From 49247458f9ede5e4073f5a38b4d6deafc20238c8 Mon Sep 17 00:00:00 2001
From: david-cortes <david.cortes.rivera@gmail.com>
Date: Wed, 3 Jan 2024 08:26:55 +0100
Subject: [PATCH] [R] Minor improvements for evaluation printing (#9940)

---
 R-package/R/callbacks.R                   | 3 ++-
 R-package/R/xgb.cv.R                      | 5 +++--
 R-package/tests/testthat/test_callbacks.R | 2 +-
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/R-package/R/callbacks.R b/R-package/R/callbacks.R
index 54f821a79..f8f3b5a30 100644
--- a/R-package/R/callbacks.R
+++ b/R-package/R/callbacks.R
@@ -770,7 +770,8 @@ xgb.gblinear.history <- function(model, class_index = NULL) {
   if (!is.null(eval_err)) {
     if (length(eval_res) != length(eval_err))
       stop('eval_res & eval_err lengths mismatch')
-    res <- paste0(sprintf("%s:%f+%f", enames, eval_res, eval_err), collapse = '\t')
+    # Note: UTF-8 code for plus/minus sign is U+00B1
+    res <- paste0(sprintf("%s:%f\U00B1%f", enames, eval_res, eval_err), collapse = '\t')
   } else {
     res <- paste0(sprintf("%s:%f", enames, eval_res), collapse = '\t')
   }
diff --git a/R-package/R/xgb.cv.R b/R-package/R/xgb.cv.R
index 1c17d86f0..b0d8c4ebe 100644
--- a/R-package/R/xgb.cv.R
+++ b/R-package/R/xgb.cv.R
@@ -244,8 +244,9 @@ xgb.cv <- function(params = list(), data, nrounds, nfold, label = NULL, missing
       )
     })
     msg <- simplify2array(msg)
-    bst_evaluation <- rowMeans(msg)
-    bst_evaluation_err <- sqrt(rowMeans(msg^2) - bst_evaluation^2) # nolint
+    # Note: these variables might look unused here, but they are used in the callbacks
+    bst_evaluation <- rowMeans(msg) # nolint
+    bst_evaluation_err <- apply(msg, 1, sd) # nolint
 
     for (f in cb$post_iter) f()
 
diff --git a/R-package/tests/testthat/test_callbacks.R b/R-package/tests/testthat/test_callbacks.R
index 63a4c3f25..de5150380 100644
--- a/R-package/tests/testthat/test_callbacks.R
+++ b/R-package/tests/testthat/test_callbacks.R
@@ -57,7 +57,7 @@ test_that("cb.print.evaluation works as expected", {
   expect_output(f5(), "\\[7\\]\ttrain-auc:0.900000\ttest-auc:0.800000")
 
   bst_evaluation_err  <- c('train-auc' = 0.1, 'test-auc' = 0.2)
-  expect_output(f1(), "\\[7\\]\ttrain-auc:0.900000\\+0.100000\ttest-auc:0.800000\\+0.200000")
+  expect_output(f1(), "\\[7\\]\ttrain-auc:0.900000±0.100000\ttest-auc:0.800000±0.200000")
 })
 
 test_that("cb.evaluation.log works as expected", {