Say I have an array in Python, e.g.:
my_array = np.array([10, -5, 4, ...])
my_indices = np.array([0, 3, 10, ...])
How can I efficiently get:
my_array that are not in my_indicesmy_array not referenced by my_indices (trivial with 1, but perhaps there is a direct way)
I might do it something like this:
>>> import numpy as np
>>> a = np.random.random(10) # set up a random array to play with
>>> a
array([ 0.20291643, 0.89973074, 0.14291639, 0.53535553, 0.21801353,
0.05582776, 0.64301145, 0.56081956, 0.85771335, 0.6032354 ])
>>>
>>> b = np.array([0,5,6,9]) # indices we *don't want*
>>> mask = np.ones(a.shape,dtype=bool)
>>> mask[b] = False # Converted to a mask array of indices we *do want*
>>> mask
array([False, True, True, True, True, False, False, True, True, False], dtype=bool)
>>>
>>> np.arange(a.shape[0])[mask] #This gets you the indices that aren't in your original
array([1, 2, 3, 4, 7, 8])
>>> a[mask] #This gets you the elements not in your original.
array([ 0.89973074, 0.14291639, 0.53535553, 0.21801353, 0.56081956,
0.85771335])
timeit to know for sure what you're best option is.For part 1, you can just use the difference between the two sets using Python's built in set class.
my_array = [1,2,3,4]
my_indices = [3,4,5]
print list(set(my_array) - set(my_indices))
Will output: [1, 2].
EDIT
In order to return the list of indices in my_array that are not in my_indices, you could use list comprehension:
my_array = [1,2,3,4]
my_indices = [0,3]
print [x for x in range(len(my_array)) if x not in my_indices]
Which can also be expressed as:
temp = []
for x in range(len(my_array)):
if x not in my_indices:
temp.append(x)
This will return the indices [1,2].
In you wanted to get the list of elements, then you can modify the statement to be:
print [my_array[x] for x in range(len(my_array)) if x not in my_indices]
Which will output [2,3].
my_array, not its elements, and in my second question I am looking for elements not referenced by my_indices. Your answer, while admittedly solves another problem, does not address these questions.
You can use list comprehensions
array_len = len(my_array)
missing_indices = [i for i in my_indices
if i < 0 or i >= array_len]
elems = [my_array[i] for i in missing_indices]
For the first question:
my_indices_set = set(my_indices)
[i for i, x in enumerate(my_array) if i not in my_indices]
For the second question:
[x for x in my_array if x not in my_indices_set]
It's more efficient if we use sets, but then there's the cost of creating the sets in the first place
