Are multiples of 0.25 exactly representable as double?

Question

I have the following code for finding quartiles:

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    double qrt[3];
    double *value;
    int count;
} t_data;

static void set_qrt(t_data *data, int qrt)
{
    int n, e;
    double d;

    d = qrt * 0.25 * data->count + 0.5;
    n = (int)d;
    e = n != d;
    data->qrt[qrt - 1] = data->value[n - 1];
    if (e) {
        data->qrt[qrt - 1] += data->value[n];
        data->qrt[qrt - 1] *= 0.5;
    }
}

static void set_qrts(t_data *data)
{
    set_qrt(data, 2);
    if (data->count > 1) {
        set_qrt(data, 1);
        set_qrt(data, 3);
    } else {
        data->qrt[0] = 0.0;
        data->qrt[2] = 0.0;
    }
}

static int comp(const void *pa, const void *pb)
{
    const double a = *(const double *)pa;
    const double b = *(const double *)pb;

    return (a > b) ? 1 : (a < b) ? -1 : 0;
}

int main(void)
{
    double values[] = {3.7, 8.9, 7.1, 5.4, 1.2, 6.8, 4.3, 2.7};
    t_data data;

    data.value = values;
    data.count = (int)(sizeof(values) / sizeof(double));
    qsort(data.value, data.count, sizeof(double), comp);
    set_qrts(&data);
    printf("Q1 = %.1f\nQ2 = %.1f\nQ3 = %.1f\n", data.qrt[0], data.qrt[1], data.qrt[2]);
}

Is

d = qrt * 0.25 * data->count + 0.5;
n = (int)d;
e = n != d;

guaranteed to work as expected? (e == isinteger(d))

Answer 1

Numbers 0.5, 0.25, 0.125 and so on represent negative powers of two, and therefore are representable exactly in IEEE 754 types. Using these numbers does not result in representation errors.