[R] lookups and joins

[Paul Sorenson]

I have some data coming from SQL sources that I wish to relate in various ways.  For reasons only known to our IT people, this can't be done in SQL at present.

I am looking for an R'ish technique for looking up new columns on a data frame.  As a simple, hardwired example I have tried the following:

# This gives me two columns, one the lookup value and the second one
# the result column, ie my lookup table.
stcl = read.csv("stockclass.csv")
stockclass = as.vector(stcl$stock_class)
# This gives me what appears to be a dictionary or map
names(stockclass) = as.vector(stcl$stock_group)

getstockclass = function(stock_group) {
	try(stockclass[[stock_group]], TRUE)
}
csg$stk_class=factor(sapply(csg$stock_group, getstockclass))

I need the try since if there is a missing value I get an exception.

I also tried something along the lines of (from memory):
getstockclass = function(stock_group) {
	stcl[which(stcl$stock_group == stock_group),]$stock_class
}

These work but I just wanted to check if there was an inbuilt way to do this kind of thing in R?  I searched on "join" without much luck.

Really what I would like is a generic function that:
	- Takes 2 data frames,
	- Some kind of specification on which column(s) to join
	- Outputs the joined frames, or perhaps a vector which is an index vector that I can use on the second data frame.

I don't really want to reinvent SQL and my data sets are not huge.

cheers