If I run the following code in Python 2.7, I get [2., 2., 2.] printed for both a and b. Why does b change together with a? Many thanks!
def test_f(x):
a = np.zeros(3)
b = a
for i in range(3):
a[i] += x
print a
print b
return 0
test_f(2)
Because b and a are referring to the same list in memory. b = a does not create a new copy of a.
Try this and see the difference:
def test_f(x):
a = np.zeros(3)
b = a.copy()
for i in range(3):
a[i] += x
print a
print b
return 0
test_f(2)
b = a.copy() will create a new copy that exactly resembles the elements of a, whereas b=a just creates a new reference to the exisiting list.
b=a.copy() or b=np.array(a, copy=True) would be better since you explicitly mention that a copy is made.
[::] only needs one colon
numpy will use a pointer to copy unless you tell it otherwise:
import numpy as np
def test_f(x):
a = np.zeros(3)
b = np.copy(a)
for i in range(3):
a[i] += x
print a
print b
return 0
test_f(2)
[ 2. 2. 2.]
[ 0. 0. 0.]
