gistfile1.r

quarterCalc <- function(interviewTime, interviewState, priceData){
  if(!inherits(interviewTime, "yearmon")){
    stop("Wrong date type")
  }
        newData <- subset(priceData, as.character(priceData$statename)==as.character(interviewState))
if(dim(subset(priceData, time==interviewTime & as.character(statename)==as.character(interviewState)))[1] > 1){return(NA)}
index = which(newData$time == interviewTime, arr.ind=TRUE)
if(length(index)>1){ stop(print(index))}
else if(index>=3){
    quarterly <- data.frame("time" = interviewTime, "state" = interviewState, "price" = mean(newData$nprc[seq((index-3), (index-1), 1)], na.rm=TRUE))
  }else{
    warning("Missing data. There are less than three prior months")
        return(NA)
  }
  return(quarterly$price[1])
}