[R] Multiply

[Val]

Thank you,  Avi and Ivan.  Worked for this particular Example.

Yes, I am looking for something with a more general purpose.
I think Ivan's suggestion works for this.

multiplication=as.matrix(dat1[,-1]) %*% as.matrix(dat2[match(dat1[,1],
dat2[,1]),-1])
Res=data.frame(ID = dat1[,1], Index = multiplication)

On Fri, Aug 4, 2023 at 10:59 AM <avi.e.gross using gmail.com> wrote:
>
> Val,
>
> A data.frame is not quite the same thing as a matrix.
>
> But as long as everything is numeric, you can convert both data.frames to
> matrices, perform the computations needed and, if you want, convert it back
> into a data.frame.
>
> BUT it must be all numeric and you violate that requirement by having a
> character column for ID. You need to eliminate that temporarily:
>
> dat1 <- read.table(text="ID, x, y, z
>  A, 10,  34, 12
>  B, 25,  42, 18
>  C, 14,  20,  8 ",sep=",",header=TRUE,stringsAsFactors=F)
>
> mat1 <- as.matrix(dat1[,2:4])
>
> The result is:
>
> > mat1
>       x  y  z
> [1,] 10 34 12
> [2,] 25 42 18
> [3,] 14 20  8
>
> Now do the second matrix, perhaps in one step:
>
> mat2 <- as.matrix(read.table(text="ID, weight, weiht2
>  A,  0.25, 0.35
>  B,  0.42, 0.52
>  C,  0.65, 0.75",sep=",",header=TRUE,stringsAsFactors=F)[,2:3])
>
>
> Do note some people use read.csv() instead of read.table, albeit it simply
> calls read.table after setting some parameters like the comma.
>
> The result is what you asked for, including spelling weight wrong once.:
>
> > mat2
>      weight weiht2
> [1,]   0.25   0.35
> [2,]   0.42   0.52
> [3,]   0.65   0.75
>
> Now you wanted to multiply as in matrix multiplication.
>
> > mat1 %*% mat2
>      weight weiht2
> [1,]  24.58  30.18
> [2,]  35.59  44.09
> [3,]  17.10  21.30
>
> Of course, you wanted different names for the columns and you can do that
> easily enough:
>
> result <- mat1 %*% mat2
>
> colnames(result) <- c("index1", "index2")
>
>
> But this is missing something:
>
> > result
>      index1 index2
> [1,]  24.58  30.18
> [2,]  35.59  44.09
> [3,]  17.10  21.30
>
> Do you want a column of ID numbers on the left? If numeric, you can keep it
> in a matrix in one of many ways but if you want to go back to the data.frame
> format and re-use the ID numbers, there are again MANY ways. But note mixing
> characters and numbers can inadvertently convert everything to characters.
>
> Here is one solution. Not the only one nor the best one but reasonable:
>
> recombined <- data.frame(index=dat1$ID,
>                          index1=result[,1],
>                          index2=result[,2])
>
>
> > recombined
>   index index1 index2
> 1     A  24.58  30.18
> 2     B  35.59  44.09
> 3     C  17.10  21.30
>
> If for some reason you need a more general purpose way to do this for
> arbitrary conformant matrices, you can write a function that does this in a
> more general way but perhaps a better idea might be a way to store your
> matrices in files in a way that can be read back in directly or to not
> include indices as character columns but as row names.
>
>
>
>
>
>
> -----Original Message-----
> From: R-help <r-help-bounces using r-project.org> On Behalf Of Val
> Sent: Friday, August 4, 2023 10:54 AM
> To: r-help using R-project.org (r-help using r-project.org) <r-help using r-project.org>
> Subject: [R] Multiply
>
> Hi all,
>
> I want to multiply two  data frames as shown below,
>
> dat1 <-read.table(text="ID, x, y, z
>  A, 10,  34, 12
>  B, 25,  42, 18
>  C, 14,  20,  8 ",sep=",",header=TRUE,stringsAsFactors=F)
>
> dat2 <-read.table(text="ID, weight, weiht2
>  A,  0.25, 0.35
>  B,  0.42, 0.52
>  C,  0.65, 0.75",sep=",",header=TRUE,stringsAsFactors=F)
>
> Desired result
>
> ID  Index1 Index2
> 1  A 24.58 30.18
> 2  B 35.59 44.09
> 3  C 17.10 21.30
>
> Here is my attempt,  but did not work
>
> dat3 <- data.frame(ID = dat1[,1], Index = apply(dat1[,-1], 1, FUN=
> function(x) {sum(x*dat2[,2:ncol(dat2)])} ), stringsAsFactors=F)
>
>
> Any help?
>
> Thank you,
>
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