[backport] [R] Fix single sample prediction. (#7524) (#7558)

R-package/R/xgb.Booster.R

@@ -435,7 +435,8 @@ predict.xgb.Booster <- function(object, newdata, missing = NA, outputmargin = FA
       lapply(seq_len(n_groups), function(g) arr[g, , ])
     } else {
       ## remove the first axis (group)
-      as.matrix(arr[1, , ])
+      dn <- dimnames(arr)
+      matrix(arr[1, , ], nrow = dim(arr)[2], ncol = dim(arr)[3], dimnames = c(dn[2], dn[3]))
     }
   } else if (predinteraction) {
     ## Predict interaction
@@ -447,7 +448,8 @@ predict.xgb.Booster <- function(object, newdata, missing = NA, outputmargin = FA
       lapply(seq_len(n_groups), function(g) arr[g, , , ])
     } else {
       ## remove the first axis (group)
-      arr[1, , , ]
+      arr <- arr[1, , , , drop = FALSE]
+      array(arr, dim = dim(arr)[2:4], dimnames(arr)[2:4])
     }
   } else {
     ## Normal prediction

R-package/tests/testthat/test_interactions.R

@@ -157,3 +157,28 @@ test_that("multiclass feature interactions work", {
   # sums WRT columns must be close to feature contributions
   expect_lt(max(abs(apply(intr, c(1, 2, 3), sum) - aperm(cont, c(3, 1, 2)))), 0.00001)
 })
+
+
+test_that("SHAP single sample works", {
+  train <- agaricus.train
+  test <- agaricus.test
+  booster <- xgboost(
+    data = train$data,
+    label = train$label,
+    max_depth = 2,
+    nrounds = 4,
+    objective = "binary:logistic",
+  )
+
+  predt <- predict(
+    booster,
+    newdata = train$data[1, , drop = FALSE], predcontrib = TRUE
+  )
+  expect_equal(dim(predt), c(1, dim(train$data)[2] + 1))
+
+  predt <- predict(
+    booster,
+    newdata = train$data[1, , drop = FALSE], predinteraction = TRUE
+  )
+  expect_equal(dim(predt), c(1, dim(train$data)[2] + 1, dim(train$data)[2] + 1))
+})