On Jun 25, 2009, at 10:02 AM, Rama Ramakrishnan wrote:
Dear R-Users,
I need to lookup values from a 2-d table using the row names and
column
names as indices. I was wondering if there's a way to do this
without an
explicit loop.
Example:
#x is the 2-d table that holds the values
x <- matrix(rnorm(26*12),nrow=26)
rownames(x) <- letters
colnames(x) <- month.name
#y is a data frame that has the "keys" I want to use as indices into x
y <- data.frame(ltrs=sample(letters,5),mnths=sample(month.name,
5),values=0)
#I want to fill in the "values" column using the "ltrs" and "mnths"
columns
as keys to look up
# the associated value from x
?apply
x$value <- apply(y, 1, function(z) x[ z['ltrs'], z['mnths'] ])
#One way to do this is with a FOR loop
for (i in 1:nrow(y)) {y$val[i] <- x[y$ltrs[i],y$mnths[i]]}
My question: Is there a more efficient way (e.g., one without using an
explicit loop) to do this?
Thanks in advance!
-Rama Ramakrishnan
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