How can I do if:
a = np.array([[1,2,3],[5,6,7]])
b = np.array([0,1])
I search to concatenate a and b so as the result would be:
np.array([1,2,3,0],[5,6,7,1])
Thanks a lot
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3 Answers
The problem is to concatenate a horizontally with b as a column vector.
<concat>( |1 2 3|, |0| )
|5 6 7| |1|
The concatentation can be done using np.hstack, and b can be converted into a column vector by adding a new axis:
>>> np.hstack([a, b[:, np.newaxis]])
array([[1, 2, 3, 0],
[5, 6, 7, 1]])
2 Comments
w-m
@Yuca: There is no distinction between a row and a column vector for a 1d array. See e.g. stackoverflow.com/a/42908123/463796 for more on that.
user3483203
@Yuca you can use the transpose with
np.atleast_2d: np.hstack([a, np.atleast_2d(b).T]).The more numpythonic way to do this is to avoid broadcasting, and use the function designed for this: numpy.column_stack:
np.column_stack([a, b])
array([[1, 2, 3, 0],
[5, 6, 7, 1]])
2 Comments
w-m
Oh, I didn't know about that! And it's implemented essentially as @Wen's answer. Fun! :)
user3483203
Yep,
column_stack is the only one of the stacking functions (I believe), that converts 1D inputs to 2D columns, so works perfectly here.Using numpy broadcast with concatenate
np.concatenate([a,b[:,None]],1)
Out[1053]:
array([[1, 2, 3, 0],
[5, 6, 7, 1]])
