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While this works, converting to a string, and then back to a float is a slow way of doing it. Check my other answer for alternatives that work.
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Hi Andrew,
1 With a modern compiler implementing move semantics it is not that slow. You can also quicken it with a template for decimal.
2 It is safe, accurate and handles negative values.
3 It is flexible. You don't have to write a new one for each rounding you may need.
All in all this is an academic discussion  Real world uses fixed point representations when needing rounded decimal values.
cheers,
AR
When the wise (person) points at the moon the fool looks at the finger (Chinese proverb)
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Are you aware that floating points number have a rounding error ? You'll never be able to represent perfectly a float in memory. For more information google for "floating point precision".
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My answer gets the nearest representation of the rounded value and produces corrrect string output.
cheers,
AR
When the wise (person) points at the moon the fool looks at the finger (Chinese proverb)
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Yes sure, you can always print a floating point with the precision you want, but it doesn't mean that the float stored in memory is rounded properly. I simply wanted to make sure that he is aware of that.
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Some of the suggested answers are ok, but are lacking correct rounding.
Luc Pattyn wrote:
double roundToOneTenth(double number) {
return 0.1*(int)(10.*number);
}
Should be:
double RoundToOneTenth(double nNumber) {
return (int)(10.0 * nNumber + 0.5) * 0.1;
}
While this solution works, it requires converting a floating point number to an integer and back to a floating point number. This is somewhat slow.
Hans Dietrich wrote:
double d = floor(1.333333 * 10.) / 10.;
Should be:
double RoundToOneTenth(double nNumber) {
return floor(nNumber * 10.0 + 0.5) * 0.1;
}
This is the solution I would recommend. Note that I changed /10 to *0.1.
It is faster to multiply than to divide. In a simple case like this the compiler would make this change for you but I am just pointing this out.
Alain Rist wrote:
#include <sstream>
#include <iomanip>
double Round(double val, size_t decimal)
{
std::ostringstream oss;
oss << std::fixed << std::setprecision(decimal) << val << std::ends;
std::istringstream(oss.str()) >> val;
return val;
}
While this works and saves you from worrying about rounding errors, this is incredibly slow and uses much more memory (although still not much) than other solutions. There is no need for using strings for such a simple operation.
Having said that, it is good that people are posting alternatives to show just how many ways there are for doing this.
Cedric Moonen wrote: Are you aware that floating points number have a rounding error ? You'll never be able to represent perfectly a float in memory.
While this is absolutely true, it only affects really big numbers or numbers with a high precision.
For instance, 1.0 / 3.0 == 0.333333333333333314829616256247390992939472198486328125 (Wikipedia[^]).
If you are concerned about this, there are macros FLT_EPSILON (for float) and DLB_EPSILON (for double) which define the minimum perfect precision of a single operation.
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..and in addition, there might be other interesting aspects of rounding. This[^] post on cplusplus.com brings a few of these up.
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Andrew Brock wrote:
double RoundToOneTenth(double nNumber)
{
return floor(nNumber * 10.0 + 0.5) * 0.1;
}This is the solution I would recommend. Note that I changed /10 to *0.1.
-5.51 => -5.4 
cheers,
AR
When the wise (person) points at the moon the fool looks at the finger (Chinese proverb)
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Alain Rist wrote: -5.51 => -5.4
Not so bad, since we're talking round numbers. 
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That rounding calculation fails with negative numbers. This will work with positives, negatives, and zero :
int RoundValue( double value )
{
int result = 0;
if( value < 0 )
result = (int)( value - 0.5 );
else
result = (int)( value + 0.5 );
return result;
}
Also - yes, it is faster to mulitply than to divide but the percentage of difference is very small (around 10% in my tests) so that is not enough that I would worry about it. Actually, since it is so close I would refrain from changing to multiplies because occasionally it can result in a loss of clarity and the compiler will do it for us when it can.
modified on Wednesday, February 23, 2011 2:02 PM
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Oops, I thought about this for the floor(), but it wasn't the case. Forgot about the casting one.
And yes, the percentage is small, but as a game developer any clock cycles I can save by simple things like this are worth it. I guess my views of what is important may differ from others, I prefer high performance over clarity.
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I can understand that - to a point. If you are that concerned with performance there are a lot of other techniques you can use to get big gains in performance. One simple example : if a complex expression or calculation is repeated then save the result in a temporary variable. I have lost count of how many times I have seen people repeat expressions in code. Not only will this have higher performance it will also help maintenance and that is one of my highest priorities. It is been frequently stated that software usually spends more time undergoing maintenance than it does being developed and in my experience this is very true so I believe in making software as maintainable as possible and clarity is very important for this.
