Qt: Interesting bug occurred convert QString to float

Asked Nov 18 '15 at 09:14

Active Nov 18 '15 at 09:14

Viewed 63 times

I found a interesting thing when i try to converting QString to float by using toFloat() function.

Here is the code:

QString src = "500.05";
int x = src.toFloat() * 100;
float y = src.toFloat() * 100;
int z = (int)(500.05 * 100);

qDebug() << x;
qDebug() << y;
qDebug() << z;

And here is the output:

50004 //x
50005 //y
50005 //z

My question is why i got x is 50004. So is this a Bug or i did something incorrect.

asked Nov 18 '15 at 09:14

JustWe

4,250
3
39
90

6

You have a *round up error*: `500.05 * 100` is actually something like `50004.999999999` – Dmitry Bychenko Nov 18 '15 at 09:16
Thanks, So is this consider as a Bug of Qt? Or i suppose use other way to convert QString to float? – JustWe Nov 18 '15 at 09:29
2

that´s *expected behaviour*. Probably you want to *round up* to the *nearest integer*: `(int)(500.05 * 100 + 0.5);` note `+0.5` – Dmitry Bychenko Nov 18 '15 at 09:35
2

[This post](http://stackoverflow.com/questions/2100490/floating-point-inaccuracy-examples) explains the issues with floating points. – TheDarkKnight Nov 18 '15 at 10:28

Qt: Interesting bug occurred convert QString to float

0 Answers0