Get Histogram from root files

Newbie
3

Hi

I must have done something wrong before but now it works if I use

TString b;
b.Form("Num%d",13);
TH1D *num11=(TH1D*)f1->Get(b);

But I still don’t understand why the sprintf and TString::Format do not work (See the commented code). For these two method I can’t get the histogram and it gives a null pointer error at the line num11->Sumw2(). Thank you very much.