The histogram probability distribution struct
The probability distribution function for a histogram consists of a set
of bins which measure the probability of an event falling into a
given range of a continuous variable x. A probability
distribution function is defined by the following struct, which actually
stores the cumulative probability distribution function. This is the
natural quantity for generating samples via the inverse transform
method, because there is a onetoone mapping between the cumulative
probability distribution and the range [0,1]. It can be shown that by
taking a uniform random number in this range and finding its
corresponding coordinate in the cumulative probability distribution we
obtain samples with the desired probability distribution.
gsl_histogram_pdf

Data Type 
size_t n
 This is the number of bins used to approximate the probability
distribution function.
double * range
 The ranges of the bins are stored in an array of n+1 elements
pointed to by range.
double * sum
 The cumulative probability for the bins is stored in an array of
n elements pointed to by sum.

The following functions allow you to create a gsl_histogram_pdf
struct which represents this probability distribution and generate
random samples from it.
gsl_histogram_pdf * gsl_histogram_pdf_alloc (size_t n)

Function 
This function allocates memory for a probability distribution with
n bins and returns a pointer to a newly initialized
gsl_histogram_pdf struct. If insufficient memory is available a
null pointer is returned and the error handler is invoked with an error
code of GSL_ENOMEM .

int gsl_histogram_pdf_init (gsl_histogram_pdf * p, const gsl_histogram * h)

Function 
This function initializes the probability distribution p with
the contents of the histogram h. If any of the bins of h are
negative then the error handler is invoked with an error code of
GSL_EDOM because a probability distribution cannot contain
negative values.

void gsl_histogram_pdf_free (gsl_histogram_pdf * p)

Function 
This function frees the probability distribution function p and
all of the memory associated with it.

double gsl_histogram_pdf_sample (const gsl_histogram_pdf * p, double r)

Function 
This function uses r, a uniform random number between zero and
one, to compute a single random sample from the probability distribution
p. The algorithm used to compute the sample s is given by
the following formula,
where i is the index which satisfies
sum[i] <= r < sum[i+1] and
delta is
(r  sum[i])/(sum[i+1]  sum[i]).
