Why does this iterative list-growing code give IndexError: list assignment index out of range?

Question

Please consider the following code:

i = [1, 2, 3, 5, 8, 13]
j = []
k = 0

for l in i:
    j[k] = l
    k += 1

print j

The output (Python 2

Answer 1

Do j.append(l) instead of j[k] = l and avoid k at all.

Answer 2

j.append(l)

Also avoid using lower-case "L's" because it is easy for them to be confused with 1's

Answer 3

Maybe you need extend()

i=[1,3,5,7]
j=[]
j.extend(i)

Answer 4

Your other option is to initialize j:

j = [None] * len(i)

Answer 5

j is an empty list, but you're attempting to write to element [0] in the first iteration, which doesn't exist yet.

Try the following instead, to add a new element to the end of the list:

for l in i:
    j.append(l)

Of course, you'd never do this in practice if all you wanted to do was to copy an existing list. You'd just do:

j = list(i)

Alternatively, if you wanted to use the Python list like an array in other languages, then you could pre-create a list with its elements set to a null value (None in the example below), and later, overwrite the values in specific positions:

i = [1, 2, 3, 5, 8, 13]
j = [None] * len(i)
#j == [None, None, None, None, None, None]
k = 0

for l in i:
   j[k] = l
   k += 1

The thing to realise is that a list object will not allow you to assign a value to an index that doesn't exist.

Answer 6

You could also use a list comprehension:

j = [l for l in i]

or make a copy of it using the statement:

j = i[:]