Set the index of a Python array

Question

I'm trying to set the index of an array in Python, but it isn't acting as expected:

theThing = []
theThing[0] = 0
'''Set theThing[0] to 0'''

This produces the following error:

Traceback (most recent call last):
  File "prog.py", line 2, in <module>
    theThing[0] = 0;
IndexError: list assignment index out of range

What would be the correct syntax for setting an index of an array in Python?

Here it is on ideone, with the same error: http://ideone.com/0IV1Sc#view_edit_box — Anderson Green, Apr 05 '13 at 02:00
`theThing = []` creates an empty array, so the index 0 doesn't exist. — Alex Gittemeier, Apr 05 '13 at 02:01
I'm relatively unfamilar with Python (coming from a JavaScript background), so I found this to be surprising. In JavaScript, you can simply do `var theThing = new Array(); theThing[0] = 0;` to set the 0th element of `theThing` to 0. — Anderson Green, Apr 05 '13 at 02:01
It turns out that it's actually possible to initialize an array with a specific size in Python: http://stackoverflow.com/questions/6142689/initialising-an-array-of-fixed-size-in-python — Anderson Green, Apr 05 '13 at 02:47
@AndersonGreen: Not really. `len(lst) == 5`. Also, if you replace `None` with a mutable object like a list in the accepted answer, things will not work the way you'd expect, as you're creating five references to the same object. — Blender, Apr 05 '13 at 02:49

Blender · Accepted Answer · 2013-04-05T02:13:08.893

5

Python lists don't have a fixed size . To set the 0th element, you need to have a 0th element:

>>> theThing = []
>>> theThing.append(12)
>>> theThing
[12]
>>> theThing[0] = 0
>>> theThing
[0]

JavaScript's array object works a bit differently than Python's, as it fills in previous values for you:

> x
[]
> x[3] = 5
5
> x
[undefined × 3, 5]

edited Apr 05 '13 at 02:13

answered Apr 05 '13 at 02:01

Blender

How does `theThing.append(12)` affect the array here? – Anderson Green Apr 05 '13 at 02:05
`theThing.append(12)` adds a 12 at the end of the (currently empty) array. – Jakob Weisblat Apr 05 '13 at 02:06
@AndersonGreen: The list is empty, so there is no `0`th element. I just added `.append(12)` to give the list one. JavaScript's syntax is probably throwing you off here. – Blender Apr 05 '13 at 02:08
Is it possible to initialize an empty array with a particular size, then (e.g., an empty array of length 10?) – Anderson Green Apr 05 '13 at 02:14
@AndersonGreen: Not really. You can do `thing = [None for i in range(10)]`, but the list will still contain 10 elements. – Blender Apr 05 '13 at 02:16
@Blender You've provided a good answer to the OP question, but I wonder if you can expand it to a more general case. Assume I have a variable myIdx, and I don't know it's value, but I want to set thing[myIdx] = myValue. Do I have to iterate from 0 to myIdx and set each of those to e.g. Nothing before I can reset the last one to the value? Is there a different datatype we should use here? – James Newton May 23 '23 at 21:56

score 1 · Answer 2 · answered Apr 05 '13 at 02:01

1

You're trying to assign to a non-existent position. If you want to add an element to the list, do

theThing.append(0)

If you really want to assign to index 0 then you must ensure the list is non-empty first.

theThing = [None]
theThing[0] = 0

answered Apr 05 '13 at 02:01

John Kugelman

1

Let us suppose that I do not know the size of final array, so I cannot specify a length, and that I want to add an item in position 4, 8, and 12. append() is not the right way. How can I do that? – Dario de Judicibus Mar 21 '23 at 16:43

score 1 · Answer 3 · answered Apr 05 '13 at 02:02

1

It depends on what you really need. First you have to read python tutorials about list. In you case you can use smth like:

lVals = [] 
lVals.append(0)
>>>[0]
lVals.append(1)
>>>[0, 1]
lVals[0] = 10
>>>[10, 1]

answered Apr 05 '13 at 02:02

Artsiom Rudzenka

3 Answers3