Nrthugu

I am new to python.
Error appears when I run this code:

list=

list2=

list.append("p")

list.append("&")

list.append("k")

print(list)

operator="&" or "|" or">" or "=" or "~"

prop="p" or "q " or "r"

#a=0

for i in list:

     if i == operator:

 #        a = list.index(i)

  #       print(a - 1)

         nextelem = list[list.index(i) + 1]

         prevelem = list[list.index(i) - 1]

         print(nextelem)

         print(prevelem)

         list.remove(i)

         list2.append(i)

     if nextelem==prop:

           print("voici",nextelem)

           list2.append(nextelem)

           list2.append(prevelem)

print(list2)

The error message specified:

Traceback (most recent call last):

File "C:/Users/PC/PycharmProjects/LIAATP1/TP01.py", line 114, in
if nextelem==prop:
NameError: name 'nextelem' is not defined

for example, with a formula: p & q | r

Expected return:

[|,&,r,p,q]

[root,leftchild,rightchild,leftchildof'&',rightchildof'&']

Strings evaluate to True in Python unless they're empty, so operator here has the value '&' (since or will return the first argument to it that evaluates to True). Testing it on the command line:

operator = "&" or "|" or ">" or "=" or "~"

print(operator)

# '&'

So every time you loop over list (which you should rename, since list is a reserved word), you check whether i == '&'. Since it never does, the code in your first if block is never executed; since that code is never executed, nextelem is never set.

Which is why you end up with this:

NameError: name 'nextelem' is not defined

Regarding the rest of your code, the logic has fundamental issues that will prevent it from doing what you expect, even if you do get it to run without throwing an exception.

Before you attempt to rewrite it, I would strongly suggest going through the official Python tutorial to gain a firm understanding of the data and control structures you'll need to implement that algorithm.

Solely for the sake of providing an example of valid list handling, and ignoring the '~' operator and operator precedence...

input_string = "p & q | r"



values = input_string.split(" ")

print(values)

# ['p', '&', 'q', '|', 'r']



print(values[-2::-2] + values[-1::-2])

# ['|', '&', 'r', 'q', 'p']

values[-1] gives the last element in a list. values[-1::-2] returns an array slice starting at the end and stepping backwards by 2 elements at a time until all elements have been processed. So, in this case, that will result in ['r', 'q', 'p'].

values[-2], then, will start at the second to last element in the list. Stepping back by two again, values[-2::-2] returns ['|', '&']. Concatenating these lists gives you the result you're looking for, though only in very specific circumstances; a general-case solution is something you'll need to work on once you're comfortable with the basics of Python (and come back to SO when you have specific questions/issues).