[R] help using outer function

[Dan Davison]

On Sun, Aug 10, 2008 at 09:02:59AM -0700, warthog29 wrote:
> 
> Hi,
> I would like to use the R's outer function on y below so that I can subtract
> elements from each other. The resulting dataframe is symmetric, save for the
                                           ^^^^^^
outer() returns a matrix, not a data frame.

> negative signs on the other half of the numbers. I would like to get only
> half of the dataframe. Here is the code I wrote (it is returning only the
> first line of the all elements I want. Please help). 
> y<-c(4,4,3.9,3.8,3.7,3.6,3.5,3.5,3.5,3.3,3.2,3.2)
> 
> b<-outer(y,y,"-")

> b<-as.matrix(by)

I assume that line was supposed to be b<-as.matrix(by). In any case
you don't need it; b is a matrix already.

> # I want to keep the elements:
> #b[1,2:12],
> #b[2,3:12],
> #.........until
> #b[11,12:12].

Use upper.tri() to get the upper-triangle:

 b[upper.tri(b, diag=FALSE)]
 [1] 0.0 0.1 0.1 0.2 0.2 0.1 0.3 0.3 0.2 0.1 0.4 0.4 0.3 0.2 0.1 0.5 0.5 0.4 0.3
[20] 0.2 0.1 0.5 0.5 0.4 0.3 0.2 0.1 0.0 0.5 0.5 0.4 0.3 0.2 0.1 0.0 0.0 0.7 0.7
[39] 0.6 0.5 0.4 0.3 0.2 0.2 0.2 0.8 0.8 0.7 0.6 0.5 0.4 0.3 0.3 0.3 0.1 0.8 0.8
[58] 0.7 0.6 0.5 0.4 0.3 0.3 0.3 0.1 0.0

Or perhaps you want to knock out the negative entries, but still keep the matrix structure:

 b[lower.tri(b)] <- NA

or perhaps you wanted 

b <- abs(outer(y,y,"-"))

in the first place?

> #Here is the function I wrote to get half of matrix:
> 
> wk<-function(p){
> for (i in 2:p){
> ri<-b[i-1,i:p]
> return(ri)
> }
> }
> wk(12)
> #[1] 0.0 0.1 0.2 0.3 0.4 0.5 0.5 0.5 0.7 0.8 0.8

I think you were intending this function to be something like this

wk<-function(p){
    ri <- NULL
    for (i in 2:p){
        ri<-c(ri, b[i-1,i:p])
    }
    return(ri)
}

Note that this function will give a different result from upper.tri(),
because you are concatenating elements in the *rows* of the matrix,
whereas the way matrices are represented in R has consecutive elements
running down the columns. I.e. look at

> A <- matrix(nrow=2,ncol=2)
> A
     [,1] [,2]
[1,]   NA   NA
[2,]   NA   NA
> A[] <- 1:4
> A
     [,1] [,2]
[1,]    1    3
[2,]    2    4

Dan

> 
> As you can see, it is only returning the first line. I would like other
> corresponding elements too, to be found in row 2 to 12. Thanks. 
> -- 
> View this message in context: http://www.nabble.com/help-using-outer-function-tp18914432p18914432.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.