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Fix R documentation for xgb.train. (#6764)

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The [general documentation](https://xgboost.readthedocs.io/en/latest/parameter.html#parameters-for-tree-booster) clearly has alpha and lambda under its "Parameters for Tree Booster" heading. Furthermore, the R package clearly uses alpha and lambda when told to use the tree booster. This update adds those two parameters to the documentation for the R package.


Closed issue #6763.
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ReeceGoding 2021-03-18 07:04:00 +00:00 committed by GitHub
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R-package/R/xgb.train.R
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@ -15,7 +15,7 @@
#'
#' 2. Booster Parameters
#'
#' 2.1. Parameter for Tree Booster
#' 2.1. Parameters for Tree Booster
#'
#' \itemize{
#' \item \code{eta} control the learning rate: scale the contribution of each tree by a factor of \code{0 < eta < 1} when it is added to the current approximation. Used to prevent overfitting by making the boosting process more conservative. Lower value for \code{eta} implies larger value for \code{nrounds}: low \code{eta} value means model more robust to overfitting but slower to compute. Default: 0.3
@ -24,12 +24,14 @@
#' \item \code{min_child_weight} minimum sum of instance weight (hessian) needed in a child. If the tree partition step results in a leaf node with the sum of instance weight less than min_child_weight, then the building process will give up further partitioning. In linear regression mode, this simply corresponds to minimum number of instances needed to be in each node. The larger, the more conservative the algorithm will be. Default: 1
#' \item \code{subsample} subsample ratio of the training instance. Setting it to 0.5 means that xgboost randomly collected half of the data instances to grow trees and this will prevent overfitting. It makes computation shorter (because less data to analyse). It is advised to use this parameter with \code{eta} and increase \code{nrounds}. Default: 1
#' \item \code{colsample_bytree} subsample ratio of columns when constructing each tree. Default: 1
#' \item \code{lambda} L2 regularization term on weights. Default: 1
#' \item \code{alpha} L1 regularization term on weights. (there is no L1 reg on bias because it is not important). Default: 0
#' \item \code{num_parallel_tree} Experimental parameter. number of trees to grow per round. Useful to test Random Forest through Xgboost (set \code{colsample_bytree < 1}, \code{subsample < 1} and \code{round = 1}) accordingly. Default: 1
#' \item \code{monotone_constraints} A numerical vector consists of \code{1}, \code{0} and \code{-1} with its length equals to the number of features in the training data. \code{1} is increasing, \code{-1} is decreasing and \code{0} is no constraint.
#' \item \code{interaction_constraints} A list of vectors specifying feature indices of permitted interactions. Each item of the list represents one permitted interaction where specified features are allowed to interact with each other. Feature index values should start from \code{0} (\code{0} references the first column). Leave argument unspecified for no interaction constraints.
#' }
#'
#' 2.2. Parameter for Linear Booster
#' 2.2. Parameters for Linear Booster
#'
#' \itemize{
#' \item \code{lambda} L2 regularization term on weights. Default: 0