Slicing at runtime
Question:
can someone explain me how to slice a numpy.array at runtime?
I don’t know the rank (number of dimensions) at ‘coding time’.
A minimal example:
import numpy as np
a = np.arange(16).reshape(4,4) # 2D matrix
targetsize = [2,3] # desired shape
b_correct = dynSlicing(a, targetsize)
b_wrong = np.resize(a, targetsize)
print a
[[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]
[12 13 14 15]]
print b_correct
[[0 1 2]
[4 5 6]]
print b_wrong
[[0 1 2]
[3 4 5]]
And my ugly dynSlicing():
def dynSlicing(data, targetsize):
ndims = len(targetsize)
if(ndims==1):
return data[:targetsize[0]],
elif(ndims==2):
return data[:targetsize[0], :targetsize[1]]
elif(ndims==3):
return data[:targetsize[0], :targetsize[1], :targetsize[2]]
elif(ndims==4):
return data[:targetsize[0], :targetsize[1], :targetsize[2], :targetsize[3]]
Resize() will not do the job since it flats the array before dropping elements.
Thanks,
Tebas
Answers:
You can directly ‘change’ it. This is due to the nature of arrays only allowing backdrop.
Instead you can copy a section, or even better create a view of the desired shape:
Link
Passing a tuple of slice objects does the job:
def dynSlicing(data, targetsize):
return data[tuple(slice(x) for x in targetsize)]
Simple solution:
b = a[tuple(map(slice,targetsize))]
can someone explain me how to slice a numpy.array at runtime?
I don’t know the rank (number of dimensions) at ‘coding time’.
A minimal example:
import numpy as np
a = np.arange(16).reshape(4,4) # 2D matrix
targetsize = [2,3] # desired shape
b_correct = dynSlicing(a, targetsize)
b_wrong = np.resize(a, targetsize)
print a
[[ 0 1 2 3]
[ 4 5 6 7]
[ 8 9 10 11]
[12 13 14 15]]
print b_correct
[[0 1 2]
[4 5 6]]
print b_wrong
[[0 1 2]
[3 4 5]]
And my ugly dynSlicing():
def dynSlicing(data, targetsize):
ndims = len(targetsize)
if(ndims==1):
return data[:targetsize[0]],
elif(ndims==2):
return data[:targetsize[0], :targetsize[1]]
elif(ndims==3):
return data[:targetsize[0], :targetsize[1], :targetsize[2]]
elif(ndims==4):
return data[:targetsize[0], :targetsize[1], :targetsize[2], :targetsize[3]]
Resize() will not do the job since it flats the array before dropping elements.
Thanks,
Tebas
You can directly ‘change’ it. This is due to the nature of arrays only allowing backdrop.
Instead you can copy a section, or even better create a view of the desired shape:
Link
Passing a tuple of slice objects does the job:
def dynSlicing(data, targetsize):
return data[tuple(slice(x) for x in targetsize)]
Simple solution:
b = a[tuple(map(slice,targetsize))]