Question

I have a dataframe df:

colour  shape
'red'   circle
'blue'  square
'blue'  circle
'green' sphere

And a double matrix m with named rows/columns

      circle square sphere  
red   1      4      7
blue  2      5      8
green 3      6      9

I’d like to add a new column to DF such that I obtain:

id  colour  shape
1   'red'   circle
5   'blue'  square
2   'blue'  circle
9   'green' sphere

I’ve tried doing this with the following code but it doesn’t seem to work:

df$id <- m[df$colour,df$shape]

I’ve also tried apply(); and similar but with no luck. Can anyone tell me the right approach to doing this without using a loop?

Answer 1

I think I might win the shortest answer contest here as long as those are character vectors rather than factors which might be more expected unless you made specifid effort to avoid. It really only adds cbind to convert the two df “character” vectors to a two column matrix expected by the [.matrix function that you were very close to success in using. (And it also seems reasonably expressive.)

# Data construct
d <- data.frame(color=c('red','blue','blue','green'), 
shape=c('circle','square','circle','sphere'), stringsAsFactors=FALSE)
 m <- matrix(1:9, 3,3, dimnames=list(c('red','blue','green'), c('circle','square','sphere')))
# Code:

 d$id <- with( d, m [ cbind(color, shape) ] )
 d
  color  shape id
1   red circle  1
2  blue square  5
3  blue circle  2
4 green sphere  9