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#' Cross Validation
#'
#' The cross validation function of xgboost
#'
#' @param params the list of parameters. The complete list of parameters is
#' available in the \href{http://xgboost.readthedocs.io/en/latest/parameter.html}{online documentation}. Below
#' is a shorter summary:
#' \itemize{
#' \item \code{objective} objective function, common ones are
#' \itemize{
#' \item \code{reg:squarederror} Regression with squared loss.
#' \item \code{binary:logistic} logistic regression for classification.
#' \item See \code{\link[=xgb.train]{xgb.train}()} for complete list of objectives.
#' }
#' \item \code{eta} step size of each boosting step
#' \item \code{max_depth} maximum depth of the tree
#' \item \code{nthread} number of thread used in training, if not set, all threads are used
#' }
#'
#' See \code{\link{xgb.train}} for further details.
#' See also demo/ for walkthrough example in R.
#' @param data takes an \code{xgb.DMatrix}, \code{matrix}, or \code{dgCMatrix} as the input.
#' @param nrounds the max number of iterations
#' @param nfold the original dataset is randomly partitioned into \code{nfold} equal size subsamples.
#' @param label vector of response values. Should be provided only when data is an R-matrix.
#' @param missing is only used when input is a dense matrix. By default is set to NA, which means
#' that NA values should be considered as 'missing' by the algorithm.
#' Sometimes, 0 or other extreme value might be used to represent missing values.
#' @param prediction A logical value indicating whether to return the test fold predictions
#' from each CV model. This parameter engages the \code{\link{cb.cv.predict}} callback.
#' @param showsd \code{boolean}, whether to show standard deviation of cross validation
#' @param metrics, list of evaluation metrics to be used in cross validation,
#' when it is not specified, the evaluation metric is chosen according to objective function.
#' Possible options are:
#' \itemize{
#' \item \code{error} binary classification error rate
#' \item \code{rmse} Rooted mean square error
#' \item \code{logloss} negative log-likelihood function
#' \item \code{mae} Mean absolute error
#' \item \code{mape} Mean absolute percentage error
#' \item \code{auc} Area under curve
#' \item \code{aucpr} Area under PR curve
#' \item \code{merror} Exact matching error, used to evaluate multi-class classification
#' }
#' @param obj customized objective function. Returns gradient and second order
#' gradient with given prediction and dtrain.
#' @param feval customized evaluation function. Returns
#' \code{list(metric='metric-name', value='metric-value')} with given
#' prediction and dtrain.
#' @param stratified a \code{boolean} indicating whether sampling of folds should be stratified
#' by the values of outcome labels.
#' @param folds \code{list} provides a possibility to use a list of pre-defined CV folds
#' (each element must be a vector of test fold's indices). When folds are supplied,
#' the \code{nfold} and \code{stratified} parameters are ignored.
#' @param train_folds \code{list} list specifying which indicies to use for training. If \code{NULL}
#' (the default) all indices not specified in \code{folds} will be used for training.
#' @param verbose \code{boolean}, print the statistics during the process
#' @param print_every_n Print each n-th iteration evaluation messages when \code{verbose>0}.
#' Default is 1 which means all messages are printed. This parameter is passed to the
#' \code{\link{cb.print.evaluation}} callback.
#' @param early_stopping_rounds If \code{NULL}, the early stopping function is not triggered.
#' If set to an integer \code{k}, training with a validation set will stop if the performance
#' doesn't improve for \code{k} rounds.
#' Setting this parameter engages the \code{\link{cb.early.stop}} callback.
#' @param maximize If \code{feval} and \code{early_stopping_rounds} are set,
#' then this parameter must be set as well.
#' When it is \code{TRUE}, it means the larger the evaluation score the better.
#' This parameter is passed to the \code{\link{cb.early.stop}} callback.
#' @param callbacks a list of callback functions to perform various task during boosting.
#' See \code{\link{callbacks}}. Some of the callbacks are automatically created depending on the
#' parameters' values. User can provide either existing or their own callback methods in order
#' to customize the training process.
#' @param ... other parameters to pass to \code{params}.
#'
#' @details
#' The original sample is randomly partitioned into \code{nfold} equal size subsamples.
#'
#' Of the \code{nfold} subsamples, a single subsample is retained as the validation data for testing the model, and the remaining \code{nfold - 1} subsamples are used as training data.
#'
#' The cross-validation process is then repeated \code{nrounds} times, with each of the \code{nfold} subsamples used exactly once as the validation data.
#'
#' All observations are used for both training and validation.
