How to subset matrix with couples of row.name and col.name

Question

I have a simple problem. I have a data frame with two columns of character variables, corresponding to row and column name "couples" from a separate matrix. I just want to use those couples to look up values in the matrix, returned in a vector.

I'm sure it is trivial, but I haven't been able to come across an answer in over an hour of googling. Here is a reproducible example:

m <- as.matrix(data.frame(a=c(1,2,3,4), 
                          b=c(14,15,16,17), 
                          c=c(27,28,29,30), 
                          d=c(43,44,45,46)))
row.names(m) <- c('w','x','y','z')

df <- data.frame(j=c('x','z','z','w','x'),
                 k=c('a','b','d','d','c'))

#I just want to "lookup" a vector of values equal to c(2,17,46,43,28)
result <- sapply(df, function(x) m[x[1],x[2]])

result
j.c k.b 
28  14

Can someone help me figure this out? sapply may not be the best approach, and I'm open to other ideas.

Answer 1

Just coerce the data.frame to be a matrix and use it as index

> m[as.matrix(df)]
[1]  2 17 46 43 28

Take a look at this document to realize why this works.