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The following program shows how to make a simple histogram of a column
of numerical data supplied on `stdin`

. The program takes three
arguments, specifying the upper and lower bounds of the histogram and
the number of bins. It then reads numbers from `stdin`

, one line at
a time, and adds them to the histogram. When there is no more data to
read it prints out the accumulated histogram using
`gsl_histogram_fprintf`

.

#include <stdio.h> #include <stdlib.h> #include <gsl/gsl_histogram.h> int main (int argc, char **argv) { double a, b; size_t n; if (argc != 4) { printf ("Usage: gsl-histogram xmin xmax n\n" "Computes a histogram of the data " "on stdin using n bins from xmin " "to xmax\n"); exit (0); } a = atof (argv[1]); b = atof (argv[2]); n = atoi (argv[3]); { double x; gsl_histogram * h = gsl_histogram_alloc (n); gsl_histogram_set_ranges_uniform (h, a, b); while (fscanf (stdin, "%lg", &x) == 1) { gsl_histogram_increment (h, x); } gsl_histogram_fprintf (stdout, h, "%g", "%g"); gsl_histogram_free (h); } exit (0); }

Here is an example of the program in use. We generate 10000 random samples from a Cauchy distribution with a width of 30 and histogram them over the range -100 to 100, using 200 bins.

$ gsl-randist 0 10000 cauchy 30 | gsl-histogram -100 100 200 > histogram.dat

A plot of the resulting histogram shows the familiar shape of the Cauchy distribution and the fluctuations caused by the finite sample size.

$ awk '{print $1, $3 ; print $2, $3}' histogram.dat | graph -T X