R help - Mar 2012 - dot products

Hello,

I need to take a dot product of each row of a dataframe and a vector.
The number of columns will be dynamic.  The way I've been doing it so
far is contorted.  Is there a better way?

    dotproduct <- function(dataf, v2) {
        apply(t(t(as.matrix(a)) * v2),1,sum) #contorted!
    }

    df = data.frame(a=c(1,2,3),b=c(4,5,6))
    vec = c(4,5)
    dotproduct(df, vec)


thanks,
allie

as.matrix(df) %*% vec

Michael 

On Mar 7, 2012, at 3:14 PM, Alexander Shenkin <ashenkin at ufl.edu> wrote:
> Hello,
> 
> I need to take a dot product of each row of a dataframe and a vector.
> The number of columns will be dynamic.  The way I've been doing it so
> far is contorted.  Is there a better way?
> 
>    dotproduct <- function(dataf, v2) {
>        apply(t(t(as.matrix(a)) * v2),1,sum) #contorted!
>    }
> 
>    df = data.frame(a=c(1,2,3),b=c(4,5,6))
>    vec = c(4,5)
>    dotproduct(df, vec)
> 
> 
> thanks,
> allie
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

On Wed, Mar 07, 2012 at 03:14:05PM -0500, Alexander Shenkin
wrote:
> Hello,
> 
> I need to take a dot product of each row of a dataframe and a vector.
> The number of columns will be dynamic.  The way I've been doing it so
> far is contorted.  Is there a better way?
> 
>     dotproduct <- function(dataf, v2) {
>         apply(t(t(as.matrix(a)) * v2),1,sum) #contorted!
>     }
> 
>     df = data.frame(a=c(1,2,3),b=c(4,5,6))
>     vec = c(4,5)
>     dotproduct(df, vec)
Hi.

If i understand correctly, then the following should
be equivalent

  as.matrix(df) %*% cbind(vec)

       [,1]
  [1,]   24
  [2,]   33
  [3,]   42

Hope this helps.

Petr Savicky.