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

Question

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?

Asked By: Shan

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Source

Answer 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=...)

Answered By: unutbu

Answer 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])

Answered By: PeterB

Answer 3

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.

Answered By: Clock ZHONG

Answer 4

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

Answered By: sandeep sharma

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

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