Changing a variable in a list does not affect the list

Question

I've been attempting to write some code to tune variables in my chess program, and I found that this code doesn't do what I expect it to do at all.

import random

# Knight value, bishop value, rook value, queen value
values = [300, 300, 500, 900]

e1vals = values
e2vals = values    

# Add a gaussian distributed random number to it
deltas = []
for i in range(0, len(values)):
    x = random.gauss(0, 20)
    deltas.append(x)

for i in range(0, len(values)):
    e1vals[i] = values[i] + deltas[i]
    e2vals[i] = values[i] - deltas[i]

print(e1vals)
print(e2vals)

Intuitively, the code here should simply add or subtract the values in deltas to e1vals and e2vals, but instead it doesn't make any change other than casting values to float.

I'm using Python 3.5.1 if that makes any difference.

@GWW No, I'm pointing out that adding to the list variables isn't changing the list. — ZirconiumX, Jun 25 '16 at 17:09
@ZirconiumX Exactly, See the dupe and create a new copy of the list by using `e1vals = values[:]`. — Bhargav Rao, Jun 25 '16 at 17:10
@Zirconium `e1values = values[:]` and `e2values = values[:]` presuming `e1vals` is supposed to be `e1values` etc.. — Padraic Cunningham, Jun 25 '16 at 17:10
@PadraicCunningham They're supposed to represent the piece values for engine 1 and engine 2 — ZirconiumX, Jun 25 '16 at 17:11
@ZirconiumX, I meant more along the lines that I don't see those names define anywhere — Padraic Cunningham, Jun 25 '16 at 17:11

score 1 · Answer 1 · answered Jun 25 '16 at 17:08

1

The problem is that e1values and e2values refer to the same list. So all your code does is add a value to each item in the list, then subtract it again, leaving you with the original value.

answered Jun 25 '16 at 17:08

Daniel Roseman

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Right, so e1values and e2values are references, not copies? – ZirconiumX Jun 25 '16 at 17:10
1

Correct, assignment always creates a reference. – Daniel Roseman Jun 25 '16 at 17:11

Changing a variable in a list does not affect the list

1 Answers1