• ROOTtips

ROOT tips & tricks

quick'n'dirty ascii plot

Just some style issues: directly after starting ROOT you might want to do the following to have some nicer looking plots:

gROOT->SetStyle("Plain");
gStyle->SetMarkerStyle(20);

To create an x-y plot reading data from an ascii file seperated by spaces user the following three lines:

t = new TTree
t->ReadFile("fitresults.txt","run:mean:mean_error:sigma:sigma_error")
t->Draw("sigma/mean:mean")

There automatically created a TCanvas object named c1 and a histogramm of type TH1F named htemp.

You want to set the labels properly?

htemp->GetXaxis()->SetTitle("Energy");
htemp->GetYaxis()->SetTitle("Sigma");
htemp->Paint();

The last statement is necessary to update the histogram.

When setting the title of the histogram itself no Paint() is necessay:

htemp->SetTitle("myPlot");

OK, now save a copy, e.g. as Postscript or PNG file:

c1->Print("myPlot.ps");
c1->Print("myPlot.png");

If you want some high quality PNG file with a transparent background, on the shell use convert from ImageMagick:

convert -density 100 -trim -transparent \#ffffff test.ps test.png

This sets all white color to transparent, so it only works when you set the background to white.

quick'n'dirty xy plot with errorbars from ascii file

TTree t;
t.ReadFile(infile,"m:me:s:se:chi2:ndf:run:energy");
t.Draw("s/m * 100:energy:sqrt((se/s)**2 + (me/m)**2)*s/m","","goff");

TGraphErrors *g = new TGraphErrors(t.GetSelectedRows(),t.GetV2(),t.GetV1(),0,t.GetV3());
g->Draw("ap");

Retrieve auto-created histogram

t->Draw("blub>>hsqrt");
TH1F *hsqrt = (TH1F*)gDirectory->Get("hsqrt");

Debugging your ROOT script with CINT

There are some well-hidden debugging functions. See User's guide 5.26, page 91 for an example and the online help of cint (enter "?" in the ROOT prompt) for a full list of commands. Here an excerpt with some additional remarks:

To start debugging:

• Load script with '.L <file name>'

• Set breakpoints (if you want)

• Run script with '<main function name>()' (up to first breakpoint) or '.s <main function name>()' (stepwise)

ROOT bloody ROOT (you know this song by Sepultura?)