[R] row*matrix

[Rolf Turner]

You need to learn that R syntax is not the same as Matlab syntax.
If you want to use R, learn R.  Read the basic introductory material,
readily available from the R web site.

In particular R treats multiplication --- i.e. "*" --- as multiplication.
Arrays are multiplied entry by entry when they are conformable.
When you multiply a vector times an array, the vector is recycled
to get the requisite number of entries.  The recycled entries are
packed into an array of the appropriate dimension in reverse
odometer order.

What you want is matrix multiplication.  In R this is denoted by
"%*%".  Note that M%*%v2 is the second column of M as you
desired.

HTH

     cheers,

         Rolf Turner

On 07/08/11 08:35, smajor 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:
>

> 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
>
> --
> View this message in context: http://r.789695.n4.nabble.com/row-matrix-tp3724023p3724023.html
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>
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