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

#recreating your data
dat <- read.table(text="colour  shape
'red'   circle
'blue'  square
'blue'  circle
'green' sphere", header=TRUE)

d2 <- matrix(c(1:9), ncol=3, nrow=3, byrow=TRUE)
dimnames(d2) <-list(c('circle', 'square', 'sphere'),
c("red", "blue", "green"))
d2<-as.table(d2)

#make a list of matching to the row and column names of the look up matrix
LIST <- list(match(dat[, 2], rownames(d2)), match(dat[, 1], colnames(d2)))
#use sapply to index the lookup matrix using the row and col values from LIST 
id <- sapply(seq_along(LIST[[1]]), function(i) d2[LIST[[1]][i], LIST[[2]][i]])
#put it all back together
data.frame(id=id, dat)