New value is copied to the whole column

Asked Apr 29 '21 at 19:41

Active Apr 29 '21 at 19:41

Viewed 19 times

First of all, I think this is not a variable address copying issue. I have this code :

x = [1, 1, 0, 0]
b_u = [[0.2 for _ in range(3)]] * 4
for i in range(4):
    # The problem is on line below. This line copying new value to the whole column.
    b_u[i][0] = (2 * x[i]) / (1 + sum(x))
print(b_u)

And you can see whats happening in lines below :

# b_u :
# Before loop : [[0.2, 0.2, 0.2], [0.2, 0.2, 0.2], [0.2, 0.2, 0.2], [0.2, 0.2, 0.2]]
# Expected : [[0.67, 0.2, 0.2], [0.67, 0.2, 0.2], [0, 0.2, 0.2], [0, 0.2, 0.2]]
# After loop : [[0.0, 0.2, 0.2], [0.0, 0.2, 0.2], [0.0, 0.2, 0.2], [0.0, 0.2, 0.2]]

Where am I mistaken? Is it a typo?

asked Apr 29 '21 at 19:41

Soroosh Noorzad

1

Don't use `*` on lists unless the contents of the list are immutable, or this behavior is wanted. – Carcigenicate Apr 29 '21 at 19:43
1

Yes... Thank you. I didn't know what can be the name of this issue? TNX anyway. – Soroosh Noorzad Apr 29 '21 at 19:44
2

I don't know a succinct name for it, but it's saving and using multiple references to the same object instead of having multiple references to multiple objects. – Carcigenicate Apr 29 '21 at 19:45
2

@Carcigenicate Maybe "reference copying instead of value copying"? Still not super succinct though. – wjandrea Apr 29 '21 at 19:48
1

@wjandrea Ya, the reference/value differentiation is probably as good as it gets. – Carcigenicate Apr 29 '21 at 19:49

New value is copied to the whole column

0 Answers0