[Tutor] Subtract a 1D Array from a 2D Array

[eryk sun]

On Sat, May 13, 2017 at 1:33 PM, Stephen P. Molnar
<s.molnar at sbcglobal.net> wrote:
> I am using Python 3.6 in Anaconda3 Spyder IDE.
>
> I have two arrays:
>
>         X           Y          Z
>                     a
>         0               0              0
>      2.059801       0          0
>     -1.203126    3.402953          0
>     -1.108639   -1.567853        2.715601
>     -0.938564   -1.32733        -2.299003
>                    a_c
>     0.4283375    0.91755         0.208299
>
> and want to subtract the value of the value of each column or the array a_c
> from the each value in the corresponding column of array c.
>
> Trying a_mm = a - a_c where a_mm is the new array results in the error
> message:
>
> ValueError: operands could not be broadcast together with shapes (3,5) (1,3)
>
> My search for a solution has not been successful.

You've presented array `a` as if it's 5x3, but the error says it's
3x5. If it were actually 5x3, there would be no problem:

    import numpy as np

    a = np.array([[0, 0, 0],
                  [2.059801, 0, 0],
                  [-1.203126, 3.402953, 0],
                  [-1.108639, -1.567853, 2.715601],
                  [-0.938564, -1.32733, -2.299003]])

    a_c = np.array([[0.4283375, 0.91755, 0.208299]])

    >>> a.shape
    (5, 3)
    >>> a_c.shape
    (1, 3)
    >>> a - a_c
    array([[-0.4283375, -0.91755  , -0.208299 ],
           [ 1.6314635, -0.91755  , -0.208299 ],
           [-1.6314635,  2.485403 , -0.208299 ],
           [-1.5369765, -2.485403 ,  2.507302 ],
           [-1.3669015, -2.24488  , -2.507302 ]])

You can use the transpose of either array. For example, use `a.T` to
get the same number of columns, or use `a_c.T` to get the same number
of rows.