how to convert 2d list to 2d numpy array?

I have a 2D list something like

a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 

and I want to convert it to a 2d numpy array. Can we do it without allocating memory like

numpy.zeros((3,3))

and the开发者_开发问答n storing values to it?

Just pass the list to np.array:

a = np.array(a)

You can also take this opportunity to set the dtype if the default is not what you desire.

a = np.array(a, dtype=...)

just use following code

c = np.matrix([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
matrix([[1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]])

Then it will give you

you can check shape and dimension of matrix by using following code

c.shape

c.ndim

np.array() is even more powerful than what unutbu said above. You also could use it to convert a list of np arrays to a higher dimention array, the following is a simple example:

aArray=np.array([1,1,1])

bArray=np.array([2,2,2])

aList=[aArray, bArray]

xArray=np.array(aList)

xArray's shape is (2,3), it's a standard np array. This operation avoids a loop programming.

I am using large data sets exported to a python file in the form

XVals1 = [.........] 
XVals2 = [.........] 

Each list is of identical length. I use

>>> a1 = np.array(SV.XVals1)

>>> a2 = np.array(SV.XVals2)

Then

>>> A = np.matrix([a1,a2])