GSL histogram issue - c++

Question

I am trying to compute the cumulative distribution function for a set of values.
I computed the histogram using gsl and I tried to computed the CDF from here, but it seems like the values are shifted by one position.
This is the code I am using:
gHist = gsl_histogram_alloc((maxRange - minRange) / 5);
gsl_histogram_set_ranges_uniform(gHist, minRange, maxRange);
for (int j = 0; j < ValidDataCount; j++)
gsl_histogram_increment (gHist, ValAdd[j]);
gsl_histogram_pdf * p = gsl_histogram_pdf_alloc(gsl_histogram_bins(gHist));
gsl_histogram_pdf_init (p, gHist);
for (int j = 0; j < gsl_histogram_bins(gHist) + 1 ; j++)
printf ("%f ", p->sum[j]);
The histogram is like this:
1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 .... goes on like this. there is a total of 20 values
And the cdf is:
0.00 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.1 0.1 ...
Why is there a 0 on the first position? Shouldn't it start with 0.05?
Thank you.

Answer 1

GSL alloc sum to be an array of size n+1, where n is the number of bins. However, only n entries are necessary to calculate the pdf. This extra allocation of one element happens because gsl defines sum[0] = 0.
in the GSL source coode "pdf.c" you can see that
gsl_histogram_pdf *gsl_histogram_pdf_alloc (const size_t n)
{
(...)
p->sum = (double *) malloc ((n + 1) * sizeof (double));
}
int gsl_histogram_pdf_init (gsl_histogram_pdf * p, const gsl_histogram * h)
{
(...)
p->sum[0] = 0;
for (i = 0; i < n; i++)
{
sum += (h->bin[i] / mean) / n;
p->sum[i + 1] = sum;
}
}