begdate<-as.Date("2006-01-01")
enddate<-as.Date("2010-12-01")
stocks<-c("IBM","C","MSFT","WMT")
d<-getMonthlyRet("^GSPC") # ^GSPC is S&P500
base<-subset(d,d[,1]>=begdate & d[,1]<=enddate)
rm(d)
for(i in 1:length(stocks)){
      d<-getMonthlyRet(stocks[i])
      base<-merge(base,d)
}
