[R] specific matrix element tranformation

[Rui Barradas]

Hello,

I've made a mistake, it's in the last if/else/if/else. Complete 
statement below.

Em 17-08-2012 06:23, Rui Barradas escreveu:
> 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)
>
>

Correction:

     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] < 0){
         warning("a[1, 1] 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]
     }

I thought it was sqrt(a[1,1]/a[1,2]).
And apparently I've also forgot to sign.

Rui Barradas

>>
>> Cheers,
>> B
>>
>>
>>
>>
>> -- 
>> View this message in context: 
>> http://r.789695.n4.nabble.com/specific-matrix-element-tranformation-tp4640550p4640554.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
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>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> ______________________________________________
> 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.