One more thing - as you said before, the compiler will convert to multiplies automatically when it can so, to me, it makes no sense to sacrifice clarity for something that is done for you transparently. In fact, I find it completely ridiculous but opinions vary.
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You're putting too much value on your solution if you ask me. 
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thanks for the tip. I hadn't noticed that.
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See also my answer to your previous question.
I must get a clever new signature for 2011.
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I require some result based on the a float value to multiply with an int value.
like 146 * (1/1.3333333) result should be 109.50 but through program it's come out with 108.95. I have also tried float( 146 * (1/1.333333)) and float(146) * float(1/1.333333) but the result is same. I am getting surprised from this result. Can anyone help me out.
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hi
for me its giving right answer. i did like this.
char ms[100];
int tk = 146;
int tl = 1;
float tm = 1.333333;
float tr = ((float)tk)*(tl/tm);
sprintf(ms, "%f", tr);
AfxMessageBox(ms);
how you are doing?
Regards,
A. Gopinath.
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Do not use floats or doubles if you want to get accuracy in the decimal portion of your values. Floating point values in computers are held in such a way that accuracy cannot be guaranteed. There are many articles on the internet that explain why this is so.
I must get a clever new signature for 2011.
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Hi everyone. I would like to know if it is possible to use Windows Forms from the System::Windows::Forms namespace in unmanaged C++. If it is possible, how is it done? Thanks.
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It is not. If you wanna create gui apps with c++, dig around win32 APIs. CreateWindow is your friend.
011011010110000101100011011010000110100101101110
0110010101110011
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I really need the datagridview tool in .net, any way of using it in Win32 API programming?
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Just an idea! Can you have the WinForms dialogs in a COM compatible dll/class library? Then you might be able to use the WinForm dialogs in your unmanaged application. This might sound easier said then done but thought of just sharing the idea with you. You might also need to investigate on the feasiblity of this.
I am a HUMAN. I have that keyword in my name........
_AnsHUMAN_
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Is there any possibility to retrieve clicked menu item text?
In this example i can only get an ID, like:
unsigned in clicked = TrackPopupMenu(hContextMenu,
TPM_RETURNCMD |
TPM_VERPOSANIMATION,
rectangle.left+2, rectangle.bottom-2,
0,
hParent,
NULL);
So "clicked" contains ID now, like:
switch(clicked)
{
case MY_ID:
{
do_something();
}
}
I am interested in getting selected item text instead, because trying to implement dynamic popup menu. Lets say, there are some items in database, they could be changed, added, removed, and when i click a button - it should show a context menu with current items from DB (there are just about 10 items max).
Is there any "easy" way to do it? Or i am better off using combobox for that particular task?
011011010110000101100011011010000110100101101110
0110010101110011
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Well, once you have the ID of the menu item, you can use GetMenuItemInfo[^] to retrieve its text. However, since -as far as i understood- you fill the menu with the items yourself you have a way to know which ID represents which text, so you could also try e.g. putting the strings in an array and store their index in some way in the menu item ID, like, 1+index, and then when you got the ID from TrackPopup..., you subtract one and get the array index and don't need to fiddle around with GetMenuItemInfo.
p.s: in case you try the array thing, don't use 0 as a menu item ID (dunno if you can do that or not) because then you would not be able to distinguish between that menu item and the user dismissing the menu without a selection. That's why i suggested index+1...
> The problem with computers is that they do what you tell them to do and not what you want them to do. <
> //TODO: Implement signature here<
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Great! Thanks. Here is the algo (my own classes but the algo is good):
case CATS_BUTTON:
{
ContextMenu *ctx = new ContextMenu();
wchar_t *err = NULL;
ctx->Create();
wchar_t **Cats = db->ListTables(DB_FILE, err);
int i;
for(i = 1; ; i++)
{
if(!IsNullOrEmpty(Cats[i - 1]))
{
ctx->AddItem(i, Cats[i - 1]);
}
else break;
}
unsigned int Clicked = ctx->TrackMenuHit2(hWnd,
Fbuttons[1]->operator HWND());
switch(Clicked)
{
case 0:
break;
default:
{
MessageBoxW(0,Cats[Clicked - 1],NULL,0);
}
break;
}
ctx->Destroy();
delete ctx;
}
break;
011011010110000101100011011010000110100101101110
0110010101110011
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