Why does the value of the variable change?-CodePudding

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?

CodePudding user response：

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()

CodePudding user response：

Reusing variable names is usually seen as a bad practice in Python.

In your case I would suggest, that you simply assign your manipulated array to a new variable. This should solve your issue, as you can then simply reference your initial values in your next calculation step.

CodePudding user response：

Numpy arrays are referenced when sliced that means modification will update all references, so copy the content

so you can either call np.copy(SP[:,0]) or SP[:,0].copy() whichever you feel easy for you.

Working Example

import numpy as np
import math as m

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

dt=0.1

SP0 = np.copy(SP[:,0])
print("SP0 = ", SP0)
SP[:,0] = SP[:,0] * m.cos(dt)   SP[:,1] * m.sin(dt)

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

Output

SP0 =  [-0.4  -0.34 -0.22  0.8   0.92]
SP0 =  [-0.4  -0.34 -0.22  0.8   0.92]