Why does >= operator work as > operator in C when compared with 0.9 [duplicate]

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Your code is equivalent to:

double x1 = 0.9;
float xf = (float)x1; // discard some precision
double x2 = (double)xf;

if (x2 >= x1) {
    // ...
}

The problem is that, after you discarded some precision from x1 by converting it to float, it can never be recovered; casting it back to double doesn’t regain that precision. So x2 is still not the same as 0.9, and due to the rounding direction, it happens to be a bit less. (Note: if you try the same thing with other values, you will sometimes find that it’s a bit more.)

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