Python Loop List Index Out of Range

Question

Given the following list  a    0  1  2  3    I d like to create a new list b  which consists of elements for which the current and next value of a are summed  It will contain 1 less element than a   Like this   b    1  3  5     from 0 1  1 2  and 2 3   Here s what I ve tried   b      for i in a     b append a i   1  - a i   b   The trouble is I keep getting this error   IndexError  list index out of range   I m pretty sure it occurs because by the time I get the the last element of a  3   I can t add it to anything because doing so goes outside of the value of it  there is no value after 3 to add   So I need to tell the code to stop at 2 while still referring to 3 for the calculation

User · Answer

Try reducing the range of the for loop to range(len(a)-1):

a = [0,1,2,3]
b = []

for i in range(len(a)-1):
    b.append(a[i]+a[i+1])

print(b)

This can also be written as a list comprehension:

b = [a[i] + a[i+1] for i in range(len(a)-1)]
print(b)

User · Answer

When you call for i in a   you are getting the actual elements  not the indexes   When we reach the last element  that is 3  b append a i 1 -a i   looks for a 4   doesn t find one and then fails   Instead  try iterating over the indexes while stopping just short of the last one  like   for i in range 0  len a -1   Do something  Your current code won t work yet for the do something part though

User · Answer

You are accessing the list elements and then using them to attempt to index your list  This is not a good idea  You already have an answer showing how you could use indexing to get your sum list  but another option would be to zip the list with a slice of itself such that you can sum the pairs    b    i   j for i  j in zip a  a 1

User · Answer

In your for loop  you re iterating through the elements of a list a  But in the body of the loop  you re using those items to index that list  when you actually want indexes  Imagine if the list a would contain 5 items  a number 100 would be among them and the for loop would reach it  You will essentially attempt to retrieve the 100th element of the list a  which obviously is not there  This will give you an IndexError      We can fix this issue by iterating over a range of indexes instead   for i in range len a     and access the a s items like that  a i   This won t give any errors    In the loop s body  you re indexing not only a i   but also a i 1   This is also a place for a potential error  If your list contains 5 items and you re iterating over it like I ve shown in the point 1  you ll get an IndexError  Why  Because range 5  is essentially 0 1 2 3 4  so when the loop reaches 4  you will attempt to get the a 5  item  Since indexing in Python starts with 0 and your list contains 5 items  the last item would have an index 4  so getting the a 5  would mean getting the sixth element which does not exist    To fix that  you should subtract 1 from len a  in order to get a range sequence 0 1 2 3  Since you re using an index i 1  you ll still get the last element  but this way you will avoid the error    There are many different ways to accomplish what you re trying to do here  Some of them are quite elegant and more  pythonic   like list comprehensions    b    a i    a i 1  for i in range len a  - 1    This does the job in only one line

[python] Python Loop: List Index Out of Range

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