Index Error: list index out range

Question

I'm working with Enthought canopy Python 2.7.9.
A very simple Python program that involves for, range, and len:

num = 10
mylist = range(num)
for i in range(len(mylist)):
  print "Size of mylist is %d" %(len(mylist))
  print "i=%d" %(i)
  print "mylist[%d] %d" %(i, mylist[i])
  mylist=mylist[:-1]

What puzzles me is that since the length of mylist has decreased to 5, why index i can still be 5, leading to the index error?

I know in Python a for-loop works with an iterable. But I do not know how this mechanism works in this example.

As a side note, you should generally use the more idiomatic for item in my_list. Check out enumerate() if you also need and index. — foresmac
– foresmac, Commented Jan 13, 2016 at 2:12

Mike Müller · Accepted Answer · 2016-01-13 01:53:44Z

2

Never change the size of a list while iterating over it. Remove this line:

mylist=mylist[:-1]

and it will work.

The length of the list is only determine once here:

for i in range(len(mylist)):

and i will take on all values from 0 through 9. When you change the size of the list later, the length will not be recalculated.

answered Jan 13, 2016 at 1:53

Mike Müller

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1 Comment

foresmac Over a year ago

To add to what Mike is saying here, at the start of the loop my_list is an identifier for a list of 10 items. Since you're using len() on that list, your loop will iterate 10 times. But inside the loop, you are reassigning the my_list identifier to point to a smaller and smaller list each iteration.

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