Question

I am currently working on two different dataframes, one of which is extremely long (long). What I need to do is to select all the rows of long whose corresponding id_type appears at least once in the other (smaller) dataset.

Suppose the two dataframes are:

long <- read.table(text = "
  id_type   x1   x2

   1       0     0  
   1       0     1
   1       1     0
   1       1     1
   2       0     0
   2       0     1
   2       1     0
   2       1     1
   3       0     0  
   3       0     1
   3       1     0
   3       1     1
   4       0     0  
   4       0     1
   4       1     0
   4       1     1", 
header=TRUE) 

and

short <- read.table(text = "
  id_type   y1   y2    

   1       5     6    
   1       5     5    
   2       7     9", 
     header=TRUE) 

In practice, what I am trying to obtain is:

 id_type   x1   x2    

  1       0     0  
  1       0     1
  1       1     0
  1       1     1
  2       0     0  
  2       0     1
  2       1     0
  2       1     1

I have tried to use out <- long[long[,"id_type"]==short[,"id_type"], ], but it is clearly wrong. How would you proceed? Thanks

Answer 1

Just use %in%:

out <- long[long$id_type %in% short$id_type, ]

Look at ?"%in%".