CODEWITHSUNDEEP
pythonlist
menrva
menrva

Reputation: 63

List changes even without touching it in for loop

So I'm doing day 13 of advent of code and ran into this problem and can't seem to understand what's happening.

Here's my code:

road = open('day13t.txt').read().strip().split('\n')

ogroad = [ list(x) for x in road ]
for i,r in enumerate(ogroad):
    for j,c in enumerate(r):
        if c == '>' or c == '<':
            ogroad[i][j] = '-'
        if c == '^' or c == 'v':
            ogroad[i][j] = '|'

rdict = {'-':'>', '\\':'v', '/':'^'}
ddict = {'|':'v', '\\':'>', '/':'<'}
ldict = {'-':'<', '\\':'^', '/':'v'}
udict = {'|':'^', '\\':'<', '/':'>'}

test = [ list(x) for x in road ]
nroad = [ list(x) for x in road ]
for i in range(3):
    for i, l in enumerate(test):
        for j, c in enumerate(l):
            if c == '>':
                ns = ogroad[i][j+1]
                nroad[i][j+1] = rdict[ns]

            if c == '<':
                ns = ogroad[i][j-1]
                nroad[i][j-1] = ldict[ns]

            if c == 'v':
                ns = ogroad[i+1][j]
                nroad[i+1][j] = ddict[ns]

            if c == '^':
                ns = ogroad[i-1][j]
                nroad[i-1][j] = udict[ns]

    test = list(nroad)
    nroad = list(ogroad)
    xroad = [ ''.join(x) for x in ogroad ]
    for l in xroad:
        print(l)

So these lists seem to have taken on a life of their own because in the last few lines of the outermost for loop I print out the contents of xroad which is basically ogroad. And I don't even touch ogroad at all in the for loops but it gives a different output every iteration.

Input I use:

/->-\        
|   |  /----\
| /-+--+-\  |
| | |  | v  |
\-+-/  \-|--/
  \------/

Upvotes: 1

Views: 61

Answers (1)

Michael Butscher
Michael Butscher

Reputation: 10969

list() only creates a flat copy. This means lists in the list aren't copied but shared. Example in shell:

>>> t=[[42]]
>>> t2 = list(t)
>>> t is t2
False
>>> t[0] is t2[0]
True
>>> t2[0][0] = 43
>>> t
[[43]]

Upvotes: 3

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