Github user felixcheung commented on a diff in the pull request:

    https://github.com/apache/spark/pull/14779#discussion_r76100314
  
    --- Diff: R/pkg/R/functions.R ---
    @@ -3194,25 +3206,34 @@ setMethod("lag",
     #' lead
     #'
     #' Window function: returns the value that is \code{offset} rows after the 
current row, and
    -#' NULL if there is less than \code{offset} rows after the current row. 
For example,
    -#' an \code{offset} of one will return the next row at any given point in 
the window partition.
    +#' \code{defaultValue} if there is less than \code{offset} rows after the 
current row.
    +#' For example, an \code{offset} of one will return the next row at any 
given point
    +#' in the window partition.
     #'
     #' This is equivalent to the \code{LEAD} function in SQL.
     #'
     #' @param x Column to compute on
    -#' @param offset Number of rows to offset
    +#' @param offset Number of rows to offset. If not specified, the default 
is 1.
     #' @param defaultValue (Optional) default value to use
     #'
     #' @rdname lead
     #' @name lead
     #' @family window_funcs
     #' @aliases lead,characterOrColumn,numeric-method
     #' @export
    -#' @examples \dontrun{lead(df$c)}
    +#' @examples \dontrun{
    +#'   df <- createDataFrame(mtcars)
    +#'
    +#'   # Partition by am (transmission) and order by hp (horsepower)
    +#'   ws <- orderBy(windowPartitionBy("am"), "hp")
    +#'
    +#'   # Lag mpg values by 1 row on the partition-and-ordered table
    --- End diff --
    
    lag -> lead?


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