Drawing Histograms with for loop with python?

ROOT
1

My code is set up such that I have a dictionary of histograms (“trigger_mass_hist”), with each key being the name of a trigger and each value being the histogram itself. I want to graph all of the histograms in the directory on one canvas (maybe 10 histograms), but when I try to graph them as follows:

canv1.cd()
canv1.SetLogx()
canv1.SetLogy()
for hist in trigger_mass_hist:
    trigger_mass_hist[hist].Draw("hist e1")
    canv1.Update()

only the last histogram shows up on the canvas. Any advice?

2

Hi,

here the idea is to add to the drawing options the string “same” for all histograms after the first one.

canv1.cd()
canv1.SetLogx()
canv1.SetLogy()
draw_opts="hist e1"
for hist in trigger_mass_hist:
    trigger_mass_hist[hist].Draw(draw_opts)
    draw_opts += " same"
    canv1.Update()

Cheers,
Danilo

3

Hi,

But then the 4th histogram will have `“hist e1 same same same” - which breaks my C++, performance-focused heart…

:wink:

Axel.

4

A hackish way would be doing it only if not present:

canv1.cd()
canv1.SetLogx()
canv1.SetLogy()
draw_opts = "hist e1"
for hist in trigger_mass_hist:
    trigger_mass_hist[hist].Draw(draw_opts)
    if "same" not in draw_opts:
        draw_opts += " same"
    canv1.Update()

But this will be slow (in is O(n)). Better do it with enumerate:

canv1.cd()
canv1.SetLogx()
canv1.SetLogy()
draw_opts = "hist e1"
for i, hist in enumerate(trigger_mass_hist):
    trigger_mass_hist[hist].Draw(draw_opts)
    if i == 0:
        draw_opts += " same"
    canv1.Update()

Or, the third option, use a THStack:

canv1.cd()
canv1.SetLogx()
canv1.SetLogy()
draw_opts = "hist e1"
stack = ROOT.THStack()
for hist in trigger_mass_hist.values():
    stack.Add(hist)
stack.Draw("nostack" + draw_opts)
canv1.Update()
1 Like
5

I draw one of them before the loop and then use the ‘same’ option in loop

from rootpy.interactive import wait
#add option or what ever
trigger_mass_hist.pop().Draw(draw_opts)
for hist in trigger_mass_hist:
      hist.Draw('same'+draw_opts)
wait()

This works for me
Cheers
Daniel

6

Hi,

of course there was a flaw in the solution I proposed :slight_smile: Anything like the previous very nice solutions of course would work!
I’d like at least to add another one to cope with the glitch above !

canv1.cd()
canv1.SetLogx()
canv1.SetLogy()
draw_opts="hist e1"

for i, hist in enumerate(trigger_mass_hist.itervalues()):
    hist.Draw(draw_opts + "" if i==0 else " same")
    canv1.Update()

which avoids the lookup in the dictionary.

Cheers,
Danilo

7

Many of these solutions work perfectly! I ended up using the second one proposed by @Graipher. Thank you!