How to do for loops without overwriting? - r

Question

I have a large data.frame called rain with information of many species mesured in different plots at different times (census), from which I want to extract the information. This data frame have many collumns, and in dataF2 I want to keep the same structure however I want to extract from rain the information of the penultimate census (Census.No is one of the collumns of rain) in each plot (Plot.Code is another one). In idx3 I have the information of the number of the penultimate census for each plot.
It's easy to do it for one plot
data1<- rain[Plot.Code==idx3[1,1] & Census.No==idx3[1,2],]
I've been trying to do for loops in R.. but I keep overwriting my data.frame and ending up just with the last loop.
dataF2<- data.frame(nrow= nrow (rain), ncol = ncol (rain))
summary (dataF2)
for (i in 1:length (idx3[,1])){
dataF2<- rain[Plot.Code==idx3[i,1] & Census.No==idx3[i,2],]
}
Here I want to extract from a data frame the information of the penultimate census in each plot (ixd3 contains this information of what was the penultimate census in each plot).
I've tried many things, like:
dataF2<- data.frame(nrow= nrow (rain), ncol = ncol (rain))
for (i in 1:length (idx3[,1])){
data1<- rainfor[Plot.Code==idx3[i,1] & Census.No==idx3[i,2],]
dataF2<- rbind (data1[i])
}
But nothing worked.. my problem is that it keeps overwithin on dataF2!
Cheers!!!

Answer 1

Your clarifications in the comments helped somewhat, but reproducible examples are always better. Let's start at the beginning:
dataF2<- data.frame(nrow= nrow (rain), ncol = ncol (rain))
This is wrong. I think that you're trying to create an empty data frame with the same dimensions as your data frame rain. If you examine dataF2 you'll see that this is far from what you have done with this line. If you read the documentation for the function ?data.frame it will become clear that there are no arguments called nrow and ncol. What you probably intended was something like this:
dataF2 <- rain
dataF2[] <- NA
Inside your for loop you are overwriting your entire data frame because....you are overwriting your entire data frame.
dataF2<- rain[Plot.Code==idx3[i,1] & Census.No==idx3[i,2],]
This assigns something to dataF2, replacing it completely. If you want to assign to just a single row of dataF2 you need to assign to that specific row:
dataF2[i,] <- rain[Plot.Code==idx3[i,1] & Census.No==idx3[i,2],]
I can't absolutely assure that this will work correctly, since you haven't provided a sufficiently detailed example, so I'm not sure that all the dimensions will coincide properly when you index on i. But this is the basic idea.