Populate a new column in a dataframe with a lookup from a double matrix

Question

I have a dataframe df:

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

And a double matrix m with named rows/

Answer 1

Another answer Using the reshape2 and plyr (optional just for join) packages.

require(plyr)
require(reshape2)

Df <- data.frame(colour = c("red", "blue", "blue", "green"), 
                  shape = c("circle", "square", "circle", "sphere"))

Mat <- matrix(1:9, dimnames = list(c("red", "blue", "green"),
                                   c("circle", "square", "sphere")), 
                    nrow = 3)

Df2 <- melt.array(Mat, varnames = c("colour", "shape"))

join(Df, Df2)
result <- join(Df, Df2)

join(Df, Df2)
Joining by: colour, shape
  colour  shape value
1    red circle     1
2   blue square     5
3   blue circle     2
4  green sphere     9

Hope this help