rt_band_get_quantiles()

rt_quantile rt_band_get_quantiles	(	rt_bandstats	stats,
		double *	quantiles,
		int	quantiles_count,
		uint32_t *	rtn_count
	)

Compute the default set of or requested quantiles for a set of data the quantile formula used is same as Excel and R default method.

Parameters

stats	: a populated stats struct for processing
quantiles	: the quantiles to be computed
quantiles_count	: the number of quantiles to be computed
rtn_count	: set to the number of quantiles being returned

Returns

the default set of or requested quantiles for a band or NULL

Definition at line 673 of file rt_statistics.c.

  {
        rt_quantile rtn;
        int init_quantiles = 0;
        int i = 0;
        double h;
        int hl;
 
#if POSTGIS_DEBUG_LEVEL > 0
        clock_t start, stop;
        double elapsed = 0;
#endif
 
        RASTER_DEBUG(3, "starting");
#if POSTGIS_DEBUG_LEVEL > 0
        start = clock();
#endif
 
        assert(NULL != stats);
        assert(NULL != rtn_count);
 
        if (stats->count < 1 || NULL == stats->values) {
                rterror("rt_band_get_quantiles: rt_bandstats object has no value");
                return NULL;
        }
 
        /* quantiles not provided */
        if (NULL == quantiles) {
                /* quantile count not specified, default to quartiles */
                if (quantiles_count < 2)
                        quantiles_count = 5;
 
                quantiles = rtalloc(sizeof(double) * quantiles_count);
                init_quantiles = 1;
                if (NULL == quantiles) {
                        rterror("rt_band_get_quantiles: Could not allocate memory for quantile input");
                        return NULL;
                }
 
                quantiles_count--;
                for (i = 0; i <= quantiles_count; i++)
                        quantiles[i] = ((double) i) / quantiles_count;
                quantiles_count++;
        }
 
        /* check quantiles */
        for (i = 0; i < quantiles_count; i++) {
                if (quantiles[i] < 0. || quantiles[i] > 1.) {
                        rterror("rt_band_get_quantiles: Quantile value not between 0 and 1");
                        if (init_quantiles) rtdealloc(quantiles);
                        return NULL;
                }
        }
        quicksort(quantiles, quantiles + quantiles_count - 1);
 
        /* initialize rt_quantile */
        rtn = rtalloc(sizeof(struct rt_quantile_t) * quantiles_count);
        if (NULL == rtn) {
                rterror("rt_band_get_quantiles: Could not allocate memory for quantile output");
                if (init_quantiles) rtdealloc(quantiles);
                return NULL;
        }
 
        /* sort values */
        if (!stats->sorted) {
                quicksort(stats->values, stats->values + stats->count - 1);
                stats->sorted = 1;
        }
 
        /*
                make quantiles
 
                formula is that used in R (method 7) and Excel from
                        http://en.wikipedia.org/wiki/Quantile
        */
        for (i = 0; i < quantiles_count; i++) {
                rtn[i].quantile = quantiles[i];
 
                h = ((stats->count - 1.) * quantiles[i]) + 1.;
                hl = floor(h);
 
                /* h greater than hl, do full equation */
                if (h > hl)
                        rtn[i].value = stats->values[hl - 1] + ((h - hl) * (stats->values[hl] - stats->values[hl - 1]));
                /* shortcut as second part of equation is zero */
                else
                        rtn[i].value = stats->values[hl - 1];
        }
 
#if POSTGIS_DEBUG_LEVEL > 0
        stop = clock();
        elapsed = ((double) (stop - start)) / CLOCKS_PER_SEC;
        RASTER_DEBUGF(3, "elapsed time = %0.4f", elapsed);
#endif
 
        *rtn_count = quantiles_count;
        if (init_quantiles) rtdealloc(quantiles);
        RASTER_DEBUG(3, "done");
        return rtn;
}

References rt_bandstats_t::count, rt_quantile_t::quantile, quicksort(), RASTER_DEBUG, RASTER_DEBUGF, rtalloc(), rtdealloc(), rterror(), rt_bandstats_t::sorted, rt_quantile_t::value, and rt_bandstats_t::values.

Referenced by RASTER_quantile(), and test_band_stats().

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