在 K&R 2-1 中解释这段代码 [英] Explain this code in K&R 2-1

问题描述

I'm trying to determine range of the various floating-point types. When I read this code:

#include <stdio.h>

main()
{
    float fl, fltest, last;
    double dbl, dbltest, dblast;

    fl = 0.0;
    fltest = 0.0;
    while (fl == 0.0) {
        last = fltest;
        fltest = fltest + 1111e28;
        fl = (fl + fltest) - fltest;
    }
    printf("Maximum range of float variable: %e
", last);

    dbl = 0.0;
    dbltest = 0.0;
    while (dbl == 0.0) {
        dblast = dbltest;
        dbltest = dbltest + 1111e297;
        dbl = (dbl + dbltest) - dbltest;
    }
    printf("Maximum range of double variable: %e
", dblast);
    return 0;
}

I don't understand why author added 1111e28 at fltest variable ?

解决方案

The loop terminates when fltest reaches +Inf, as at that point fl = (fl + fltest) - fltest becomes NaN, which is unequal to 0.0. last contains a value which when added to 1111e28 produces +Inf and so is close to the upper limit of float.

1111e28 is chosen to reach +Inf reasonably quickly; it also needs to be large enough that when added to large values the loop continues to progress i.e. it is at least as large as the gap between the largest and second-largest non-infinite float values.

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