R help - Jul 2004 - How to do a "go to " in a loop in R

Hi there,

I'm writing a function which involves a loop. What to write in the
"?"
place would allow it skips the "for loop" and goes to 
"a[i]<-0".
 
 
 a<-b[s>3,]

 if (nrow(a)==0) ?????????????
                 

	for (i in 1:nrow(a)){
  		a[i]<-1
	}
 a[i]<-0


Lisa Wang
Cancer Informatics,
Ontario Cancer Institute/Princess Margaret Hospital, University Health
Network;
Email: lisawang at uhnres.utoronto.ca
Phone: 416 946 4501 ext. 5201
Fax: 416 946 4619

Lisa Wang <lisawang <at> uhnres.utoronto.ca> writes:

: 
: Hi there,
: 
: I'm writing a function which involves a loop. What to write in the
"?"
: place would allow it skips the "for loop" and goes to 
"a[i]<-0".
: 
:  a<-b[s>3,]
: 
:  if (nrow(a)==0) ?????????????
: 
: 	for (i in 1:nrow(a)){
:   		a[i]<-1
: 	}
:  a[i]<-0

There are a number of errors in the above but I assume your question
is how to set up a loop over an index of the rows of  a  so that the
loop executes zero times if a has no rows.  Try this:

  for(i in seq(len=nrow(a))) ...

Also, depending on what you want to do, you might consider whether you 
need a loop in the first place.

Well, there ain't no such thing in R ....  And your code doesn't
really make sense anyway.  You talk about ``a[i]'' where a is
apparently a ***matrix*** (with the same number of columns as the
matrix b, but with fewer --- possibly 0 --- rows).  And in such as
setting a[i] has a meaning, but probably not what you want.  And
talking about ``a[i] <- 0'' when i is a running index, running in
this case over the empty set (!!!) doesn't make sense either.

Perhaps what you want to accomplish is along something like the
following lines:

a<-b[s>3,]
if(nrow(a)==0) {
	result <- 0
} else {
	result <- numeric(nrow(a))
        for (i in 1:nrow(a)){
		result[i] <- 1
        }
}

Of course if that's what you're really trying to do, the for-loop
is a silly waste of time.  Instead do

a<-b[s>3,]
if(nrow(a)==0) {
        result <- 0
} else result <- rep(1,nrow(a))

or slightly more elegantly

a<-b[s>3,]
result <- if(nrow(a)==0) 0 else rep(1,nrow(a))

				cheers,

					Rolf Turner
					rolf at math.unb.ca

===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+==You
wrote:
> I'm writing a function which involves a loop. What to write in the
"?"
> place would allow it skips the "for loop" and goes to 
"a[i]<-0".
>  
>  
>  a<-b[s>3,]
> 
>  if (nrow(a)==0) ?????????????
>                  
> 
> 	for (i in 1:nrow(a)){
>   		a[i]<-1
> 	}
>  a[i]<-0
> 
> 
> Lisa Wang
> Cancer Informatics,
> Ontario Cancer Institute/Princess Margaret Hospital, University Health
> Network;
> Email: lisawang at uhnres.utoronto.ca
> Phone: 416 946 4501 ext. 5201
> Fax: 416 946 4619