R help - Jul 2009 - z[i,j] = x[i,j] * y(i+j) ?

For the life of me I couldn't work out what to searc

I have an m*n numeric matrix x and a numeric vector y (of length m+n-1)

How do I do a calculation like this?

z[i,j] = x[i,j] * y[i+j] ?


Well, one can write a pair of loops, or write a single loop 
within which we calculate a vector at a time, but ...

is there a "neat" way to do it?

tiny example:
x<-matrix(data=c(32,46,21,28,58,60,33,32,16,NA,10,15),nrow=4)
y<-c(1.1,1.09,1.08,1.06,1.05,1.02)
z<-matrix(nrow=4,ncol=3)

z[,1]=x[,1]*y[1:4]
z[,2]=x[,2]*y[2:5]
z[,3]=x[,3]*y[3:6]
 
which produces:
> x
     [,1] [,2] [,3]
[1,]   32   58   16
[2,]   46   60   NA
[3,]   21   33   10
[4,]   28   32   15
> y
[1] 1.10 1.09 1.08 1.06 1.05 1.02
> z
      [,1]  [,2]  [,3]
[1,] 35.20 63.22 17.28
[2,] 50.14 64.80    NA
[3,] 22.68 34.98 10.50
[4,] 29.68 33.60 15.30
> z/x
     [,1] [,2] [,3]
[1,] 1.10 1.09 1.08
[2,] 1.09 1.08   NA
[3,] 1.08 1.06 1.05
[4,] 1.06 1.05 1.02

(this last to indicate what each element of x was multiplied by to produce
z... well, apart from the NA)

Thanks for any pointers

Glen

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Try this:

z <- x * y[row(x) + col(x)]

z2 <- x
for(i in 1:nr) for(j in 1:nc) z2[i,j] <- x[i,j] * y[i+j]

all.equal(z, z2) # TRUE



On Wed, Jul 29, 2009 at 11:56 PM, glen_b<glnbrntt at gmail.com>
wrote:
>
>
> For the life of me I couldn't work out what to searc
>
> I have an m*n numeric matrix x and a numeric vector y (of length m+n-1)
>
> How do I do a calculation like this?
>
> z[i,j] = x[i,j] * y[i+j] ?
>
>
> Well, one can write a pair of loops, or write a single loop
> within which we calculate a vector at a time, but ...
>
> is there a "neat" way to do it?
>
> tiny example:
> x<-matrix(data=c(32,46,21,28,58,60,33,32,16,NA,10,15),nrow=4)
> y<-c(1.1,1.09,1.08,1.06,1.05,1.02)
> z<-matrix(nrow=4,ncol=3)
>
> z[,1]=x[,1]*y[1:4]
> z[,2]=x[,2]*y[2:5]
> z[,3]=x[,3]*y[3:6]
>
> which produces:
>
>> x
> ? ? [,1] [,2] [,3]
> [1,] ? 32 ? 58 ? 16
> [2,] ? 46 ? 60 ? NA
> [3,] ? 21 ? 33 ? 10
> [4,] ? 28 ? 32 ? 15
>
>> y
> [1] 1.10 1.09 1.08 1.06 1.05 1.02
>
>> z
> ? ? ?[,1] ?[,2] ?[,3]
> [1,] 35.20 63.22 17.28
> [2,] 50.14 64.80 ? ?NA
> [3,] 22.68 34.98 10.50
> [4,] 29.68 33.60 15.30
>
>> z/x
> ? ? [,1] [,2] [,3]
> [1,] 1.10 1.09 1.08
> [2,] 1.09 1.08 ? NA
> [3,] 1.08 1.06 1.05
> [4,] 1.06 1.05 1.02
>
> (this last to indicate what each element of x was multiplied by to produce
> z... well, apart from the NA)
>
> Thanks for any pointers
>
> Glen
>
> --
> View this message in context:
http://www.nabble.com/z-i%2Cj--%3D-x-i%2Cj--*-y%28i%2Bj%29---tp24731799p24731799.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> 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.
>

I take it you mean

z[i,j] = x[i,j]*y[i+j-1]

(as you had it z[1,1] = x[1,1]*y[2], whereas your example suggests is should be
y[1])

Here is a suggestion:
> z <- x * y[outer(1:nrow(x), 1:ncol(x), "+")-1]
> z
      [,1]  [,2]  [,3]
[1,] 35.20 63.22 17.28
[2,] 50.14 64.80    NA
[3,] 22.68 34.98 10.50
[4,] 29.68 33.60 15.30
>  

Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of glen_b
Sent: Thursday, 30 July 2009 1:57 PM
To: r-help at r-project.org
Subject: [R] z[i,j] = x[i,j] * y(i+j) ?



For the life of me I couldn't work out what to searc

I have an m*n numeric matrix x and a numeric vector y (of length m+n-1)

How do I do a calculation like this?

z[i,j] = x[i,j] * y[i+j] ?


Well, one can write a pair of loops, or write a single loop 
within which we calculate a vector at a time, but ...

is there a "neat" way to do it?

tiny example:
x<-matrix(data=c(32,46,21,28,58,60,33,32,16,NA,10,15),nrow=4)
y<-c(1.1,1.09,1.08,1.06,1.05,1.02)
z<-matrix(nrow=4,ncol=3)

z[,1]=x[,1]*y[1:4]
z[,2]=x[,2]*y[2:5]
z[,3]=x[,3]*y[3:6]
 
which produces:
> x
     [,1] [,2] [,3]
[1,]   32   58   16
[2,]   46   60   NA
[3,]   21   33   10
[4,]   28   32   15
> y
[1] 1.10 1.09 1.08 1.06 1.05 1.02
> z
      [,1]  [,2]  [,3]
[1,] 35.20 63.22 17.28
[2,] 50.14 64.80    NA
[3,] 22.68 34.98 10.50
[4,] 29.68 33.60 15.30
> z/x
     [,1] [,2] [,3]
[1,] 1.10 1.09 1.08
[2,] 1.09 1.08   NA
[3,] 1.08 1.06 1.05
[4,] 1.06 1.05 1.02

(this last to indicate what each element of x was multiplied by to produce
z... well, apart from the NA)

Thanks for any pointers

Glen

-- 
View this message in context:
http://www.nabble.com/z-i%2Cj--%3D-x-i%2Cj--*-y%28i%2Bj%29---tp24731799p24731799.html
Sent from the R help mailing list archive at Nabble.com.

______________________________________________
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.