dplyr helper functions

code.r

#' Grouped mutate
#'
#' Group by columns, then mutate, and then regroup to previous groups.
#' @param .data data frame
#' @param .by character vector with columns to group by
#' @param ... arguments passed to mutate(...)
#' @export
grouped_mutate <- function(.data, .by, ...) {
  pre <- groups(.data)
  if (is.null(pre)) pre <- list()
  regroup(.data, lapply(.by, as.symbol)) %>% mutate(...) %>% regroup(pre)
}

#' Relative difference between dates
#' 
#' ...
#' 
#' @param date_col date column
#' @param value_col value column
#' @param date_diff date difference, e.g. lubridate::days(7)
#'
#' @export
relative_by_date <- function(date_col, value_col, date_diff = lubridate::days(7)) {
  sapply(as.Date(date_col), function(i){
    p0 <- value_col[which(date_col == i - date_diff)]
  	p1 <- value_col[which(date_col == i)]
    x <- (p1 - p0) / p0
    if (length(x) != 0) x else NA
  })
}