p1) # hide
> all max values
> p1max = numpy.max (p1, axis=zaxis) # 2nd
> highest values for zaxis
> # additional code to analyze p0max and p1max goes here
> # ------------------------------------------------------
>
> I would appreciate feedback on a simpler approach -- e.g., one that does
> not require masked arrays and or use of magic values like NaN.
>
> Thanks,
> -- jv
>
Here's a way that only uses argsort and fancy indexing:
>>>a = np.random.randint(10, size=(3,3,3))
>>>print a
[[[0 3 8]
[4 2 8]
[8 6 3]]
[[0 6 7]
[0 3 9]
[0 9 1]]
[[7 9 7]
[5 2 9]
[9 3 3]]]
>>>am = a.argsort(axis=2)
>>>maxs = a[np.arange(a.shape[0])[:,None], np.arange(a.shape[1])[None],
am[:,:,-1]]
>>>print maxs
[[8 8 8]
[7 9 9]
[9 9 9]]
>>>seconds = a[np.arange(a.shape[0])[:,None], np.arange(a.shape[1])[None],
am[:,:,-2]]
>>>print seconds
[[3 4 6]
[6 3 1]
[7 5 3]]
And to double check:
>>>i, j = 0, 1
>>>l = a[i, j,:]
>>>print l
[4 2 8]
>>>print np.max(a[i,j,:]), maxs[i,j]
8 8
>>>print l[np.argsort(l)][-2], second[i,j]
4 4
Good luck.
Angus.
--
AJC McMorland
Post-doctoral research fellow
Neurobiology, University of Pittsburgh
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