nickpick nickpick - 1 month ago 34
Python Question

Multiple indices for numpy array: IndexError: failed to coerce slice entry of type numpy.ndarray to integer

Is there a way to do multiple indexing in a numpy array as described below?

arr=np.array([55, 2, 3, 4, 5, 6, 7, 8, 9])
arr[np.arange(0,2):np.arange(5,7)]

output:
IndexError: too many indices for array

Desired output:
array([55,2,3,4,5],[2,3,4,5,6])


This problem might be similar to calculating a moving average over an array (but I want to do it without any function that is provided).

Answer

Here's an approach using strides -

start_index = np.arange(0,2)
L = 5     # Interval length
n = arr.strides[0]
strided = np.lib.stride_tricks.as_strided
out = strided(arr[start_index[0]:],shape=(len(start_index),L),strides=(n,n))

Sample run -

In [976]: arr
Out[976]: array([55, 52, 13, 64, 25, 76, 47, 18, 69, 88])

In [977]: start_index
Out[977]: array([2, 3, 4])

In [978]: L = 5

In [979]: out
Out[979]: 
array([[13, 64, 25, 76, 47],
       [64, 25, 76, 47, 18],
       [25, 76, 47, 18, 69]])