IndexError: list assignment index out of range - Python with an array

Question

I recently started to use python and i am still newbie with many things of the language. This piece of code should print a serie rows (Ex.:[47.815, 47.54, 48.065, 57.45]) that i take as imput from several text files (called 2t1,...,2t19,3t1,...,3t19) and the name of the file after it's over. Every file has 80 elements that must be saved (for a total of ~12000 elements, ~80 rows for each file, 4 elements each row)

for i in range(2,3):
for j in range(1,19):
    #Open a text file and convert the row content to float:
    for line in open('[Path]/%dt%d.txt' % (i,j)):
    # Ignore comments:
        if not line.startswith('#'):
            # Got a valid data line:
                row[i][j] = [float(item) for item in line.split()]
                print (row[i][j])
    print (' %dt%d \n' %(i,j))

It is able to do what i ask it, but just until 2t5 and then it is shown this message:

Traceback (most recent call last):

File "< tmp 1>", line 8, in < module>

row[i][j] = [float(item) for item in line.split()]

IndexError: list assignment index out of range

I can't really figure out what's the problem, i guess the list isn't filled to it's max (i read online that it has 500.000.000 spaces), and i can't see what may be wrong . That error appears when the list you are working on is empty, but i doublechecked and there is every file, but i also tryed with row[i][j].append(float(item) for item in line.split()) (that is the solution for the most common mistakes) and it returns me a really long list of this:

< generator object < genexpr> at 0x000000000XXXXXXX>

(Where the X are different numbers) ending then with the same error.

P.s. If you need the data i'm working on just ask and i'll upload it.

How are you initially assigning row? Evidently, it doesn't have enough entries. — interjay
– interjay, Commented Dec 3, 2014 at 9:02
@interjay I didn't initialize/declare it. The code is not 100% mine and it was like that from where i took it. Also i guess python doesn't need variables to be declared (that's what my manual says). — Protolando
– Protolando, Commented Dec 3, 2014 at 9:08
@Protolando There has to be a line that says row = something somewhere, or the error you get would be different. — interjay
– interjay, Commented Dec 3, 2014 at 9:22

Greg · Accepted Answer · 2014-12-03 09:05:29Z

3

It seems like your problem is pretty straightforward. You probably have a list named row and you're trying to assign values to non-existent indices

>>> row = []
>>> row[0] = 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range

If however you appended a new list into the row list like I think you want to do

>>> row = []
>>> row.append([1, 2, 3, 4])
>>> row
[[1, 2, 3, 4]]
>>> row[0][0]
1

This would work fine. If you're trying to triple nest things

>>> row = []
>>> row.append([[1,2,3,4]])
>>> row
[[[1, 2, 3, 4]]]
>>> row[0].append([-1, -2, -3, -4])
>>> row
[[[1, 2, 3, 4], [-1, -2, -3, -4]]]

You can use the same syntax

answered Dec 3, 2014 at 9:05

Greg

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