[R] Matrix element-by-element multiplication

[Rolf Turner]

On 30/09/15 12:49, waddawanna wrote:
> Hello Steven,
>
> It looks like, there is no in-built function that can do GAUSS ".*"
> element-wise multiplication.
> Now, if you want to make the desired computations in R, it is actually
> preatty straightforward.
>
>> a<-c(1,2,3)
>> b<-matrix(rep(1:9,1),3,3,byrow=TRUE)
>> a*b
>
> That, should work fine. But, suppose that for some reason you have following
> situation, which can make you trip for hours of sleepless nights. That is,
> you have a matrix "b", where number of columns equal to number of columns of
> your vector "a". That is
>
>> b<-matrix(rep(1:9,1),3,3,byrow=TRUE);b
>       [,1] [,2] [,3]
> [1,]    1    2    3
> [2,]    4    5    6
> [3,]    7    8    9
>
>> a <- matrix(rep(1:3,1),1,3,byrow=TRUE)
>       [,1] [,2] [,3]
> [1,]    1    2    3
>
> If you try to do elementwise multilication, i.e., of those two
>> b*a
>
> You get an error that they are not comfomable, that is why, you have to
> write your own function (here,
> I just write the for-loop):
>> for ( i in 1:3 ) {
>       foo[ ,i] = ( foo[ ,i] * bar[1,i] ) ;
>     }
>         [,1] [,2] [,3]
> [1,]    1    4    9
> [2,]    4   10   18
> [3,]    7   16   27
>
> I hope that this helped.


(1) You're making heavy weather by using rep() totally unnecessarily.


(2) The example you give can be done much more succinctly; for-loops
are unnecessary:

    t(as.vector(a)*t(b))

(3) Please don't post to R-help via nabble (WTF ever that is).  It 
messes up everything.

cheers,

Rolf Turner

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