# R function to read stock prices from Yahoo and convert to excess returns
capmRead <- function(symbols,start = "1998-01-01"){
require(tseries)
require(zoo)
fom <-function(x) as.Date(sub("..$","01",format(x)))

# Riskfree Data
x <- get.hist.quote("^irx",start=start,quote="Close")
x <- aggregate(x,fom,head,1)
rkfree <- diff(log(x))

# Market Data 
x <- get.hist.quote("^gspc",start=start,quote="Close")
x <- aggregate(x,fom,head,1)
market <- diff(log(x))

capm <- market - rkfree
for(firm in symbols){
	x <- get.hist.quote(firm, start = start, quote = "AdjClose")
	x <- aggregate(x, fom, head, 1)
	x <- diff(log(x)) - rkfree
	capm <- merge(capm,x) 
	}
capm <- merge(capm,rkfree)
dimnames(capm)[[2]] <- c("market",symbols,"rkfree")
as.data.frame(capm)
}