Why is states changing when I increment an index in banks?

Question

def maxdex(banks):
  high = 0
  maxdex = 0
  for i in enumerate(banks):
    if(i[1] > high):
      maxdex,high = i[0],i[1]
  return([maxdex,high])
banks = [14,0,15,12,11,11,3,5,1,6,8,4,9,1,8,4]
states = []
cycles = 0
while True:
  states = states + [banks]
  maxer = maxdex(banks)
  blocks = maxer[1]
  target = (maxer[0]+1)%len(banks)
  while blocks > 0:
    banks[target] = banks[target]+1
    print(banks)
    print(states)
    target = (target+1)%len(banks)
    blocks -= 1
  cycles += 1
  if(banks in states):
    break
print(cycles)

When I run the code, states is changing when I try to increment a specific index in banks. What's going on?

(Btw, I'm solving a problem from Advent of code)

`states = states + [banks]` This does not make a _copy_ of banks; it uses the _actual object_ banks. So of course when you change banks, states also changes. — John Gordon, May 11 '21 at 17:01

score 0 · Answer 1 · edited Jun 01 '21 at 20:10

How about this?

def maxdex(banks):
  high = 0
  maxdex = 0
  for i in enumerate(banks):
    if(i[1] > high):
      maxdex,high = i[0],i[1]
  return([maxdex,high])
banks = [14,0,15,12,11,11,3,5,1,6,8,4,9,1,8,4]
states = []
cycles = 0
while True:
    #list.copy implemented
  states = states + list.copy(banks)
  maxer = maxdex(banks)
  blocks = maxer[1]
  target = (maxer[0]+1)%len(banks)
  while blocks > 0:
    banks[target] = banks[target]+1
    print(banks)
    print(states)
    target = (target+1)%len(banks)
    blocks -= 1
  cycles += 1
  if(banks in states):
    break
print(cycles)

Why is states changing when I increment an index in banks?

1 Answers1