I am playing with the NumPy library and there is a behaviour that I do not understand:
I defined a simple vector Y:
Y = np.array([6,7,8],dtype = np.int16)
Then, I tried to add an additional element at the end of the array using the np.insert() method. I specified the position to be at -1:
Z = np.insert(Y,(-1),(19))
My element was inserted not on the last, but on the penultimate position:
Could somebody explain my why this element was not inserted at the end of my array?
Negative indexes are calculated using len(l) + i (with l being the object and i being the negative index).
For example, with a list x = [1, 2, 3, 4] the index -2 is calculated as 4-2 = 2, so x[-2] returns 3
In this example, the index -1 is calculated as index 2
Therefore, 19 is inserted at index 2, the penultimate position. To fix this, use Z = np.insert(Y, len(Y), 19).
PS - this is the same with list.insert, not just with numpy arrays
Edit (suggested by Homer512): np.append may be easier than np.insert if you want to add to the end of an array: Z = np.append(Y, 19)
