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

问题:

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 then storing values to it?

回答1:

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=...) 

回答2:

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]) 

回答3:

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

回答4:

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.

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