How to subtract matrices in Python properly?-CodePudding

I've been trying to subtract two matrices (not sure if they are really matrices since one of them is pandas series) but the result is not true. I added the code and outputs, how can I get the correct 200X1 shape result?

*X is 200x4 and w is 4x1

CodePudding user response：

Maybe it could help:

rng = np.random.default_rng(2022)
df = pd.DataFrame(rng.integers(0, 10, (5, 4)))
sr = pd.Series(rng.integers(0, 10, (5, )))

>>> df
   0  1  2  3
0  7  2  7  0
1  1  6  9  0
2  0  6  8  7
3  8  1  5  0
4  0  4  8  9

>>> sr
0    3
1    9
2    0
3    2
4    6
dtype: int64

>>> df - sr  # does not work
   0  1  2  3   4
0  4 -7  7 -2 NaN
1 -2 -3  9 -2 NaN
2 -3 -3  8  5 NaN
3  5 -8  5 -2 NaN
4 -3 -5  8  7 NaN

>>> df.sub(sr, axis=0)  # work
   0  1  2  3
0  4 -1  4 -3
1 -8 -3  0 -9
2  0  6  8  7
3  6 -1  3 -2
4 -6 -2  2  3

CodePudding user response：

You can reshape y_hat :

y - y_hat.reshape(-1,1)

The reason you get (200,200) is because of numpy broadcasting, it treats y_hat as (1,200), so in order for the shapes to match, numpy broadcasts y into (200,200) and y_hat into (200,200) and then does the substraction.