[R] [FORGED] Not seeing the results of a function

[Rolf Turner]

On 30/03/20 7:55 am, David wrote:

> I’m trying to write a function that will add items to two vectors, and 
> then to create a third vector that is of the form 1, 2, 3, …, length of 
> one of the newly modified vectors.  My problem is that what I’ve written 
> doesn’t seem to return any of those modifications.  How can I get the 
> new values to be returned? Here’s the function below.  I want to add a 
> value, stp, to the end of the stpos vector, a value mcp to the end of 
> the mcpos vector, and the to create that days vector.  When I call this 
> function with numerical values for stp and mcp, and then ask to see 
> stpos, mcpos, and days, nothing has been changed.  How can I fix this?
> 
> Here’s the function:
>  > addday <- function(stp,mcp,stpos,mcpos){stpos<-c(stpos,stp)
> + mcpos<-c(mcpos,mcp)
> + days<-c(1:length(stpos))}

You are probably thinking in terms of "macro languages" rather than in R 
terms.

An R function returns, essentially, the last assigned value.  So your 
function will return the "days" vector; all else disappears into a black 
hole in cyberspace.

Something like the following should do what you want:

addday <- function(stp,mcp,stpos,mcpos){
     stpos<-c(stpos,stp)
     mcpos<-c(mcpos,mcp)
     days<-c(1:length(stpos))
     rslt <- list(stpos=stpos,mcpos=mcpos,days=days)
     rslt
}

As is so often said on this list, you would do well to spend some time 
reading and inwardly digesting a basic R tutorial, such as "An 
Introduction to R", readily available from the R web page (under "Manuals").

HTH

cheers,

Rolf Turner

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276