Python two multidimensional array creation

Question

My program uses the code below to insert an object named Field to a multidimensional array that is created on the fly (at least I thought it would be created):

for x in range(self.width):
    for y in range(self.height):
            self.board_fields[x][y] = Field()

So board_fields wasn't created on the fly and I got the error:

AttributeError: 'Board' object has no attribute 'board_fields'

How should I define the multidimensional array before putting values into it?

Are you sure that the above mentioned code got executed? Because, the error makes me feel that, python doesn't even know what board_fields is. — thefourtheye
– thefourtheye, Commented Jul 2, 2013 at 12:54

bogatron · Accepted Answer · 2013-07-02 12:43:13Z

2

self.board_fields = [[Field() for j in range(self.height)] for i in range(self.width)]

answered Jul 2, 2013 at 12:43

bogatron

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2 Comments

thefourtheye Over a year ago

Upvoted :) Is there any other way in which I can create multi-dimensional arrays, with just default python libraries?

bogatron Over a year ago

Aside from list comprehensions (which I used above), you could use nested for loops (like @astex's solution). For an arbitrary number of dimensions, you could write a recursive function that accepts a list of dimensions.

astex · Accepted Answer · 2013-07-02 12:40:04Z

1

These aren't really arrays in the classical sense, but lists of lists. While this is a semantic difference in some contexts, here it affects where within the lists assignment can be made. The append statement allocates additional memory to the list and assigns its argument to that new 'slot'.

self.board_fields = []
for x in range( self.width ):
    board_fields_sub = []
    for y in range( self.height ):
        board_fields_sub.append( Field() )
    self.board_fields.append( board_fields_sub )

answered Jul 2, 2013 at 12:40

astex

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2 Comments

Tom Over a year ago

if you mean to add the line self.board_fields =[] then I get the error self.board_fields[x][y] = Field() IndexError: list index out of range

poke Over a year ago

@Tom No, that’s not enough. An empty list has still no items so indexing will fail. astex creates empty lists and then appends the items to that.

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