Take random sample by group

Question

I have a data frame made by almost 50,000 rows spread in 15 different IDs (every ID has thousands of observations). Data frame looks like:

        ID  Year    Temp    ph
1       P1  1996    11.3    6.80
2       P1  1996    9.7     6.90
3       P1  1997    9.8     7.10
...
2000    P2  1997    10.5    6.90
2001    P2  1997    9.9     7.00
2002    P2  1997    10.0    6.93

I want to take 500 random rows for every ID (so 500 for P1, 500 for P2,....) and create a new df. I try:

new_df<-df[df$ID %in% sample(unique(dfID),500),]

But it takes randomly one ID, while I need 500 random rows for every ID.

Answer 1

Try this:

library(plyr)
ddply(df,.(ID),function(x) x[sample(nrow(x),500),])