IndexError: list index out of range

[python101]

My code

s=['aaaa','cccccccc','dddd','ssss','xxxxx','aaaa','ssx','ssx','cc','cc']
 
for k in range(len(s)):
    st=s.count(s[k])
    if st>=2:
       print s.pop(s.index(s[k]))
 
Traceback (most recent call last):
  File "C:/Python30/test.py", line 4, in <module>
    st=s.count(s[k])
IndexError: list index out of range
 

how can I fix the error
IndexError: list index out of range

[bartonc]

My code

s=['aaaa','cccccccc','dddd','ssss','xxxxx','aaaa','ssx','ssx','cc','cc']
 
for k in range(len(s)):
    st=s.count(s[k])
    if st>=2:
       print s.pop(s.index(s[k]))
 
Traceback (most recent call last):
  File "C:/Python30/test.py", line 4, in <module>
    st=s.count(s[k])
IndexError: list index out of range
 

how can I fix the error
IndexError: list index out of range

That would be because of the pop(). When you shorten the list that you are indexing on you cause the error.

By using a slice for beginning to end, you work on a copy. I threw in some other options which avoid the index all together:

>>> s=['aaaa','cccccccc','dddd','ssss','xxxxx','aaaa','ss  x','ssx','cc','cc']
>>> for item in s[:]:
...     n = s.count(item)
...     if n >= 2:
...         print item
...         s.remove(item)
...         
aaaa
cc
>>> s
['cccccccc', 'dddd', 'ssss', 'xxxxx', 'aaaa', 'ss  x', 'ssx', 'cc']
>>> 

[Motoma]

That would be because of the pop(). When you shorten the list that you are indexing on you cause the error.

By using a slice for beginning to end, you work on a copy. I threw in some other options which avoid the index all together:

>>> s=['aaaa','cccccccc','dddd','ssss','xxxxx','aaaa','ss  x','ssx','cc','cc']
>>> for item in s[:]:
...     n = s.count(item)
...     if n >= 2:
...         print item
...         s.remove(item)
...         
aaaa
cc
>>> s
['cccccccc', 'dddd', 'ssss', 'xxxxx', 'aaaa', 'ss  x', 'ssx', 'cc']
>>> 

Just as a note:

for item in s[:]:
 

is exactly the same as

for item in s:
 

The only reason I mention this is that when learning Python, I often stumbled when I would encounter the whole array-subarray-step-bracket-thingies.

[KaezarRex]

My code

s=['aaaa','cccccccc','dddd','ssss','xxxxx','aaaa','ssx','ssx','cc','cc']
 
for k in range(len(s)):
    st=s.count(s[k])
    if st>=2:
       print s.pop(s.index(s[k]))
 
Traceback (most recent call last):
  File "C:/Python30/test.py", line 4, in <module>
    st=s.count(s[k])
IndexError: list index out of range
 

how can I fix the error
IndexError: list index out of range

Here is a way to get rid of duplicate items in a list that probably isn't good form, but I thought it was interesting. I put everything in a dictionary, which can't have duplicate keys, and then retrieved the list of keys.

>>> s=['aaaa','cccccccc','dddd','ssss','xxxxx','aaaa','ssx','ssx','cc','cc']
>>> d=dict([(s[i], i) for i in range(len(s))])
>>> s=d.keys()
>>> print s
['cccccccc', 'ssss', 'ssx', 'cc', 'aaaa', 'dddd', 'xxxxx']

If you were using an ordered list this way wont work because the dictionary keys are in an unordered list.

[KaezarRex]

Just as a note:

for item in s[:]:
 

is exactly the same as

for item in s:
 

The only reason I mention this is that when learning Python, I often stumbled when I would encounter the whole array-subarray-step-bracket-thingies.

It's actually different (and very useful) because "s[:]" creates a whole new list (a deep copy) to iterate through, instead of iterating over "s" itself. That way the loop can modify "s" without throwing an error.

[Motoma]

It's actually different (and very useful) because "s[:]" creates a whole new list (a deep copy) to iterate through, instead of iterating over "s" itself. That way the loop can modify "s" without throwing an error.

Oh man...Owned.

You learn something new every day. Thanks for the lesson KaesarRex!

[bartonc]

It's actually different (and very useful) because "s[:]" creates a whole new list (a deep copy) to iterate through, instead of iterating over "s" itself. That way the loop can modify "s" without throwing an error.

Actually, Motoma has a point. When working with arrays, the elements are passed by reference (even in a slice). Lists and arrays are not interchangeable that way!

[KaezarRex]

Actually, Motoma has a point. When working with arrays, the elements are passed by reference (even in a slice). Lists and arrays are not interchangeable that way!

Looks like I've learned something too. Thanks for the clarification.