std::cout does not print float as excepted

Summarizing my comments:

• The number 1.0000001 cannot be represented by a float.
• The compiler will pick a float value.
• It can round however it wants (up, down or closest), that's implementation-defined.

So if you play a bit with this converter, you can see that after 1, the next float values are:

• 1
• 1.00000011920928955078125
• 1.0000002384185791015625
• 1.00000035762786865234375
• 1.000000476837158203125
• ...

1.0000001 falls in between the first two, so your compiler can choose either. In your case it seems your compiler picked 1.

(also for printing operations, cout operations will truncate digits after a defined precision, so it does not show the full numbers I put above - that precision defaults to 6 digits).

The default precision I believe is 6 digits for you, but they are all zeroes. So they don't print.

#include <iostream>
#include <iomanip>

int main()
{
    std::cout << std::setprecision(10);

    // this is what you had
    float f1 = 1.0000001f;
    float f2 = f1 * 2;
    std::cout << f1 << std::endl;
    std::cout << f2 << std::endl;

    // any more zeroes and it would have been idistinguishable from 1.0
    f1 = 1.00000001f;
    f2 = f1 * 2;
    std::cout << f1 << std::endl;
    std::cout << f2 << std::endl;
}

1.000000119
2.000000238
1
2