[Qt-creator] tofloat() function gives inappropriate results

[David Dibben]

Hi Ashish,


On 10/07/2010 19:58, Ashish Vats wrote:
> I am using QT4.5 , I want to convert the string value to float so I have
> used function tofloat(). But the results i am getting are not accurate.
> For example : If the string value is 7.6 then the output of function to
> float is 7.5999...

Firstly, I am think this might not be the right list for this question. 
I would try Qt-Interest instead.

But to answer your question. The problem is not with QString::toFloat 
but the nature of floating point numbers. A float cannot represent 7.6 
exactly, so you will always get this type of error

For example, try this little program:

int main()
｛
　　　	float x = 7.6f;
	printf(" %16.15f \n", x);

	double y = 7.6;
	printf(" %16.15f \n", y);

	reurn 0;
｝

This does not use Qt's QString at all, and I get:
7.599999904632568
7.600000000000000

When using a float, you won't get 7.6. I would try using a double 
instead. That will give a better representation, but there will still be 
strings that do not get converted exactly. Depending on what you want to 
do with the numbers that may or may not matter.


-- 
David Dibben

Web: http://www.daviddibben.com
Mail: dibben at ieee.org