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pythonlistexception

Why can't I build a list by assigning each element in turn? How can I add (append) the elements without getting an IndexError?


I tried writing some code like:

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

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

But I get an error message that says IndexError: list assignment index out of range, referring to the j[k] = l line of code. Why does this occur? How can I fix it?


Solution

  • 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.