#'
#' Adapted from \url{https://en.wikipedia.org/wiki/Cross-validation_\%28statistics\%29}
#'
#' @return
#' An object of class \code{xgb.cv.synchronous} with the following elements:
#' \itemize{
#' \item \code{call} a function call.
#' \item \code{params} parameters that were passed to the xgboost library. Note that it does not
#' capture parameters changed by the \code{\link{cb.reset.parameters}} callback.
#' \item \code{callbacks} callback functions that were either automatically assigned or
#' explicitly passed.
#' \item \code{evaluation_log} evaluation history stored as a \code{data.table} with the
#' first column corresponding to iteration number and the rest corresponding to the
#' CV-based evaluation means and standard deviations for the training and test CV-sets.
#' It is created by the \code{\link{cb.evaluation.log}} callback.
#' \item \code{niter} number of boosting iterations.
#' \item \code{nfeatures} number of features in training data.
#' \item \code{folds} the list of CV folds' indices - either those passed through the \code{folds}
#' parameter or randomly generated.
#' \item \code{best_iteration} iteration number with the best evaluation metric value
#' (only available with early stopping).
#' \item \code{best_ntreelimit} and the \code{ntreelimit} Deprecated attributes, use \code{best_iteration} instead.
#' \item \code{pred} CV prediction values available when \code{prediction} is set.
#' It is either vector or matrix (see \code{\link{cb.cv.predict}}).
#' \item \code{models} a list of the CV folds' models. It is only available with the explicit
#' setting of the \code{cb.cv.predict(save_models = TRUE)} callback.
#' }
#'
#' @examples
#' data(agaricus.train, package='xgboost')
#' dtrain <- with(agaricus.train, xgb.DMatrix(data, label = label, nthread = 2))
#' cv <- xgb.cv(data = dtrain, nrounds = 3, nthread = 2, nfold = 5, metrics = list("rmse","auc"),
#' max_depth = 3, eta = 1, objective = "binary:logistic")
#' print(cv)
#' print(cv, verbose=TRUE)
#'
#' @export
xgb.cv <- function(params=list(), data, nrounds, nfold, label = NULL, missing = NA,
prediction = FALSE, showsd = TRUE, metrics=list(),
obj = NULL, feval = NULL, stratified = TRUE, folds = NULL, train_folds = NULL,
verbose = TRUE, print_every_n=1L,
early_stopping_rounds = NULL, maximize = NULL, callbacks = list(), ...) {
check.deprecation(...)
params <- check.booster.params(params, ...)
# TODO: should we deprecate the redundant 'metrics' parameter?
for (m in metrics)
params <- c(params, list("eval_metric" = m))
check.custom.obj()
check.custom.eval()
#if (is.null(params[['eval_metric']]) && is.null(feval))
# stop("Either 'eval_metric' or 'feval' must be provided for CV")
# Check the labels
if ((inherits(data, 'xgb.DMatrix') && is.null(getinfo(data, 'label'))) ||
(!inherits(data, 'xgb.DMatrix') && is.null(label))) {
stop("Labels must be provided for CV either through xgb.DMatrix, or through 'label=' when 'data' is matrix")
} else if (inherits(data, 'xgb.DMatrix')) {
if (!is.null(label))
warning("xgb.cv: label will be ignored, since data is of type xgb.DMatrix")
cv_label <- getinfo(data, 'label')
} else {
cv_label <- label
}
# CV folds
if (!is.null(folds)) {
if (!is.list(folds) || length(folds) < 2)
stop("'folds' must be a list with 2 or more elements that are vectors of indices for each CV-fold")
nfold <- length(folds)
} else {
if (nfold <= 1)
stop("'nfold' must be > 1")
folds <- generate.cv.folds(nfold, nrow(data), stratified, cv_label, params)
}
# Potential TODO: sequential CV
#if (strategy == 'sequential')
# stop('Sequential CV strategy is not yet implemented')
# verbosity & evaluation printing callback:
params <- c(params, list(silent = 1))
print_every_n <- max(as.integer(print_every_n), 1L)
if (!has.callbacks(callbacks, 'cb.print.evaluation') && verbose) {
callbacks <- add.cb(callbacks, cb.print.evaluation(print_every_n, showsd = showsd))
}
# evaluation log callback: always is on in CV
evaluation_log <- list()
if (!has.callbacks(callbacks, 'cb.evaluation.log')) {
