list index out of range: python

Question

I have a list which has these elements(list consists str(elements)):

['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '13', '14', '15']

and I want to process it to have an output like this

['-0', '1#', '15#']

If the element is -xi want to leave it there so I take the last 2 elements and if the gap is 1 then remove the element before the last element. Here is the code:

            for k in range(len(l1)):
                if "-" in (l1[-k] or l1[-k-1]):
                    print("debuggggg")
                    pass
                elif(int(l1[-k]) - int(l1[-k-1])== 1 and int(l1[-k]) - int(l1[-k-2])== 2) :
                    a= l1[-2]
                    print(a)
                    l1.remove(a)
                    #print("debug 2")
                elif(int(l1[-k]) - int(l1[-k-1])== 1):
                    a= l1[-2]
                    l1.remove(a)
                    l1[-2] = l1[-2] +"#"
                    l1[-1] = l1[-1] +"#"
                    print("3")
                #elif(type(l1[-2]) is str):
                    #pass

Problem is here :

 debuggggg
    14
    13
    12
    11
    10
    9
    8

The last 2 elements do not include char "-" but it seems that they do. Furthermore after the 8 loop script crashes:

 elif(int(l1[-k]) - int(l1[-k-1])== 1 and int(l1[-k]) - int(l1[-k-2])== 2) :
IndexError: list index out of range

but it is not out the range of the list. What is the problem?

Answer 1

You're removing items from the list. If you start with l1 containing ten items then k is going to go from 0 to 9, but if you remove an item from l1 then l1[9] no longer exists.

Also I think if "-" in (l1[-k] or l1[-k-1]): is testing whether "-" is in the logical OR of l1[-k] and l1[-k-1] , which I don't think is what you intended?

Answer 2

You should rather have a source list that you iterate over and a results list that you insert elements into.

source_list = ['-1', '1', '2', '3']
results_list = []

for item in source_list:
    # Do some tests, and put items into the results list if you want them.
    # I think this is what you want, although I couldn't understand from your question.
    if '-' in item:
        results_list.append(item)
    else:
        results_list.append(item + '#')

If there is some other requirement then let me know, and I will try to adapt my example.

Answer 3

Your second question

but it is not out the range of the list. What is the problem?

It is just out of range error.

l1[-len(l1)-1]

raises this error. You are doing this in your code.

for k in range(len(l1)):
    ...
    elif(int(l1[-k]) - int(l1[-k-1])== 1 and int(l1[-k]) - int(l1[-k-2])== 2) :#<--here(l1[-k-2])

Answer 4

yes It is IndexError: list index out of range. Because you are trying to access l1[-10] at line 5, which does not exist in l1 after 8 iterations.

See the following debug info:

>>> l1 = ['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '13', '14', '15']
>>> for k in range(len(l1)):
...     if "-" in (l1[-k] or l1[-k-1]):
...         print("debuggggg")
...         pass
...     elif(int(l1[-k]) - int(l1[-k-1])== 1 and int(l1[-k]) - int(l1[-k-2])== 2) :
...         a= l1[-2]
...         print(a)
...         l1.remove(a)
...         print l1
...         #print("debug 2")
...     elif(int(l1[-k]) - int(l1[-k-1])== 1):
...         a= l1[-2]
...         l1.remove(a)
...         l1[-2] = l1[-2] +"#"
...         l1[-1] = l1[-1] +"#"
...         print("3")
...     #elif(type(l1[-2]) is str):
...         #pass
... 
debuggggg
14
['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '13', '15']
13
['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '12', '15']
12
['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '11', '15']
11
['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '10', '15']
10
['-0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '15']
9
['-0', '1', '2', '3', '4', '5', '6', '7', '8', '15']
8
['-0', '1', '2', '3', '4', '5', '6', '7', '15']
Traceback (most recent call last):
  File "<stdin>", line 5, in <module>
IndexError: list index out of range

I have just printed list l1. After 8 iterations,int(l1[-k-2] is not accessible. This is the reason why u are getting Index error