Creating a factor from a combination of vectors

Dear list,

Here's a little problem I already solved with my own coding style, but I 
feel there is a more efficient and cleaner way to write it, but had no 
success finding the "clever" solution.

I want to produce a factor from a subset of the combination of two 
vectors. I have the vectors a et b in a data-frame :

df <- expand.grid(a=c(0, 5, 10, 25, 50), b=c(0, 25, 50, 100, 200))
fac.df

   a   b
1   0   0
2   5   0
3  10   0
4  25   0
5  50   0
6   0  25
7   5  25
<snip>

and want to create a factor which levels correspond to particular 
combinations of a and b (let's say Low for a=0 & b=0, Medium for a=10 & 
b=50, High for a=50 & b=200, others levels set to NA), reading them from a 
data-frame which describes the desired subset and corresponding levels.

Here's my own solution (inputs are data-frames df and cas, output is the 
sub factor):

cas <- as.data.frame(matrix(c(0, 10,50, 0, 50, 200), 3, 

2,dimnames=list(c("Low", "Medium", "High"), c("a", "b"))))

cas

       a   b
Low     0   0
Medium 10  50
High   50 200

sub <- character(length(df$a))
for (i in 1:length(df$a)) {

+   temp <- rownames(cas)[cas$a==df$a[i] & cas$b==df$b[i]]
+   sub[i] <- ifelse(length(temp)>0, temp, NA)
+ }

sub <- ordered(sub, levels=c("Low", "Medium", "High"))
sub

[1] Low    <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>
<NA>   <NA>   <NA>   Medium <NA>   <NA>   <NA>   <NA> [18] 
<NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   High Levels: Low < Medium 
< High

I was looking for a vectorized solution (apply style) binding data-frames 
df and cas, but didn't succeed avoiding the for loop. Could anybody bring 
me the ligths over the darkness of my ignorance ? Thank you very much in 
advance.

--
Ir. Yves BROSTAUX
Unit?? de Statistique et Informatique
Facult?? universitaire des Sciences agronomiques de Gembloux (FUSAGx)
8, avenue de la Facult??
B-5030 Gembloux
Belgique
T??l: +32 81 62 24 69
Email: brostaux.y at fsagx.ac.be