Create legend for histogram created in for loop

Newbie
1

ROOT Version: 6.18/04
Platform: Linux

I am trying to add a legend to a plot that consists of 3 histograms that were created in a for loop, they are called ratio[i], where i goes from 0 to 2, and refers to the elements in the following vector:

std::vector<std::string> Cut = {"L_J20", "L_J30", "L_J40"}

I have given each histogram a different colour by writing:

if (i == 0) ratio[i]->SetLineColor(kRed);
else if (i == 1) ratio[i]->SetLineColor(kBlue);
else ratio[i]->SetLineColor(kYellow);

This works, but I was wondering if there is a quicker way to do this (in the form of a loop perhaps). But then my main question is, how do I now create a legend for these histograms? I have tried:

auto legend = new TLegend (.1, .7, .3, .9);
legend->AddEntry(ratio[0], "J20")
legend->AddEntry(ratio[1], "J30")
legend->AddEntry(ratio[2], "J40")
legend->Draw("same")

This doesn’t work (it crashes, but I get no error). Anyone got any idea on how to solve this?

2 
{
   TH1D *h[3];

   for (int i=0; i<3; i++) {
      h[i] = new TH1D(Form("h%d",i), Form("h%d",i), 100, -1, 1);
      h[i]->SetLineColor(i+3);
   }

   h[0]->Draw();
   h[1]->Draw("SAME");
   h[2]->Draw("SAME");

   auto legend = new TLegend(0.1,0.7,0.28,0.9);

   for (int i=0; i<3; i++) {
      legend->AddEntry(h[i], Form("J%d0",i+3), "l");
   }

   legend->Draw();
}

Screenshot 2020-03-16 at 16.02.32
Screenshot 2020-03-16 at 16.02.321342×928 28.1 KB

2 Likes
3

That works! Thanks! What would I do, if I don’t want the J’s to be consecutive though? So, what would I do if I want to call my histograms J20, J40 and J75 for example?

4

If there is no logic in the legend’s comments you want tu use, it will not be possible to use the logic I showed you. You need to make an array of comments instead.

1 Like