when I save one array of a matrix into another variable, when I change the value of matrix, the vaule of another variable was also changed. I don't know why.
import numpy as np
a = np.array([[1,2,3],[4,5,6]])
b = a[1,:]
print a
print b
a[1,:] = a[1,:] - a[0,:]
print a
print b
the results is
[[1 2 3]
[4 5 6]]
[4 5 6]
[[1 2 3]
[3 3 3]]
[3 3 3]
in this script, the value of b was also changed when a changed.
You need read more about shallow copy:
Assignment statements in Python do not copy objects, they create bindings between a target and an object. For collections that are mutable or contain mutable items, a copy is sometimes needed so one can change one copy without changing the other. This module provides generic shallow and deep copy operations.
The difference between shallow and deep copying is only relevant for compound objects (objects that contain other objects, like lists or class instances):
A shallow copy constructs a new compound object and then (to the extent possible) inserts references into it to the objects found in the original.
- A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.
Example:
>>> lst1 = ['a','b',['ab','ba']]
>>> lst2 = lst1[:]
>>> lst2[0] = 'c'
>>> lst2[2][1] = 'd'
>>> print(lst1)
['c', 'b', ['ab', 'd']]
Now, You must use deepcopy, for example:
>>>from copy import deepcopy
>>>lst1 = ['a','b',['ab','ba']]
>>>lst2 = deepcopy(lst1)
>>>lst2[2][1] = "d"
>>>lst2[0] = "c";
>>>print lst2
>>>print lst1
['c', 'b', ['ab', 'd']]
['a', 'b', ['ab', 'ba']]
You can read more about Shallow and Deep Copy.