# 2x2 matrix multiplied by a 2x1 column vector gives erratic results

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Timothy Goldsmith am 6 Jun. 2016
Kommentiert: Stephen23 am 7 Jun. 2016
For example: A=[3,-2;2,-2] times v=[1;-1] works, but fails if A=[1,2;3,4]. The problem seems to be that in Matlab matrix multiplication the elements in row A are multiplied by the corresponding columns in B. Here B has only one column, and needs that the column elements in A be multiplied by the corresponding row elements in B. I have circumvented this problem by writing a function that does the latter, but as the need is for applying a vector to a transformation matrix, I am surprised to discover that the standard matrix multiplication algorithm cannot be relied upon. When it fails it takes A=[a1,b1;a2,b2] and computes v1a1+v2b1;v1a2+v2b2] instead of v1(a1+a2);v2(b1+b2). Is there away around this problem other than my function?
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Stephen23 am 7 Jun. 2016
Bearbeitet: Stephen23 am 7 Jun. 2016
"doing the correct calculation" is an interesting definition of "fail": MATLAB correctly returns the mathematically universal matrix product. How is this very basic mathematical operation either "erratic" or a "fail" ?

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### Antworten (1)

Moe_2015 am 6 Jun. 2016
Bearbeitet: Moe_2015 am 6 Jun. 2016
That is not how matrix multiplication works at all. For a matrix and a vector:
A= [1 2 v= [1
3 4] -1]
A*v= [1*1+2*(-1)
3*1+4*(-1)]
MATLAB does not fail. It does it correctly.
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Stephen23 am 7 Jun. 2016
@Timothy Goldsmith: please give us complete examples of when "I find that A*v sometimes gives me the answer I want, and sometimes it gives the 'correct' answer". We need to see complete working examples of this, i.e. the input and output matrices, and the actual and expected output matrices.

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