Meshgrid a N-columned matrix in Numpy (or smth else)

Question

Python version: 2.7

I have the following numpy 2d array:

array([[ -5.05000000e+01,  -1.05000000e+01],
   [ -4.04000000e+01,  -8.40000000e+00],
   [ -3.03000000e+01,  -6.30000000e+00],
   [ -2.02000000e+01,  -4.20000000e+00],
   [ -1.01000000e+01,  -2.10000000e+00],
   [  7.10542736e-15,  -1.77635684e-15],
   [  1.01000000e+01,   2.10000000e+00],
   [  2.02000000e+01,   4.20000000e+00],
   [  3.03000000e+01,   6.30000000e+00],
   [  4.04000000e+01,   8.40000000e+00]])

If I wanted to find all the combinations of the first and the second columns, I would use np.array(np.meshgrid(first_column, second_column)).T.reshape(-1,2). As a result, I would get a 100*1 matrix with 10*10 = 100 data points. However, my matrix can have 3, 4, or more columns, so I have a problem of using this numpy function.

Question: how can I make an automatically meshgridded matrix with 3+ columns?

UPD: for example, I have the initial array:

[[-50.5 -10.5]
 [  0.    0. ]]

As a result, I want to have the output array like this:

array([[-10.5, -50.5],
   [-10.5,   0. ],
   [  0. , -50.5],
   [  0. ,   0. ]])

or this:

array([[-50.5, -10.5],
   [-50.5,   0. ],
   [  0. , -10.5],
   [  0. ,   0. ]])

By `3, 4, or more dimensional`, do you mean having `3, 4, or more number of columns` rather? To clarify that could you add sample for 3 "dimensional" data? Also, `np.meshgrid(first_column, second_column)` would give you a list of two arrays each having 100 elems. So, how would you have that `100*1 matrix`? — Divakar, Feb 26 '17 at 14:48

score 1 · Accepted Answer · edited May 23 '17 at 12:09

1

You could use * operator on the transposed array version that unpacks those columns sequentially. Finally, a swap axes operation is needed to merge the output grid arrays as one array.

Thus, one generic solution would be -

np.swapaxes(np.meshgrid(*arr.T),0,2)

Sample run -

In [44]: arr
Out[44]: 
array([[-50.5, -10.5],
       [  0. ,   0. ]])

In [45]: np.swapaxes(np.meshgrid(*arr.T),0,2)
Out[45]: 
array([[[-50.5, -10.5],
        [-50.5,   0. ]],

       [[  0. , -10.5],
        [  0. ,   0. ]]])

edited May 23 '17 at 12:09

Community

1
1

answered Feb 26 '17 at 15:50

Divakar

218,885
19
262
358

I am afraid that does not work well. If I have a 2x2 matrix like this: [[-50.5 -10.5] [ 0. 0. ]] After typing `X_total = np.meshgrid(*X_total.T)` I would get: [array([[-50.5, 0. ], [-50.5, 0. ]]), array([[-10.5, -10.5], [ 0. , 0. ]])] Instead I want: array([[-10.5, -50.5], [-10.5, 0. ], [ 0. , -50.5], [ 0. , 0. ]]) – Alex Feb 26 '17 at 16:30

Meshgrid a N-columned matrix in Numpy (or smth else)

1 Answers1