[R] specific matrix element tranformation

[Rui Barradas]

Hello,

Inline

Em 17-08-2012 04:39, bantex escreveu:
> Hi guys,
>
> After a long while I came up with this :
>
> set.seed(2)
> a<-matrix(rnorm(4),ncol=2)
> abc<-function(a)
> {
> b=matrix(NA,nrow=3,ncol=3)
> b[1,1]=a[1,1]+1
> b[1,2]=a[2,1]*a[2,2]
> b[1,3]=a[2,2]+a[1,1]
> b[2,1]=a[1,1]-5
> b[2,2]=sqrt(a[2,2])
> b[2,3]=a[1,1]/a[2,2]
> b[3,1]=a[2,2]-3
> b[3,2]=a[1,1]*(a[1,2]+a[2,2])
> b[3,3]=sqrt(a[1,1])/a[1,2]
>
> return(b=matrix(c(b[1,1],b[1,2],b[1,3],b[2,1],b[2,2],b[2,3],
> b[3,1],b[3,2],b[3,3]),ncol=3))

You don't need to say that 'b' is a matrix a second time! And this form 
is wrong, you should add 'byrow = TRUE' since you're using row order.

> }
>
> How can I improve on it?

With your data, you would have two errors in applying function abc to 
matrix 'a'. Try the following.

fun <- function(a){
     b <- matrix(nrow = 3, ncol = 3)
     #let's say these are the transformations i wish to perform
     b[1,1] <- a[1,1]+1
     b[1,2] <- a[2,1]*a[2,2]
     b[1,3] <- a[2,2]+a[1,1]
     b[2,1] <- a[1,1]-5
     if(a[2,2] < 0){
         warning("a[2, 2] is negative, using zero for its square root 
b[2,2].")
         b[2,2] <- 0
     }else{
         b[2,2] <- sqrt(a[2,2])
     }
     if(a[2,2] == 0){
         warning("a[2, 2] is zero, using zero for division result b[2,3].")
         b[2,3] <- 0
     }else{
         b[2,3] <- a[1,1]/a[2,2]
     }
     b[3,1] <- a[2,2]-3
     b[3,2] <- a[1,1]*(a[1,2]+a[2,2])
     if(a[1,2] == 0){
         warning("a[1, 2] is zero, using zero for division result b[3,3].")
         b[3,3] <- 0
     }else if(a[1,1]/a[1,2] < 0){
         warning("a[1,1]/a[1,2] is negative, using zero for its square 
root b[3,3].")
         b[3,3] <- 0
     }else{
         b[3,3] <- sqrt(a[1,1]/a[1,2])
     }
     b
}

set.seed(2)
a <- matrix(rnorm(4),ncol = 2)
fun(a)


>
> Cheers,
> B
>
>
>
>
> --
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