numpy slicing using user defined input - arrays

Question

I have (in a larger project) data contained in numpy.array.
Based on user input I need to move a selected axis (dimAxisNr) to the first dimension of the array and slice one or more (including the first) dimension based on user input (such as Select2 and Select0 in the example).
Using this input I generate a DataSelect which contains the information needed to slice. But the output size of the sliced array is different from the one using inline indexing. So basically I need a way to generate the '37:40:2' and '0:2' from an input list.
import numpy as np
dimAxisNr = 1
Select2 = [37,39]
Select0 = [0,1]
plotData = np.random.random((102,72,145,2))
DataSetSize = np.shape(plotData)
DataSelect = [slice(0,item) for item in DataSetSize]
DataSelect[2] = np.array(Select2)
DataSelect[0] = np.array(Select0)
def shift(seq, n):
n = n % len(seq)
return seq[n:] + seq[:n]
#Sort and Slice the data
print(np.shape(plotData))
print(DataSelect)
plotData = np.transpose(plotData, np.roll(range(plotData.ndim),-dimAxisNr))
DataSelect = shift(DataSelect,dimAxisNr)
print(DataSelect)
print(np.shape(plotData))
plotData = plotData[DataSelect]
print(np.shape(plotData))
plotDataDirect = plotData[slice(0, 72, None), 37:40:2, slice(0, 2, None), 0:2]
print(np.shape(plotDataDirect))

Answer 1

I'm not sure I've understood your question at all...
But if the question is "How do I generate a slice based on a list of indices like [37,39,40,23] ?"
then I would answer : you don't have to, just use the list as is to select the right indices, like so :
a = np.random.rand(4,5)
print(a)
indices = [2,3,1]
print(a[0:2,indices])
Note that the sorting of the list matters: [2,3,1] yields a different result from [1,2,3]
Output :
>>> a
array([[ 0.47814802, 0.42069094, 0.96244966, 0.23886243, 0.86159478],
[ 0.09248812, 0.85569145, 0.63619014, 0.65814667, 0.45387509],
[ 0.25933109, 0.84525826, 0.31608609, 0.99326598, 0.40698516],
[ 0.20685221, 0.1415642 , 0.21723372, 0.62213483, 0.28025124]])
>>> a[0:2,[2,3,1]]
array([[ 0.96244966, 0.23886243, 0.42069094],
[ 0.63619014, 0.65814667, 0.85569145]])

Answer 2

I have found the answer to my question. I need to use numpy.ix_.
Here is the working code:
import numpy as np
dimAxisNr = 1
Select2 = [37,39]
Select0 = [0,1]
plotData = np.random.random((102,72,145,2))
DataSetSize = np.shape(plotData)
DataSelect = [np.arange(0,item) for item in DataSetSize]
DataSelect[2] = Select2
DataSelect[0] = Select0
#print(list(37:40:2))
def shift(seq, n):
n = n % len(seq)
return seq[n:] + seq[:n]
#Sort and Slice the data
print(np.shape(plotData))
print(DataSelect)
plotData = np.transpose(plotData, np.roll(range(plotData.ndim),-dimAxisNr))
DataSelect = shift(DataSelect,dimAxisNr)
plotDataSlice = plotData[np.ix_(*DataSelect)]
print(np.shape(plotDataSlice))
plotDataDirect = plotData[slice(0, 72, None), 37:40:2, slice(0, 2, None), 0:1]
print(np.shape(plotDataDirect))