- vs -= operators with numpy

Question

I\'m having some strange behavior in my python code related to - and -=. I\'m writing a QR decomposition using numpy, and have the following line o

Answer 1

When v is a slice, then v -= X and v = v - X produce very different results. Consider

>>> x = np.arange(6)
>>> v = x[1:4]
>>> v -= 1
>>> v
array([0, 1, 2])
>>> x
array([0, 0, 1, 2, 4, 5])

where v -= 1 updates the slice, and therefore the array that it views, in-place, vs.

>>> x = np.arange(6)
>>> v = x[1:4]
>>> v = v - 1
>>> v
array([0, 1, 2])
>>> x
array([0, 1, 2, 3, 4, 5])

where v = v - 1 resets the variable v while leaving x untouched. To obtain the former result without -=, you'd have to do

v[:] = v - 1