help recoding

hi R-listers,

I would like some help recoding a variable. I have a dataframe 'cause' 
that translates between a set of codes:

acc	nds
-	-
1	2
3	4	
5	8
...	...

the desired result for dataframe 'p':
a
-
1
5
5

would be:
a	b
-	-
1	2
5	8
5	9

I have tried:

transform(p, b=cause$nds[cause$acc==p$a])

but for some reason it complains about the difference in length between 
the translation and result dataframes.

This prints the desired result, but I am unsure how to collect the 
results into a variable.

for (i in p) print cause$nds[cause$acc==i]

Any help greatly appreciated.

Selwyn