[hist] Fix error calculation in THnBase::ProjectionAny.

Thanks to jgrosseo!

Fix #19241

(cherry picked from commit f386fc2)

Author: hageboeck

hist/hist/src/THnBase.cxx

@@ -697,6 +697,9 @@ TObject* THnBase::ProjectionAny(Int_t ndim, const Int_t* dim,
    Bool_t haveErrors = GetCalculateErrors();
    Bool_t wantErrors = haveErrors || (option && (strchr(option, 'E') || strchr(option, 'e')));
 
+   if (wantNDim && wantErrors)
+      hn->Sumw2();
+
    Int_t* bins  = new Int_t[ndim];
    Long64_t myLinBin = 0;
 

hist/hist/test/THn.cxx

@@ -144,3 +144,33 @@ TEST(THn, GetBinCenter)
    EXPECT_DOUBLE_EQ(centers.at(0), 2.5);
    EXPECT_DOUBLE_EQ(centers.at(1), -1.5);
 }
+
+TEST(THn, ErrorsOfProjection)
+{
+   const int bins[] = {10, 10, 10, 10};
+   const double xmin[] = {0, 0, 0, 0};
+   const double xmax[] = {10, 10, 10, 10};
+   THnF thn("thn", "", 4, bins, xmin, xmax);
+   thn.Sumw2();
+
+   const double coordinates1[]{0.5, 0.5, 0.5, 0.5};
+
+   for (int i = 0; i < 9; i++) {
+      thn.Fill(coordinates1, 0.1);
+      const double coordinates2[]{1.5, 1.5, 0.5 + i, 0.5 + i};
+      thn.Fill(coordinates2, 2.);
+   }
+
+   const Int_t dimensions[] = {0, 1};
+   // Despite the option "E", the errors are resetted to sqrt(N) instead of keeping the original ones
+   std::unique_ptr<THnBase> proj{thn.ProjectionND(2, dimensions, "E")};
+
+   const auto projectedBin = proj->GetBin(coordinates1);
+   EXPECT_FLOAT_EQ(proj->GetBinContent(projectedBin), 0.9);
+   EXPECT_FLOAT_EQ(proj->GetBinError(projectedBin), 0.3);
+
+   const double coordinates2[]{1.5, 1.5};
+   const auto projectedBin2 = proj->GetBin(coordinates2);
+   EXPECT_FLOAT_EQ(proj->GetBinContent(projectedBin2), 18.);
+   EXPECT_FLOAT_EQ(proj->GetBinError(projectedBin2), 6.);
+}