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I would like to have a numpy matrix that looks like this [int, [[int,int]]] I receive an error that looks like this "ValueError: setting an array element with a sequence."

below is the declaration

def __init__(self):
    self.path=np.zeros((1, 2))

I attempt to assign a value to this in the line below 

routes_traveled.path[0, 1]=[loc]

loc is a list and routes_traveled is the object

BadProgrammer
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  • Take a look at the docs here: http://docs.scipy.org/doc/numpy/reference/generated/numpy.zeros.html. If you want to specify the shape (1,2) you need to pass that in as a sequence, like @unutbu says in his answer. Otherwise it will assume the '2' is the optional datatype. – Pencil Von Radiergummi Apr 18 '15 at 22:13

2 Answers2

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Do you want a higher dimensional array, say 3d, or do you really want a 2d array whose elements are Python lists. Real lists, not numpy arrays?

One way to put lists in to an array is to use dtype=object:

In [71]: routes=np.zeros((1,2),dtype=object)
In [72]: routes[0,1]=[1,2,3]
In [73]: routes[0,0]=[4,5]

In [74]: routes
Out[74]: array([[[4, 5], [1, 2, 3]]], dtype=object)

One term of this array is 2 element list, the other a 3 element list.

I could have created the same thing directly:

In [76]: np.array([[[4,5],[1,2,3]]]) 
Out[76]: array([[[4, 5], [1, 2, 3]]], dtype=object)

But if I'd given it 2 lists of the same length, I'd get a 3d array:

In [77]: routes1=np.array([[[4,5,6],[1,2,3]]])
Out[77]: 
array([[[4, 5, 6],
        [1, 2, 3]]])

I could index the last, routes1[0,1], and get an array: array([1, 2, 3]), where as routes[0,1] gives [1, 2, 3].

In this case you need to be clear where you talking about arrays, subarrays, and Python lists.

With dtype=object, the elements can be anything - lists, dictionaries, numbers, strings

In [84]: routes[0,0]=3
In [85]: routes
Out[85]: array([[3, [1, 2, 3]]], dtype=object)

Just be ware that such an array looses a lot of the functionality that a purely numeric array has. What the array actually contains is pointers to Python objects - just a slight generalization of Python lists.

hpaulj
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  • your first suggestion I think will work. Doea it matter that my first column is just an int and my second is an object? – BadProgrammer Apr 18 '15 at 23:29
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Did you want to create an array of zeros with shape (1, 2)? In that case use np.zeros((1, 2)).

In [118]: np.zeros((1, 2))
Out[118]: array([[ 0.,  0.]])

In contrast, np.zeros(1, 2) raises TypeError:

In [117]: np.zeros(1, 2)
TypeError: data type not understood

because the second argument to np.zeros is supposed to be the dtype, and 2 is not a value dtype.

Or, to create a 1-dimensional array with a custom dtype consisting of an int and a pair of ints, you could use

In [120]: np.zeros((2,), dtype=[('x', 'i4'), ('y', '2i4')])
Out[120]: 
array([(0, [0, 0]), (0, [0, 0])], 
      dtype=[('x', '<i4'), ('y', '<i4', (2,))])

I wouldn't recommend this though. If the values are all ints, I think you would be better off with a simple ndarray with homogeneous integer dtype, perhaps of shape (nrows, 3):

In [121]: np.zeros((2, 3), dtype='<i4')
Out[121]: 
array([[0, 0, 0],
       [0, 0, 0]], dtype=int32)

Generally I find using an array with a simple dtype makes many operations from building the array to slicing and reshaping easier.

unutbu
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  • I think you misunderstood my question. I am not looking for an matrix of ints. I need a matrix that looks like [int, [[int,int]]]. A matrix with first column being an int and the second being a list of lists of ints – BadProgrammer Apr 18 '15 at 22:24
  • Refresh your page. I addressed that in the second part of my answer. – unutbu Apr 18 '15 at 22:24
  • This is closer to what I'm looking for. However I would like for the second column to support a list of of the points. While it currently only supports 1 point. – BadProgrammer Apr 18 '15 at 22:30
  • Unless I did not seperate each of the points into their own list – BadProgrammer Apr 18 '15 at 22:30
  • I think the answer you gave for this question might work. http://stackoverflow.com/questions/2106287/pythonic-way-to-create-a-numpy-array-from-a-list-of-numpy-arrays – BadProgrammer Apr 18 '15 at 22:40