ValueError: invalid literal for int() with base 10: '' - string to int conversion

Question

I am trying to convert a numeric string to int, but I keep getting the following:

Traceback (most recent call last):
  File "C:/Python34/Euler36.py", line 16, in <module>
    rev = int(rev)
ValueError: invalid literal for int() with base 10: ''

I've looked at several threads for similar errors, but none that seem to be having trouble with an empty string value: ''

I'm just using ''.join() to concatenate the list into a string, and thought I could use int() to convert the string to int. Seemingly not. Here's the code:

num = 0
temp = 0
add_digit = 0
rev = []
length = 100 #All numbers less than this

while num < length:
    temp = num
    rev = []
    while temp > 0:
        add_digit = temp % 10
        temp /= 10
        temp = int(temp)
        rev.append(str(add_digit))
    rev = ''.join(rev)
    rev = int(rev)
    print(rev)
    num += 1

print("Done.")

Answer 1

You start with num = 0 . You then set temp = num , so temp is zero. Thus your while temp > 0 loop never runs, and so nothing is ever added to rev . So when you try to use join , it just joins an empty list and produces an empty string.

How to fix this is unclear, since you don't say what you want the code to do.

Answer 2

num in the first iteration is 0, and your while is not entered. Thus rev is an empty string.