TGraphErrors Reading Input from File

ROOT
1

According to the documentation for TGraphErrors, there is a constructor that reads input text files. The documentation, http://root.cern.ch/root/html/TGraphErrors.html#TGraphErrors:TGraphErrors@8, states that a format of “%lg %lg %lg” will read the first three columns a as X, Y, and EY. When running the following script, errors are read into EX. As shown in the plot.

TGraphErrors* g= new TGraphErrors("test.csv", "%lg %lg %lg", ","); g->Draw("AP");

Is this a mistake in documentation of implementation.

wrongErrors.png696×472 2.68 KB

2

can you post the csv file ?

3

Ahh, I knew I forgot something.
test.csv (198 Bytes)

4

All if fine for me. I did the follwoing test:

{
TGraphErrors* g= new TGraphErrors("test.csv", "%lg %lg %lg", ",");
Double_t *x=g->GetX();
Double_t *y=g->GetY();
Double_t *ex=g->GetEX();

for (int i=0; i<8; i++) printf("%g %g %g\n",x[i],y[i],ex[i]);

}

which gives:

root [0] .x csv.C
240.27 -3.88522 0.126246
266.67 -3.768 0.0526381
319.8 -3.76976 0.0353706
349.54 -3.62117 0.0572677
481.94 -3.7652 0.106624
554.4 -4.08538 0.302589
975.8 -4.69185 0.0815721
1048.1 -4.97544 0.208396

which is exactly your file …

5

I think that what ksmith meant is that in the documentation, it is written

This is not EY but EX

6

Ah yes but looking at the code it seems that this feature applies only in the case there is no delimiter defined.

replace all “,” by " "in your csv file (global edit) and it will work as you expect

7 
root [0] .x csv.C
240.27 -3.88522 0.126246
266.67 -3.768 0.0526381
319.8 -3.76976 0.0353706
349.54 -3.62117 0.0572677
481.94 -3.7652 0.106624
554.4 -4.08538 0.302589
975.8 -4.69185 0.0815721
1048.1 -4.97544 0.208396
root [1] 

{
TGraphErrors* g= new TGraphErrors("test.csv", "%lg %lg %lg" );
Double_t *x=g->GetX();
Double_t *y=g->GetY();
Double_t *ey=g->GetEY();

for (int i=0; i<8; i++) printf("%g %g %g\n",x[i],y[i],ey[i]);

}
$ cat test.csv 
240.27 -3.88522 0.126246
266.67 -3.768 0.0526381
319.8 -3.76976 0.0353706
349.54 -3.62117 0.0572677
481.94 -3.7652 0.106624
554.4 -4.08538 0.302589
975.8 -4.69185 0.0815721
1048.1 -4.97544 0.208396
8

Exactly, the documentation does not make it clear that the functionality of this method changes when a delimiter is specified. Is this a documentation or implementation mistake?

9

It is a documentation issue. I will try to improve.

1 Like
10

It would be nice to have this feature for csv files. Is this difficult to implement?

11

The code is totally different. You can have a look if you want. I am not sure why the author did not implement it that way, but looking at the code it looks more complex.

Any way a global edit or a small “sed” command on your csv file to replace ; by blanck will do the job.

If you come with a working implementation we will be please to include it.