Rounding, and converting double to int

[dannn]

Hello there,

I am trying to convert a double type into an integer; which in itself isn't too much of a problem.

The problem I am facing is trying to round my numbers, I would use the round() function however, when a number is exactly halfway (1.5) for example I would like it to round down instead of up (which is really the correct way!)

I found a tutorial, however that seems to show what I want to do; however I can't get the thing to work, and cannot for the life of me get my head around it!

Thanks for the help guys.

[helios]

which is really the correct way!

No, it isn't.

[Zhuge]

What method of rounding are you using? It certainly doesn't match any method of rounding I've ever seen. Rounding .5 up is one of the most common methods used.

[firedraco]

(1.5) for example I would like it to round down instead of up

OK

(which is really the correct way!)

???

Anyway, couldn't use just use round() for all numbers except for those within a very small distance of 1.5?

[TheBear]

Hi dannn,

simply subtract 0.5 from your number and then compute the ceiling of the result:

int
roundHalfwayDown(double d)
{
	return ceil(d - 0.5);
}

This works fine with g++ 4.4.1 (ubuntu).

Cheers,
TheBear

[Duthomhas]

http://www.cplusplus.com/forum/articles/3638/

[tummychow]

helios +1

[Duthomhas]

There is no "one true way to round." All that matters is the kind of bias you want. See the article I linked.

[edit] We were all taught in gradeschool "arithmetic rounding", or common rounding, which is x>=.5 round up, x<.5 round down. [/edit]

[tummychow]

Exactly, arithmetic rounding is the mathematically correct way to round. That's why I ++'d helios. He was right. Of course you have ceiling() and floor() but if you can round in either direction, .5 and up should be rounded up.

[helios]

Of course you have ceiling() and floor()

That's not rounding. That's truncation.