#' Calculate cumulative miles for each individual by week
#'
#' For each individual, calculate the miles run that week and the
#' cumulative miles run up to and including that week.
#'
#' @param d A \code{data.frame} containing all the card and day information.
#' @param miles_per_card An integer specifying how many miles a card is worth.
#' @return A \code{data.frame} with the cumulative miles for each individual by week.
#' @import dplyr
#' @export
#' @examples
#' \dontrun{
#' read_cards() %>% cumulative_miles_by_id
#' }
cumulative_miles_by_id = function(d, miles_per_card=5) {
d %>%
group_by_(~id) %>%
arrange_(~week) %>%
mutate_(one = 1,
cumulative_miles = ~miles_per_card*cumsum(one)) %>%
select_(~ -one)
}
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