This program demonstrates two features of two-dimensional histograms. First a 10 by 10 2d-histogram is created with x and y running from 0 to 1. Then a few sample points are added to the histogram, at (0.3,0.3) with a height of 1, at (0.8,0.1) with a height of 5 and at (0.7,0.9) with a height of 0.5. This histogram with three events is used to generate a random sample of 1000 simulated events, which are printed out.

#include <stdio.h> #include <gsl/gsl_rng.h> #include <gsl/gsl_histogram2d.h> int main (void) { const gsl_rng_type * T; gsl_rng * r; gsl_histogram2d * h = gsl_histogram2d_alloc (10, 10) gsl_histogram2d_set_ranges_uniform (h, 0.0, 1.0, 0.0, 1.0); gsl_histogram2d_accumulate (h, 0.3, 0.3, 1); gsl_histogram2d_accumulate (h, 0.8, 0.1, 5); gsl_histogram2d_accumulate (h, 0.7, 0.9, 0.5); gsl_rng_env_setup(); T = gsl_rng_default; r = gsl_rng_alloc(T); { int i; gsl_histogram2d_pdf * p = gsl_histogram2d_pdf_alloc (h->n); gsl_histogram2d_pdf_init (p, h); for (i = 0; i < 1000; i++) { double x, y; double u = gsl_rng_uniform (r); double v = gsl_rng_uniform (r); int status = gsl_histogram2d_pdf_sample (p, u, v, &x, &y); printf("%g %g\n", x, y); } } return 0; }

The following plot shows the distribution of the simulated events. Using a higher resolution grid we can see the original underlying histogram and also the statistical fluctuations caused by the events being uniformly distributed over the the area of the original bins.

@image{histogram2d,4in}

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