c++ - 从 float 到 int 的奇怪转换

Question

for(float i = 40; i <= 42; i+=0.1f)
{
    float offset = (i-40) * 10 * sizeof(float);
    int wtf = offset;
    printf("%.1f =?= %d\n", offset, wtf);
    fseek(fin, offset, SEEK_SET);
    fread(&tf, sizeof(float), 1, fin);
    //printf("%.1f %.4f\n", i, tf);
}

输出:

0.0 =?= 0 4.0 =?= 3 8.0 =?= 7 12.0 =?= 11 16.0 =?= 15 20.0 =?= 19 24.0 =?= 23 28.0 =?= 27 32.0 =?= 31 36.0 =?= 35 40.0 =?= 39 44.0 =?= 43 48.0 =?= 47 52.0 =?= 51 56.0 =?= 55 60.0 =?= 59 64.0 =?= 63 68.0 =?= 67 72.0 =?= 71 76.0 =?= 75 80.0 =?= 79

我不明白这个，你能帮我吗？

Answer 1

问题是 0.1 不能完全表示为二进制 float 。每个 float 值都略小于您的预期，并且转换为 int 会向下舍入，给出的值比您预期的小 1。

您可以通过四舍五入到最接近的整数来获得期望值:

int wtf = std::round(offset);