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  1. from copy import deepcopy
  2. class Stack:
  3.     def __init__(self):
  4.         self._values = []
  5.         return
  6.  
  7.     def is_empty(self):
  8.         if self._values == []:
  9.             r = True
  10.         else:
  11.             r=False
  12.         return r
  13.  
  14.     def push(self, value):
  15.         self._values.append(deepcopy(value))
  16.         return
  17.  
  18.     def pop(self):
  19.         assert len(self._values) > 0, "Cannot pop from an empty stack"
  20.         value=self._values.pop()
  21.         return value
  22.  
  23.     def peek(self):
  24.         assert len(self._values) > 0, "Cannot peek at an empty stack"
  25.         value=self._values[-1]
  26.         return value
  27.  
  28. def reroute(opstring, values_in):
  29.     """
  30.     -------------------------------------------------------
  31.     Reroutes values in a list according to a operating string and
  32.     returns a new list of values. values_in is unchanged.
  33.     In opstring, 'X' means push onto stack,
  34.     'S' means pop from stack into values_out.
  35.     Use: values_out = reroute(opstring, values_in)
  36.     -------------------------------------------------------
  37.     Preconditions:
  38.         opstring - String containing only 'S' and 'X's (str)
  39.         values_in - A valid list (list of ?)
  40.     Postconditions:
  41.         returns
  42.         values_out - if opstring is valid then values_out contains a
  43.             reordered version of values_in, otherwise returns 
  44.             None (list of ?)
  45.     -------------------------------------------------------
  46.     """
  47.     st=Stack()
  48.     vo=[]
  49.     i=0
  50.     j=0
  51.     while i<len(opstring):
  52.     	if opstring[i]=='X':
  53.     		st.push(values_in[j])
  54.     		j+=1
  55.     	elif opstring[i]=='S':
  56.     		if st.is_empty()==False:
  57.     			vo=vo.append(st.pop())
  58.     		else:
  59.     			return None
  60.     	i+=1
  61.     return vo
  62.  
  63. out=[]
  64. out= reroute("XXSSXXSS",[1,2,3,4])
  65. print out
Runtime error #stdin #stdout #stderr 0.01s 9008KB
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stdin
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Standard input is empty
stdout
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Standard output is empty
stderr
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Traceback (most recent call last):
  File "prog.py", line 64, in <module>
  File "prog.py", line 57, in reroute
AttributeError: 'NoneType' object has no attribute 'append'
https://ideone.com/xVUuh0
language:
Python (cpython 2.7.16)
created:
7 years ago
visibility:
public

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