[R] row*matrix

[Joshua Wiley]

Hi,

You are using the regular multiplication operator, when you want the
matrix multiplication operator.  Compare:

> M * v2
     [,1] [,2] [,3]
[1,]    0    0    0
[2,]    4    5    6
[3,]    0    0    0
> M %*% v2
     [,1]
[1,]    2
[2,]    5
[3,]    8

See ?"*" and ?"%*%"

HTH,

Josh

On Sat, Aug 6, 2011 at 1:35 PM, smajor <semajor at ncsu.edu> wrote:
> Hi all, I'm learning R. I'm used to Matlab and am having the following issue:
>
> I define a column vector and matrix and then multiply them.  The result is
> not what I expect:
> v2 <- c(0,1,0)
> M <- cbind(c(1,4,7),c(2,5,8),c(3,6,9))
> M*v2
>
>     [,1] [,2] [,3]
> [1,]    0    0    0
> [2,]    4    5    6
> [3,]    0    0    0
>
> I expect the result to be a column, specifically the 2nd column of M.
>
> Now I want to left multiply the matrix by a row vector, and I get errors:
> t(v2)*M
> Error in t(v2) * M : non-conformable arrays
>> V2 <- t(v2)
>> V2*M
> Error in V2 * M : non-conformable arrays
> I would expect to get the 2nd row of M as an output.
>
> Why am I not getting a column for the first case?  Why is the second case
> causing errors?  Any help would be greatly appreciated.  I can get the
>
> --
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>
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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
Programmer Analyst II, ATS Statistical Consulting Group
University of California, Los Angeles
https://joshuawiley.com/