Adding elements to a 3d array in python

Question

I am new to Python and I am trying to add elements to a 3D list.

def getHoughSpace(self):
        space = [[[]]]
        for i in range(0, len(self.xs)):
            rho = self.getRho(self.xs[i], self.ys[i], self.zs[i])
            space.append([Cell(rho, 0)])
            for j in range(0, len(self.ys)):
                theta = self.getTheta(self.xs[j], self.ys[j], self.zs[j])
                space[i].append([Cell(theta, 0)])
                for k in range(0, len(self.zs)):
                    phi = self.getPhi(self.xs[k], self.ys[k])
                    space[i][j].append([Cell(phi, 0)]) # error happens here

However, when i=1, j=0, and k=0, my program crashes with the error:

AttributeError: Cell instance has no attribute 'append'

Why is this happening? How can I get passed i=0, j=0, and k=0 but not when i=1? Obviously, I am accessing a Cell at space[i][j], but I want to access the list of Cells.

An interesting note is that if I comment out the error line, it runs. So why does the problem occur only in the 3rd dimension?

nitishagar · Accepted Answer · 2014-12-21 04:06:32Z

1

You might want to append elements at the end, for the three dimensional list:

space = []
space.append([])
space[i].append([])
space[i][j].append(<element>)

Or you can use numpy:

import numpy as np
space = np.empty((len(self.xs), len(self.ys), len(self.zs)), dtype=object)

edited Dec 21, 2014 at 4:06

answered Dec 21, 2014 at 3:45

nitishagar

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2 Comments

Trevor Hutto Over a year ago

I used numpy to get it working. I would note that you cannot use append anymore sense you have allocated an array. You must use space[i][j][k] = Cell(phi, 0).

nitishagar Over a year ago

@TrevorHutto The problem, with original program is: space.append([Cell(rho, 0)]) which disturbs the dimensionality of list, if you print it you can see another list being added as element instead of list of list.

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