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I'm a student and am just beginning to learn code. Right now I'm working with Python and have a program I think should work, but just returns some errors that I don't understand:

Traceback (most recent call last): File "C:\Program Files\Notepad++\1913lab3.py", line 23, in print(most(odd, []))

File "C:\Program Files\Notepad++\1913lab3.py", line 9, in most N = S[i] UnboundLocalError: local variable 'i' referenced before assignment

I don't understand what the first error tells me, but the I think I understand the second one, but I don't understand why I'm getting it. I don't think i is a local variable as I defined it right away in the beginning. Here's the code:

t = 0
f = 0
i = 0

def odd(N):
    return N % 2 != 0

def most(P, S):
    N = S[i]
    if P == True:
        t += 1
    else:
        f += 1
    i += 1
    if i < len(S):
        most(P, S)
    else:
        if t > f:
            return 'True'
        else:
            return 'False'

print(most(odd, []))
print(most(odd, [0]))
print(most(odd, [1]))
print(most(odd, [1, 2]))
print(most(odd, [1, 2, 3]))

The assignment is to define a recursive function (most()). The function takes one function and one list as its arguments (P and S). I can't use loops or local variables. Here's a quote from the assignment:

"P is a function of one argument that returns either True or False, and S is a list. The function most calls P on each element of S. It must return True if P returns True more often than it returns False. It must return False otherwise."

The 5 print commands are just 5 examples that I need to work for credit, but this program needs to work for all lists. If anyone can help me fix these errors, that'd be great. (Also, any general Python tips would be welcome.)

Austin Chase
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  • Take a look at this answer: http://stackoverflow.com/questions/10851906/python-3-unboundlocalerror-local-variable-referenced-before-assignment#10852003 – Hedegare Feb 14 '17 at 01:05
  • Check out this question: (http://stackoverflow.com/questions/423379/using-global-variables-in-a-function-other-than-the-one-that-created-them ). Your problem is that you're trying to modify global variables inside your functions, but since python doesn't make you declare your variables before you use them, python has no way of knowing if your use of the variables inside the function should be interpreted as creating a new variable that only exists inside the function, or if you mean the global variable with that name. – Christopher Shroba Feb 14 '17 at 01:05
  • @Amorpheuses But he defined `i` globally and this probably lead him to believe that he was using the "global" `i` variable. Also, your comment comes across as a bit rude... – Christian Dean Feb 14 '17 at 01:07

1 Answers1

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By default any variable that is modified within a function is assumed to be local to that function. So if you have i += 1, i must be defined within the function first. Or you have to declare i as a global first (global i) so that python knows it is refering to the i you have defined (first) outside the function. But beware with globals, they are often dangerous (as they make it hard to keep track what is going on) and should be avoided if possible.

Julien
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  • Thank you for the quick response! So it seems that I can solve the issue but inserting "global i" to the top of my function, but doing so is frowned upon and could be problematic. How could I get around using the global command, then? The lab assignment only allows me to use two inputs, P and S, for the function most. – Austin Chase Feb 14 '17 at 01:15
  • You could for example define a helper recursive function that returns the difference between the number of True and False. And then just check the sign of this difference. – Julien Feb 14 '17 at 01:22