Rdata frame, problems returning a RDataFrame from a function

[Adrianodelvincio]

Hi guys, nice evening

I am trying to fill a RDataframe with some vectors. To improve the code organization, I have defined a function that takes as input an RDataFrame, and fill it:

void FillTry(ROOT::RDataFrame &d1, TString datafileName){
	
	int j(0); 	// Variable for loop
	d1.Define("id", [&j](){		// Id of the events
		j++;
		return j;
		}); 
}

Something like that. I tried to run the function like this:

	ROOT::RDataFrame d(100); // PDF2

	FillTry(d1,"blabla");
	d1.Snapshot("myTree", "a.root");

But I receive the following errors:

terminate called after throwing an instance of 'std::runtime_error'
  what():  : there is no column available to match.

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ROOT Version: ROOT 6.26/10
Platform: Ubuntu

[Danilo]

Hi Adriano,

Thanks for this interesting question.

The issue is that Define returns a node of the processing graph enriched with the new column, it does not add the column to the node it is called from (see here)

Here I tried to propose a slightly modified version of your code that works:

auto FillTry(ROOT::RDataFrame &d1){
	int j = 0; 	// Variable for loop
	return d1.Define("id", [&j](){return j++;});	// Id of the events
}

void test() {

    ROOT::RDataFrame d(100); // PDF2
	auto filledFrame = FillTry(d);
	filledFrame.Snapshot("myTree", "a.root");

    // Print the content of the file, for the sake of the example
    TFile f("a.root");
    f.ls();
}

A final small remark about performance: you pass by value a TString in your original FillTry function. This triggers a copy and should be perhaps replaced by a const reference for performance reasons, i.e. void FillTry(ROOT::RDataFrame &d1, const TString &datafileName)

Don’t hesitate to write back in case of further questions!

Cheers,
Danilo

[Adrianodelvincio]

Thank you for the advice and of course for your solution! It’s working know and the code is simple and asy to read

Cheers