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Projecting n-tuple to 2D histogram, and calling gsl_ntuple_project() wit

From: Nathaniel Roth
Subject: Projecting n-tuple to 2D histogram, and calling gsl_ntuple_project() within a loop
Date: Wed, 1 Jan 2020 19:32:48 -0500

Dear gsl help,

  This question concerns a c++ program I am writing that makes use of gsl,
compiled with g++.

I have a dataset organized as several million rows (events) with about 12
numbers (columns) associated to each event, and I'd like to create 1D and
2D histograms that bin those events based on the values in one or two of
those columns.

  I've successfully created such 1D histograms using gsl n-tuples and
histograms.  My question is about finding an efficient way to create 2D
histograms this way.

  Based on the online documentation, gsl_ntuple_project() seems defined for
1d histograms, and not 2d histograms. To work around this I've created a
loop in which I create as many 1D histograms as I require, each using the
"horizontal" axis binning, to successively assemble the second (vertical)
dimension of my histogram, using the selection function to apply my binning
criteria for the vertical binning dimension during each pass through the

The loop in question looks like this:

for (int iy = 0; iy < num_vertical_bins; iy++)
      S.params = &bin_params; // vertical binning criteria enter here

      big_ntuple = gsl_ntuple_open (filename, &ntuple_row, sizeof

      gsl_ntuple_project(horizontal_histogram, big_ntuple,&V,&S);

      gsl_histogram_fprintf(outfile, horiontal_histogram, "%g", "%g");



This provides the desired behavior. However, calling gsl_ntuple_open and
gsl_ntuple_close during each pass through the loop slows things down
significantly. If I try to move the two lines that call gsl_ntuple_open and
gsl_ntuple_close outside the loop (before and after, respectively), the
code executes much faster, but it no longer correctly fills the vertical
dimension of the histogram: only the first histogram written out (during
the first pass through the loop) has nonzero bin counts, and the subsequent
histograms that are written out are filled with zero counts.

My question is, is it indeed necessary that I call the gsl_ntuple_open
and gsl_ntuple_close functions from inside the loop? If not, how can I
avoid the behavior I'm seeing where all the except for the first get filled
with zero bin counts? Or, is there generally a better way to create the
sort of 2D histogram I am describing?

Thank you,

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