[FIXED] Why does the value of the variable change?

Issue

Here is the minimal version of my code:

SP=np.array([[-0.4,.2,1],[-.34,.31,1],[-.22,-.2,0],[.8,.25,0],[.92,-.42,1]])

dt=0.1

SP0=SP[:,0]
print("SP0 = ",SP0)
SP[:,0]=SP[:,0]*cos(dt)+SP[:,1]*sin(dt)
print("SP0 = ", SP0)
SP[:,1]=SP[:,1]*cos(dt)-SP0*sin(dt)

OUTPUT:

SP0 =  [-0.4  -0.34 -0.22  0.8   0.92]
SP0 =  [-0.37803498 -0.30735306 -0.2388676   0.82096169  0.8734738 ]

As can be seen above, I am trying to evolve SP[:,0] and SP[:,1] depending on their previous values but in order to put the previous value of SP[:,0] in SP[:,1] (since it gets changed before moving on to SP[:,1] ) I first store it in SP0.

But this doesn't seem to make any difference because the value of SP0 also changes to the new SP[:,0] after I change SP[:,0] later. But this is weird since SP[:,0] only changes after I store it's value in SP0.

Why is this happening and how do I evolve the SP[:,1] and SP[:,0] correctly?

Solution

Numpy slices don't create new arrays; they return views into the original array. Modifying either will affect the other.

To keep the previous value, call .copy() on the view:

SP0=SP[:,0].copy()

Answered By - Anonymous12358