I have a simple numpy array, and I want to make a separate array that takes every two elements per two indices
For example:
x = np.arange(0,20)
print(x)
[ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19]
My goal is to get an array of
[2 3 6 7 10 11 14 15 18 19]
How might I do that? I tried this:
print(x[1:len(x)-1:2])
[ 1 3 5 7 9 11 13 15 17]
but I only get every other index.
You can simply do this using the traditional start:stop:step convention without any modulo by reshaping your array, indexing, and then flattening it back. Try this -
x.reshape(-1,2)[1::2].flatten()
array([ 2, 3, 6, 7, 10, 11, 14, 15, 18, 19])
This should be significantly faster than approaches where mathematical operations are being used to check each value since this is just reshaping and indexing.
You can do something like this:
print([x[i] for i in range(len(x)) if not(((i - 2) % 4) and ((i - 3) % 4))])
You could use the modulo % to find the latter 2 elements of every group of four:
>>> x[np.isin(x % 4, [2,3])]
array([ 2, 3, 6, 7, 10, 11, 14, 15, 18, 19])
If your elements aren't a range (and thus their value isn't equal to their index), you could take a new arange:
>>> x = x[::-1] # reverse x
>>> x[np.isin(np.arange(len(x)) % 4, [2,3])]
array([17, 16, 13, 12, 9, 8, 5, 4, 1, 0])
You can simply do this to get these numbers in one line:
print([x[i] for i in range(len(x-2)) if (i+2)%4==0 or (i+1)%4==0])