Convert a 1D array to a 2D array in numpy


Question

I want to convert a 1-dimensional array into a 2-dimensional array by specifying the number of columns in the 2D array. Something that would work like this:

> import numpy as np
> A = np.array([1,2,3,4,5,6])
> B = vec2matrix(A,ncol=2)
> B
array([[1, 2],
       [3, 4],
       [5, 6]])

Does numpy have a function that works like my made-up function "vec2matrix"? (I understand that you can index a 1D array like a 2D array, but that isn't an option in the code I have - I need to make this conversion.)

1
86
7/18/2018 4:40:42 AM

Accepted Answer

You want to reshape the array.

B = np.reshape(A, (-1, 2))
124
9/25/2012 1:44:23 PM

You have two options:

  • If you no longer want the original shape, the easiest is just to assign a new shape to the array

    a.shape = (a.size//ncols, ncols)
    

    You can switch the a.size//ncols by -1 to compute the proper shape automatically. Make sure that a.shape[0]*a.shape[1]=a.size, else you'll run into some problem.

  • You can get a new array with the np.reshape function, that works mostly like the version presented above

    new = np.reshape(a, (-1, ncols))
    

    When it's possible, new will be just a view of the initial array a, meaning that the data are shared. In some cases, though, new array will be acopy instead. Note that np.reshape also accepts an optional keyword order that lets you switch from row-major C order to column-major Fortran order. np.reshape is the function version of the a.reshape method.

If you can't respect the requirement a.shape[0]*a.shape[1]=a.size, you're stuck with having to create a new array. You can use the np.resize function and mixing it with np.reshape, such as

>>> a =np.arange(9)
>>> np.resize(a, 10).reshape(5,2)

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