[R] How do I multiply labeled vectors of numbers?

[Marc Schwartz]

on 12/10/2008 05:17 PM Bill McNeill (UW) wrote:
> I'm an R newbie trying to do an operation that seems like it should be very
> simple, but after much playing around with the interface and reading "An
> Introduction to R" I can figure out how to do it.
> 
> I have two text files.  File1 contains:
> 
> type    n
> dog        2
> cat        4
> horse    6
> 
> File2 contains:
> 
> type    n
> horse    1
> dog        3
> cat        9
> mouse 11
> 
> I want to read both files into R and multiply corresponding values of n so
> that I end up with:
> 
> dog       2*3 = 6
> cat        4*9 = 36
> horse    6*1 = 6
> mouse  0*11 = 0
> 
> Note that the type rows are not necessarily in the same order in the two
> files, and that a missing type gets a default n = 0 value.
> 
> I figure the way to do this is use read.table and do the appropriate
> manipulation of the factors, but I can't figure out what those manipulations
> would be.  Basically I want to use a factor table as a hash table, but I
> haven't been able to figure out how to do this, which makes me think I'm not
> conceptualizing this is the correct R-ish way.
> 
> Thanks.


F3 <- merge(F1, F2, by = "type", all = TRUE)

> F3
   type n.x n.y
1   cat   4   9
2   dog   2   3
3 horse   6   1
4 mouse  NA  11


# Convert NA's to 0

F3[is.na(F3)] <- 0

> F3
   type n.x n.y
1   cat   4   9
2   dog   2   3
3 horse   6   1
4 mouse   0  11


F3$prod <- F3$n.x * F3$n.y

> F3
   type n.x n.y prod
1   cat   4   9   36
2   dog   2   3    6
3 horse   6   1    6
4 mouse   0  11    0


See ?merge

HTH,

Marc Schwartz