Python: Index Error for 3D list

Question

I've never worked with 2D or 3D arrays before but i'm trying to make a maze. In my snippet, squares is a list with each instance of a cell (so in a 3x4 maze, there would be 12 instances in squares) I am then trying to append to row, a list of all the squares in a row, so row[0] would contain the first four square instances, row[1] would be the next four, etc. the row[x].append(squares[y+z]) throws the IndexError, i'm guessing it's the row[x] part, but i'm not sure what to do to fix it. I tried using extend instead of append.

numberOfRows = 3
numberOfColumns = 4
z = 0

for x in range(numberOfRows):
    for y in range(numberOfColumns):
        row[x].append(squares[y+z])
    z += 4

How is row defined? You need to give us enough code to actually see it fail. Meanwhile, ave you verified that squares[y+z] is not throwing the IndexError? (You can add another line that just says squares[y+z] above the append line and see if that throws.) — abarnert
– abarnert, Commented Dec 6, 2012 at 1:00
If you want multidimensional arrays, I'd suggest you look closely at numpy. — Chinmay Kanchi
– Chinmay Kanchi, Commented Dec 6, 2012 at 1:39

mmgp · Accepted Answer · 2012-12-06 02:07:34Z

1

If I'm guessing it right, you want:

numberOfRows = 3
numberOfColumns = 4
z = 0

squares = range(numberOfRows * numberOfColumns)

row = [[] for _ in xrange(numberOfRows)]
for x in range(numberOfRows):
    for y in range(numberOfColumns):
        row[x].append(squares[y+z])
    z += 4

print row

i.e., you were only missing the row definition.

EDIT:

After reading OP's comments, it seems that considering the following alternative is worth for the situation:

row = []
for x in range(numberOfRows):
    row.append([squares[y+z] for y in range(numberOfColumns)])
    z += numberOfColumns

So you don't create all the lists in row beforehand.

edited Dec 6, 2012 at 2:07

answered Dec 6, 2012 at 1:04

mmgp

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5 Comments

user1880866 Over a year ago

mmgp's solution worked now the _ is just a temporary variable, right? like i used x and y? so the row definition is creating lists in the row list?

abarnert Over a year ago

If the OP were actually missing the row definition, he'd get a NameError, not an IndexError. Which means he clearly hasn't shown us his actual code, so this really is just a guess at what he wants—and to explain why his original code actually failed would require even more guesswork. It seems like you've guessed well, but why should you have to guess in the first place?

mmgp Over a year ago

@user1880866 _ is a valid identifier, it is commonly used to denote some variable that you don't care about. In the Python shell it has a special use, you could try discovering yourself. Now, the row definition is simply creating n empty lists, where n is numberOfRows.

user1880866 Over a year ago

abarnert: i had defined row as just an empty list, row = [] i didn't know i had to define the lists in the list. you're right, i should've included more so you knew what i was talking about. though i hardly know what i'm talking about, which makes it difficult to discern what's important. thank you to the people who went ahead and unraveled my mess

abarnert Over a year ago

@user1880866: It's worth understanding why you have to create the lists. Think about what you defined: row = []. So row is an empty list, with 0 elements. That means row[x].append(whatever) is going to fail, for any x, even 0. There is no row[x] to append to, or access in any other way. That's what the IndexError means.

Aesthete · Accepted Answer · 2012-12-06 01:29:33Z

0

This can be simplified to the following:

>>> [squares[i:i+numberOfColumns] for i in range(0, len(squares), numberOfColumns)]
[[0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]]

edited Dec 6, 2012 at 1:29

answered Dec 6, 2012 at 0:51

Aesthete

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