[R] Check for is.object

Phil Spector spector at stat.berkeley.edu
Mon Nov 22 17:58:30 CET 2010

Santosh -
    The simple answer to your question is to initialize 
NewObject to NULL before the loop, i.e.

    newObject = NULL

However, I should point out this is one of the most 
inefficient ways to program in R.  A far better way is
to allocate enough space for NewObject outside your
loop, and "fill it in" in the loop.  Here's a simple
example to give you an idea of the difference in time
the two methods require:

> system.time({answer = matrix(NA,1000,5);
+              for(i in 1:1000)answer[i,] <- sample(10,5)})
    user  system elapsed
   0.020   0.000   0.017 
> system.time({answer=NULL;
+              for(i in 1:1000)answer=rbind(answer,sample(10,5))})
    user  system elapsed
   0.072   0.000   0.070

However, it gets even worse if the sample size is larger:

> system.time({answer = matrix(NA,10000,5);
+              for(i in 1:10000)answer[i,] <- sample(10,5)})
    user  system elapsed
   0.184   0.000   0.184 
> system.time({answer=NULL;for(i in 1:10000)
+              answer=rbind(answer,sample(10,5))})
    user  system elapsed
   5.492   0.032   5.562

Even if you don't know how big your newObject matrix will
become, it's still far more efficient to overallocate the 
matrix and then truncate it at the end.

I'd strongly recommend that you avoid building your matrix
incrementally inside a loop!

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

On Mon, 22 Nov 2010, Santosh Srinivas wrote:

> Hello,
> I am trying to recursively append some data from multiple files into a
> common object
> For this, I am using in a loop
> NewObject <- rbind(NewObject,tempObject)
> For the first loop, obviously there is no NewObject ... so I wanted to do
> NewObject <- tempObject[0,]
> Now when it loops again I want to put the statement do "NewObject <-
> tempObject[0,]" inside a if statement ... so that it does I can skip it once
> NewObject has been initialized.
> But, is.object doesn't seem to work.
> What is the alternative check that I can do? And is there a better way to
> achieve what I want?
> Thanks,
> S
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