First pass at reorder

NAMESPACE

@@ -2,3 +2,5 @@
 
 export(fct_collapse)
 export(fct_relevel)
+export(fct_reorder)
+importFrom(stats,median)

R/reorder.R

@@ -0,0 +1,28 @@
+#' Reorder the levels of a function according to another variable
+#'
+#' \code{fct_reorder} is useful for 1d displays where the factor is mapped to
+#' position; \code{fct_reorder2} for 2d displays where the factor is mapped to
+#' a non-position aesthetic.
+#'
+#' \code{fct_reorder} is currently an almost exact copy of
+#' \code{\link[stats]{reorder}} but it may change in the future.
+#'
+#' @param x A factor
+#' @param y,fun fun The levels of \code{x} will be reordered so that \code{fun}
+#'    apply to each group is in ascending order.
+#' @param ... Other arguments passed on to \code{fun}. A common argument is
+#'   \code{na.rm = TRUE}.
+#' @importFrom stats median
+#' @export
+#' @examples
+#' boxplot(Sepal.Width ~ Species, data = iris)
+#' boxplot(Sepal.Width ~ fct_reorder(Species, Sepal.Width), data = iris)
+fct_reorder <- function(x, y, fun = median, ...) {
+
+  summary <- tapply(y, x, fun, ...)
+  if (!is.numeric(summary) || length(summary) != nlevels(x)) {
+    stop("`fun` must return a single number per group", call. = FALSE)
+  }
+
+  factor(x, levels = levels(x)[order(summary)])
+}