trying to program in PostgreSQL C a statistics function

SUMMARY
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I am trying to program in PostgreSQL a statistics function.

My old method in SQL is here.

select ( avg(measure) - 0.0 ) / nullif(stddev_pop(case when measure > 0.00 then 0.00 else measure end ),0) sortino_true from TABLE/VIEW;

The logic is based on
SORTINO RATIO: ARE YOU CALCULATING IT WRONG?
SEPTEMBER 11, 2013
https://www.rcmalternatives.com/2013/09/sortino-ratio-are-you-calculating-it-wrong/

In PostgreSQL C, how do I get access to the 'X' (data) variable?

DETAILS
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I see a similar function with other variables in here.
But I am not trying to re-program 'stddev_samp'.

float8_stddev_samp ( lines 2741 through 2767 )
https://github.com/postgres/postgres/blob/9a34123bc315e55b33038464422ef1cd2b67dab2/src/backend/utils/adt/float.c

N = transvalues[0];
sumX = transvalues[1];
sumX2 = transvalues[2];

The following article explains it and this makes sense.

Sum of Squares Formula Shortcut
https://www.thoughtco.com/sum-of-squares-formula-shortcut-3126266

I can match article symbols to PostgreSQL C varaibles.

numerator = N * sumX2 - sumX * sumX;
N * SIGMA(X**2) - (SIGMA(X))**2

BUT I NEED ...
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However in my case,
I have this adjustement

"case when measure > 0.00 then 0.00 else measure end"

So the case seems that I need access to 'X'
but access to sumX and sumX2 are only shown.

How would I get accress to X?

May anyone point me to a simple good working example?

Thanks,
Andre Mikulec
Andre_Mikulec(at)Hotmail(dot)com