I am having a hard time understanding how matrix works.
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a = [ 1 0; 2 1] ;
b= [ -1 2 ; 0 1] ;
n= a*b
N= a.*b
what is the mathematical differnce between a.*b and a*b.
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Réponses (3)
Jan
le 22 Fév 2022
Modifié(e) : Jan
le 26 Fév 2022
.* is the elementwise multiplication, while * is the matrix multiplication:
[a,b; c,d] .* [e,f; g,h] =
[a*e, b*f; c*g, d*h]
[a,b; c,d] * [e,f; g,h] = % [EDITED, typos fixed]
[a*e + b*g, a*f + b*h; c*e + d*g, c*f + d*h]
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Walter Roberson
le 26 Fév 2022
Modifié(e) : Walter Roberson
le 26 Fév 2022
Jan, if the forum will run symbolic toolbox code for you then you are authorized to use the toolbox within the context of the forum; it is a service being provided by Mathworks.
It has the usual 55 second limit, and only about 5 gigabytes of ram, is about half the speed of your desktop, debugger and interactive input is disabled...
Stephen23
le 26 Fév 2022
@Jan: my pleasure! I also spent ten minutes trying to catch those flies, going through the steps of multiplying: however I am slow and those tiny flies are hard to catch. It was only when I was about to post the comment that it occured to me to try the symbolic toolbox, which luckily gave the same result :)
Walter Roberson
le 22 Fév 2022
The operator .* is element-by-element multiplication. N = a.*b means that N(J,K) = a(J,K) times b(J,K) with no other elements of a or b influencing the outcome.
The operator * is algebraic matrix multiplication, also known as "inner product". n = a*b means that n(J,K) = dot(a(J,:), b(:,K)) which is an operation that involves a complete row of one matrix and a complete column of the other matrix. Carefully chosen matrix multiplication can express rotation, translation, and scale
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Arthur Reis
le 22 Fév 2022
Modifié(e) : Arthur Reis
le 22 Fév 2022
'*' is the matrix multiplication as is usual in Linear Algebra
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/903340/image.png)
'.*' is element-wise multiplication. Each element is multiplied by its correspondent
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/903345/image.png)
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