R help - Sep 2009 - Collecting data in a data structure

Friends

I cannot find this documented anyplace.

I have a data frame with, say, 2 cols

d<-data.frame(x=c(1:5), y=seq(from=2, to=10, by=2))
> d
  x  y
1 1  2
2 2  4
3 3  6
4 4  8
5 5 10

And I want to collect each row where the first col is prime.  To this end I
have a function is.prime()

I want to say something like...

q<-data.frame()
qi<-1
for(i in 1:5){
  if(is.prime(d[i, 1])){
    q[qi,]<-d[i,]
    qi<-qi+1
  }
}



but I get a "Warning in `[<-.data.frame`(`*tmp*`, qi, , value = list(x =
3L,
y = 6)) :
  provided 2 variables to replace 0 variables
" message.

How can I accomplish this?

cheers
Worik

	[[alternative HTML version deleted]]

There are two issues here.

The specifics of the error message are because you declared
a data.frame with 0 columns and then attempted to change two
columns.

 > data.frame()
data frame with 0 columns and 0 rows
 > q <- data.frame()
 > q[1,] <- data.frame(a=3, b=6)
Warning in `[<-.data.frame`(`*tmp*`, 1, , value = list(a = 3, b = 6)) :
   provided 2 variables to replace 0 variables
 >


The second issue is that you need to read about logical subscripting.
  ?`[`


Assuming your function is.prime is vectorized, meaning
 > is.prime(1:10) ## 2 3 5 7
[1]  FALSE TRUE TRUE FALSE TRUE FALSE TRUE FALSE FALSE FALSE

then this single statement will do what you want

qq <- d[is.prime(d[,1]),]

I recommend you not use "q" as a variable name.
It will cause you trouble later when you confuse it with q() for
quitting an R session.

Rich