Multiplying numpy 2d Array with 1d Array

Question

I have a numpy 2d array:

[[1,1,1],
[1,1,1],
[1,1,1],
[1,1,1]]

How can I get it so that it multiplies the indices from top to bottom with the corresponding values from a 1d array when the row length of the 2d array is smaller than length of 1d array ? For example multiply above with this:

[10, 20, 30, 40]

to get this:

 [[10, 10, 10],
 [20, 20, 20],
 [30, 30, 30]
 [40, 40, 40]]

Probably a duplicate, but I couldnt find exact thing I am looking for. Thanks in advance.

Akavall · Accepted Answer · 2017-06-05T03:24:36.720

2

* in numpy does element-wise multiplication, for example multiply 1d array by another 1d array:

In [52]: np.array([3,4,5]) * np.array([1,2,3])
Out[52]: array([ 3,  8, 15])

When you multiply a 2d array by 1d array, same thing happens for every row of 2d array:

In [53]: np.array([[3,4,5],[4,5,6]]) * np.array([1,2,3])
Out[53]:
array([[ 3,  8, 15],
       [ 4, 10, 18]])

For your specific example:

In [66]: ones = np.ones(12, dtype=np.int).reshape(4,3)

In [67]: a = np.array([10, 20, 30, 40])

In [68]: (ones.T * a).T
Out[68]:
array([[10, 10, 10],
       [20, 20, 20],
       [30, 30, 30],
       [40, 40, 40]])

edited Jun 05 '17 at 03:24

answered Jun 05 '17 at 00:14

Akavall

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Had to look up what the .T meant but I see now, thanks a lot. – user21398 Jun 05 '17 at 00:30
But wait what happens when your row length is smaller than your 1d Array? Does transpose still work then? – user21398 Jun 05 '17 at 00:40
@user21398, If row length is smaller (or larger) than 1d array, you would not be able to multiply. – Akavall Jun 05 '17 at 00:45
I see, ill have to rephrase question then. But how could you do it then? – user21398 Jun 05 '17 at 00:47
Perfect, thanks for taking the time! – user21398 Jun 05 '17 at 00:56

Multiplying numpy 2d Array with 1d Array

1 Answers1