i want to extract every 4th element in the numpy array
The array is
a = [0,1,3,4,5,6,7,8, 9]
final_array shuld be [0,5,9]
The array is ofocurse very huge, so just the indices (0,4,8) would not be possible i guess.
Is there a quicker way to do it instead of iterating using for loop.
You can use numpy's slicing, simply start:stop:step.
>>> xs
array([1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4])
>>> xs[1::4]
array([2, 2, 2])
This creates a view of the the original data, so it's constant time. It'll also reflect changes to the original array and keep the whole original array in memory:
>>> a
array([1, 2, 3, 4, 5])
>>> b = a[::2] # O(1), constant time
>>> b[:] = 0 # modifying the view changes original array
>>> a # original array is modified
array([0, 2, 0, 4, 0])
so if either of the above things are a problem, you can make a copy explicitly:
>>> a
array([1, 2, 3, 4, 5])
>>> b = a[::2].copy() # explicit copy, O(n)
>>> b[:] = 0 # modifying the copy
>>> a # original is intact
array([1, 2, 3, 4, 5])
This isn't constant time, but the result isn't tied to the original array. The copy also contiguous in memory, which can make some operations on it faster.
a[::4]. Cf, say, this.