A function for summing a variable across quarters

quarter_sum.R

#' A function for summing a variable across quarters.
#' 
#'  @param data a data frame where the variables can be found.
#'  @param Var a character string naming the variable to sum.
#'  @param TimeVar a character string nameing the variable with the time variable. Note, must contain the month and the year.
#'  @param NewVar a character string with the summed variable's name
#'  
#'  @return a data frame with Quarter and summed variable columns.
#'  @importFrom plyr ddply
#'  @export


quarter_sum <- function(data, Var, TimeVar, NewVar = NULL)
{
  require(lubridate)
  require(plyr)
  data$Quarter <- quarter(data[, TimeVar], with_year = TRUE)
  data <- data[, c("Quarter", Var)]
  names(data) <- c("Quarter", "TempVar")
  SumTemp <- ddply(data, .(Quarter), transform, SumVar = sum(TempVar))
  SumTemp <- SumTemp[, c(1, 3)]
  SumTemp <- SumTemp[!duplicated(SumTemp[, 1], SumTemp[, 2]), ]
  if (!is.null(NewVar)){
    names(SumTemp) <- c("Quarter", NewVar)
    return(SumTemp)
  } else
    return(SumTemp)
}