R help - Jan 2011 - return object from loop inside a function

Hi All,

I have a for loop inside the function and I cannot get UUU to give me an 
updated grid.dens object when I run the function (it does update when I 
run just the for loop). Here's a simplified version of my function:

UUU=function(pop, grid.dens) {

for (i in 1:10){
Food=grid.dens[pop$yloc[i],pop$xloc[i]]   #use initial grid.dens values
Consumed=(pop$weight[i]*0.25)
Left=Food-Consumed
grid.dens[pop$yloc[i],pop$xloc[i]]=Left   #update grid.dens values on i
pop$birth[i]=pop$birth[i]+1
}
return(pop)
return(grid.dens)
}

I get an updated pop, but not an updated grid.dens.  What am I doing wrong?

Thanks!

Nico

Nicolas -
    Along with returning a value, the return function passes
control back to the caller of the function.  So any statement
after a return statement will just be ignored.
    In R the way this is handled is by returning a list.  Change

return(pop)
return(grid.dens)

to

return(list(pop=pop,grid.dens))

and you can access the two parts like this:

result = UUU(pop,grid.dens)

result$pop        # updated pop
result$grid.dens  # updated grid.dens

Hope this helps.

 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Tue, 25 Jan 2011, Nicolas Gutierrez wrote:
> Hi All,
>
> I have a for loop inside the function and I cannot get UUU to give me an 
> updated grid.dens object when I run the function (it does update when I run
> just the for loop). Here's a simplified version of my function:
>
> UUU=function(pop, grid.dens) {
>
> for (i in 1:10){
> Food=grid.dens[pop$yloc[i],pop$xloc[i]]   #use initial grid.dens values
> Consumed=(pop$weight[i]*0.25)
> Left=Food-Consumed
> grid.dens[pop$yloc[i],pop$xloc[i]]=Left   #update grid.dens values on i
> pop$birth[i]=pop$birth[i]+1
> }
> return(pop)
> return(grid.dens)
> }
>
> I get an updated pop, but not an updated grid.dens.  What am I doing wrong?
>
> Thanks!
>
> Nico
>
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