CODEWITHSUNDEEP
pythonpython-2.7
Patrick Poitras
Patrick Poitras

Reputation: 376

Operating on one list also changes the other

I solved Project Euler's Problem 67 a few months back with a wildly inefficient algorithm. Today, I realized how this could have been done better, and while trying to make a more efficient algorithm, I ran into some problems.

The code is this:

r_prev = [59,0,0]
r_load = [73,41,0]
r = [0,0,0]
zero_r = [0,0,0]
rows = 3
while rows != 0:
    position = 0
    for x in r_load:
        try:
            if r_prev[position] > r_prev[position-1]:
                r[position] = r_prev[position] + r_load[position]
            else:
                r[position] = r_prev[position-1] + r_load[position]
        except IndexError:
            r[position] = r_prev[position] + r_load[position]
        position += 1
    print r
    rows -= 1
    r_load = input("Enter next row")
    r_prev = r
    r_prev.append(0)
    zero_r.append(0)
    r = zero_r

It works by keeping only the sum of the highest path to get to a point. I am feeding it input manually at this point (via the console). When fed the first line, it performs as expect, but when fed the second line, it considers r_prev and r as the same thing, and every operation that is performed on r_prev is also performed on r.

How do I solve this problem?

Upvotes: 0

Views: 70

Answers (1)

BrenBarn
BrenBarn

Reputation: 251355

When you do r_prev = r you are not copying the list. You are giving the same list an additional name. To make a copy, do r_prev = list(r).

Upvotes: 4

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