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This chapter describes functions for creating and manipulating ntuples, sets of values associated with events. The ntuples are stored in files. Their values can be extracted in any combination and booked in an histogram using a selection function.
The values to be stored are held in a user-defined data structure, and an ntuple is created associating this data structure with a file. The values are then written to the file (normally inside a loop) using the ntuple functions described below.
A histogram can be created from ntuple data by providing a selection function and a value function. The selection function specifies whether an event should be included in the subset to be analyzed or not. The value function computes the entry to be added to the histogram entry for each event.
All the ntuple functions are defined in the header file `gsl_ntuple.h'
Ntuples are manipulated using the gsl_ntuple
struct. This struct
contains information on the file where the ntuple data is stored, a
pointer to the current ntuple data row and the size of the user-defined
ntuple data struct.
typedef struct { FILE * file; void * ntuple_data; size_t size; } gsl_ntuple;
gsl_ntuple_write
Once an ntuple has been created its contents can be histogrammed in
various ways using the function gsl_ntuple_project
. Two
user-defined functions must be provided, a function to select events and
a function to compute scalar values. The selection function and the
value function both accept the ntuple row as a first argument and other
parameters as a second argument.
The selection function determines which ntuple rows are selected for histogramming. It is defined by the following struct,
typedef struct { int (* function) (void * ntuple_data, void * params); void * params; } gsl_ntuple_select_fn;
The struct component function should return a non-zero value for each ntuple row that is to be included in the histogram.
The value function computes scalar values for those ntuple rows selected by the selection function,
typedef struct { double (* function) (void * ntuple_data, void * params); void * params; } gsl_ntuple_value_fn;
In this case the struct component function should return the value to be added to the histogram for the ntuple row.
The following example programs demonstrate the use of ntuples in managing a large dataset. The first program creates a set of 100,000 simulated "events", each with 3 associated values (x,y,z). These are generated from a gaussian distribution with unit variance, for demonstration purposes, and written to the ntuple file `test.dat'.
#include <config.h> #include <gsl/gsl_ntuple.h> #include <gsl/gsl_rng.h> #include <gsl/gsl_randist.h> struct data { double x; double y; double z; }; int main (void) { const gsl_rng_type * T; gsl_rng * r; struct data ntuple_row; int i; gsl_ntuple *ntuple = gsl_ntuple_create ("test.dat", &ntuple_row, sizeof (ntuple_row)); gsl_rng_env_setup(); T = gsl_rng_default; r = gsl_rng_alloc (T); for (i = 0; i < 10000; i++) { ntuple_row.x = gsl_ran_ugaussian (r); ntuple_row.y = gsl_ran_ugaussian (r); ntuple_row.z = gsl_ran_ugaussian (r); gsl_ntuple_write (ntuple); } gsl_ntuple_close(ntuple); return 0; }
The next program analyses the ntuple data in the file `test.dat'. The analysis procedure is to compute the squared-magnitude of each event, E^2=x^2+y^2+z^2, and select only those which exceed a lower limit of 1.5. The selected events are then histogrammed using their E^2 values.
#include <config.h> #include <math.h> #include <gsl/gsl_ntuple.h> #include <gsl/gsl_histogram.h> struct data { double x; double y; double z; }; int sel_func (void *ntuple_data, void *params); double val_func (void *ntuple_data, void *params); int main (void) { struct data ntuple_row; int i; gsl_ntuple *ntuple = gsl_ntuple_open ("test.dat", &ntuple_row, sizeof (ntuple_row)); double lower = 1.5; gsl_ntuple_select_fn S; gsl_ntuple_value_fn V; gsl_histogram *h = gsl_histogram_alloc (100); gsl_histogram_set_ranges_uniform(h, 0.0, 10.0); S.function = &sel_func; S.params = &lower; V.function = &val_func; V.params = 0; gsl_ntuple_project (h, ntuple, &V, &S); gsl_histogram_fprintf (stdout, h, "%f", "%f"); gsl_histogram_free (h); gsl_ntuple_close (ntuple); return 0; } int sel_func (void *ntuple_data, void *params) { double x, y, z, E, scale; scale = *(double *) params; x = ((struct data *) ntuple_data)->x; y = ((struct data *) ntuple_data)->y; z = ((struct data *) ntuple_data)->z; E2 = x * x + y * y + z * z; return E2 > scale; } double val_func (void *ntuple_data, void *params) { double x, y, z; x = ((struct data *) ntuple_data)->x; y = ((struct data *) ntuple_data)->y; z = ((struct data *) ntuple_data)->z; return x * x + y * y + z * z; }
The following plot shows the distribution of the selected events. Note the cut-off at the lower bound.
Further information on the use of ntuples can be found in the documentation for the CERN packages PAW and HBOOK (available online).
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