try this:

cincirin wrote:

try this:

1. intVariable = static_cast<int>(doubleVariable + .5f) * 100

In other words (int)1.1f = 1 and (int)1.9f = 1

I think this piece of code will give you 400 with an input value of 4.2… The multiplication has to be inside the static_cast as well. I’d rather try :

Note that I also changed .5f into .5, since doubleVariable is (I think) a double and not a float.

As Johan Solo said:

4.2 -> int = 4 *100 = 400;

Correctly arranged it comes to this:

That looks like the same solution as to add 0.001. The question is why does it cast a double like 420.0 to the int 419? And how can this cast error else be resolved instead of adding half or a bit of an int?

The problem is because of floating point representation in any programming language. The conversion from double to int is just truncating without rounding.

luggi wrote:

That looks like the same solution as to add 0.001. The question is why does it cast a double like 420 to the int 419? And how can this cast error else be resolved instead of adding half or a bit of an int?

It’s a bit more clever that just adding .001, it ensures that the rounding will be done properly. When you cast a double / float value into a integer, no rounding is performed, the biggest integer value smaller than your floating point value is returned (the floor function [cplusplus.com]). Another complication is that some numbers don’t have an exact representation on a float / double. For instance 0.1 is 0.00011001100110011… (see here [learncpp.com]).

Okay thx. So finally I can assume that to add 0.5 is the best method to solve this problem.

luggi wrote:

Okay thx. So finally I can assume that to add 0.5 is the best method to solve this problem.

You might want to be careful. Just adding 0.5 is not always the solution. For negative numbers you run into the opposite problem.
A typical solution for rounding is:

Isn’t there any method in Qt that does exactly this? I use this quiet a lot and it would be useful if I can apply it directly to the double.

I cannot tell whether such a function exists in Qt, but I think you should just define a function which uses koahnig’s code, maybe you want the function to return an int directly.

Not to my knowledge. I guess a lot of programmers are using the line directly or have it somewhere in there headers. Rounding is nothing Qt specific. It is more basic programming in any programming language.

Look at qRound() and qRound64() in qglobal.h.

That’s it, thx!