R/plotcp.R

In itree: Tools for classification and regression trees, with an emphasis on interpretability.

# SCCS @(#)plotcp.s	1.1 02/08/98
# Contributed by B.D. Ripley 97/07/17
#
plotcp <- function(x, minline = TRUE, lty = 3, col = 1,
		   upper = c("size", "splits", "none"), ...)
{
  dots <- list(...)
  if(!inherits(x, "itree")) stop("Not legitimate itree object")
  if(x$method %in% c("class_purity","class_extremes","regression_purity","regression_extremes"))
  	stop("cp not defined for this method!")
 
  if(!is.null(x$penalty)){
    	warning("cp is impacted by using penalties and is NOT comparable to unpenalized cp's.") 
    }
  	
  	
  upper <- match.arg(upper)
  p.rpart <- x$cptable
  if(ncol(p.rpart) < 5L)
    stop("cptable does not contain cross-validation results")
  xstd <- p.rpart[, 5L]
  xerror <- p.rpart[, 4L]
  nsplit <- p.rpart[, 2L]
  ns <- seq_along(nsplit)
  cp0 <- p.rpart[ , 1L]
  cp <- sqrt(cp0 * c(Inf, cp0[-length(cp0)]))
  if(! "ylim" %in% names(dots)) dots$ylim <-
      c(min(xerror - xstd) - 0.1, max(xerror + xstd) + 0.1)
  do.call(plot, c(list(ns, xerror, axes = FALSE, xlab = "cp", ylab =
                       "X-val Relative Error", type = "o"), dots))
  box()
  axis(2, ...)
  segments(ns, xerror - xstd, ns, xerror + xstd)
  axis(1L, at = ns, labels = as.character(signif(cp, 2L)), ...)
  switch(upper,
	 size = {
           axis(3L, at = ns, labels = as.character(nsplit+1), ...)
           mtext("size of tree", side=3, line=3)
	 },
	 splits = {
           axis(3L, at = ns, labels = as.character(nsplit), ...)
           mtext("number of splits", side=3, line=3)
	 },)
  minpos <- min(seq_along(xerror)[xerror==min(xerror)])
  if(minline) abline(h=(xerror+xstd)[minpos], lty=lty, col=col)
  invisible()
}