Matrix Multiplication gives a wrong answer

Asked Sep 08 '17 at 15:26

Active Sep 08 '17 at 15:26

Viewed 180 times

I'm trying to multiply a matrix 4x1 with a vector 1x4. I'm supposed to get a matrix 4x4. When I try to multiply both matrices I get an error, incompatible shapes. It doesn't make any sense because the shapes are compatible so probablyh I'm doing something wrong:

B = {{3619.3}, {3619.3}, {-29800}, {0}} 
Cc = {0, 0, 0, 1} 
B.Cc

Dot::dotsh: Tensors {{3619.3},{3619.3},{-29800},{0}} and {0,0,0,1} have incompatible shapes.

I tried doing similar multiplications but sometimes Matheamtica doesn't give any error but gives me wrong a answer which is worse. What Am I doing wrong?

asked Sep 08 '17 at 15:26

Miguel Duran Diaz

1

You probably want Outer[Times, {3619.3, 3619.3, -29800, 0}, {0, 0, 0, 1}]. – J. M.'s missing motivation Sep 08 '17 at 15:29
Have a look at the this tutorial and related ones. The error message you get is very explicit about what went wrong. – b.gates.you.know.what Sep 08 '17 at 15:33
Outer works. As far as I know, outer matchs the usual matrix multiplication when multiplying m x 1 * n x 1 vectors. Am I supposed to always use "outer" when I want to do that? How do I know that mathematica returns a correct result with general matrices A={mxn}, B={nxp} when I try to do "A.B" ? – Miguel Duran Diaz Sep 08 '17 at 15:34
3

You could do B . {Cc} instead. – Carl Woll Sep 08 '17 at 15:48
1

You might want to read this. – J. M.'s missing motivation Sep 08 '17 at 15:54
1

TIL: As explained in the answer JM links, one might also do {{3619.3}, {3619.3}, {-29800}, {0}}.{{0, 0, 0, 1}}, which you might find more satisfying. Essentially to force Mathematica to think of it as a row vector, you have to make it a 1xn matrix. (Edit I now realize Carl Woll also said something similar above.) – jjc385 Sep 08 '17 at 16:31

Matrix Multiplication gives a wrong answer

0 Answers0