Question

I have a data frame as follows:

Category Name Value

How would I select say, 5 random names per category? Using sample returns random rows using all rows as possible candidates. However, I want to specify the number of random rows per category. Any suggestions?

Update: I am open to using ddply

Answer 1

Best guess in absence of test cases:

  do.call( rbind, lapply( split(dfrm, df$cat) ,
                         function(df) df[sample(nrow(df), 5) , ] )
          )

Tested with Jonathan’s data:

> do.call( rbind, lapply( split(df, df$Category) ,
+                          function(df) df[sample(nrow(df), 5) , ] )
+           )

      Category Name      Value   
1.8          1    8 -0.2496109   #  useful side-effect of labeling source group
1.15         1   15 -0.4037368
1.17         1   17 -0.4223724
1.12         1   12 -0.9359026
1.18         1   18  0.3741184
2.37         2   37  0.3033610
2.34         2   34 -0.4517738
2.36         2   36 -0.7695923
snipped remainder