callbacks <- add.cb(callbacks, cb.evaluation.log())
}
# Early stopping callback
stop_condition <- FALSE
if (!is.null(early_stopping_rounds) &&
!has.callbacks(callbacks, 'cb.early.stop')) {
callbacks <- add.cb(callbacks, cb.early.stop(early_stopping_rounds,
maximize = maximize, verbose = verbose))
}
# CV-predictions callback
if (prediction &&
!has.callbacks(callbacks, 'cb.cv.predict')) {
callbacks <- add.cb(callbacks, cb.cv.predict(save_models = FALSE))
}
# Sort the callbacks into categories
cb <- categorize.callbacks(callbacks)
# create the booster-folds
# train_folds
dall <- xgb.get.DMatrix(data, label, missing, nthread = params$nthread)
bst_folds <- lapply(seq_along(folds), function(k) {
dtest <- slice(dall, folds[[k]])
# code originally contributed by @RolandASc on stackoverflow
if (is.null(train_folds))
dtrain <- slice(dall, unlist(folds[-k]))
else
dtrain <- slice(dall, train_folds[[k]])
handle <- xgb.Booster.handle(params, list(dtrain, dtest))
list(dtrain = dtrain, bst = handle, watchlist = list(train = dtrain, test = dtest), index = folds[[k]])
})
rm(dall)
# a "basket" to collect some results from callbacks
basket <- list()
# extract parameters that can affect the relationship b/w #trees and #iterations
num_class <- max(as.numeric(NVL(params[['num_class']], 1)), 1) # nolint
num_parallel_tree <- max(as.numeric(NVL(params[['num_parallel_tree']], 1)), 1) # nolint
# those are fixed for CV (no training continuation)
begin_iteration <- 1
end_iteration <- nrounds
# synchronous CV boosting: run CV folds' models within each iteration
for (iteration in begin_iteration:end_iteration) {
for (f in cb$pre_iter) f()
msg <- lapply(bst_folds, function(fd) {
xgb.iter.update(fd$bst, fd$dtrain, iteration - 1, obj)
xgb.iter.eval(fd$bst, fd$watchlist, iteration - 1, feval)
})
msg <- simplify2array(msg)
bst_evaluation <- rowMeans(msg)
bst_evaluation_err <- sqrt(rowMeans(msg^2) - bst_evaluation^2) # nolint
for (f in cb$post_iter) f()
if (stop_condition) break
}
for (f in cb$finalize) f(finalize = TRUE)
# the CV result
ret <- list(
call = match.call(),
params = params,
callbacks = callbacks,
evaluation_log = evaluation_log,
niter = end_iteration,
nfeatures = ncol(data),
folds = folds
)
ret <- c(ret, basket)
class(ret) <- 'xgb.cv.synchronous'
invisible(ret)
}
#' Print xgb.cv result
#'
#' Prints formatted results of \code{xgb.cv}.
#'
#' @param x an \code{xgb.cv.synchronous} object
#' @param verbose whether to print detailed data
#' @param ... passed to \code{data.table.print}
#'
#' @details
#' When not verbose, it would only print the evaluation results,
#' including the best iteration (when available).
#'
#' @examples
#' data(agaricus.train, package='xgboost')
#' train <- agaricus.train
#' cv <- xgb.cv(data = train$data, label = train$label, nfold = 5, max_depth = 2,
#' eta = 1, nthread = 2, nrounds = 2, objective = "binary:logistic")
#' print(cv)
#' print(cv, verbose=TRUE)
#'
#' @rdname print.xgb.cv
#' @method print xgb.cv.synchronous
#' @export
print.xgb.cv.synchronous <- function(x, verbose = FALSE, ...) {
cat('##### xgb.cv ', length(x$folds), '-folds\n', sep = '')
if (verbose) {
if (!is.null(x$call)) {
cat('call:\n ')
print(x$call)
}
if (!is.null(x$params)) {
cat('params (as set within xgb.cv):\n')
cat(' ',
paste(names(x$params),
paste0('"', unlist(x$params), '"'),
sep = ' = ', collapse = ', '), '\n', sep = '')
}
if (!is.null(x$callbacks) && length(x$callbacks) > 0) {
cat('callbacks:\n')
lapply(callback.calls(x$callbacks), function(x) {
cat(' ')
print(x)
})
}
for (n in c('niter', 'best_iteration', 'best_ntreelimit')) {
if (is.null(x[[n]]))
next
cat(n, ': ', x[[n]], '\n', sep = '')
}
if (!is.null(x$pred)) {
cat('pred:\n')
str(x$pred)
}
}
if (verbose)
cat('evaluation_log:\n')
print(x$evaluation_log, row.names = FALSE, ...)
if (!is.null(x$best_iteration)) {
cat('Best iteration:\n')
print(x$evaluation_log[x$best_iteration], row.names = FALSE, ...)
}
invisible(x)
}
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