c++, static_cast <int> of float point calculation and possible of losing integers

Question

With the following code:

    int ten{ 1 };
    double zeroPnine{ 0.9 };

    cout << ten - zeroPnine << endl; // 0.1
    cout << (ten - zeroPnine) * 10 << endl; // 1
    cout << static_cast <int>(ten - zeroPnine) << endl; // 0    
    cout << static_cast <int>((ten - zeroPnine) * 10 )<< endl; // 1

I am expecting the last line to output 1, but actual output is in actually 0, how come?

Full output: 0.1
1
0
0

Because of the imprecisions of the floating point format on computers, which can cause rounding errors. — Some programmer dude
– Some programmer dude, Commented Feb 19, 2015 at 6:56
If you print out the last line without the static cast, it shows 1, or at least something close to 1. But if you say "((ten - zeroPnine) * 10) == 1", it says false for the reason Joachim states. The values are imprecise. Floating points should be relied upon for relative values, not discrete points. — Silvio Mayolo
– Silvio Mayolo, Commented Feb 19, 2015 at 6:57
I understand 0.9 isn't integer, but 1 - 0.9 = 0.1, and 0.1 * 10 should give me 1, why is it a problem? — user97662
– user97662, Commented Feb 19, 2015 at 7:12
Due to how floating point numbers work, 0.9 is actually 0.90000000000000002220446049250313080847263336181640625 — molbdnilo
– molbdnilo, Commented Feb 19, 2015 at 7:45

just_another_person · Accepted Answer · 2015-02-20 07:43:14Z

1

Best advice would be to avoid using static_cast to convert double.

Try converting double to int before doing the calculation if you want the answer to be integers.

answered Feb 20, 2015 at 7:43

just_another_person

1342 silver badges6 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

c++, static_cast <int> of float point calculation and possible of losing integers

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related