Eddie
No, you should be truncating to an int, not using Math.Round. Here's an
example:
using System;
namespace DoubleRound
{
class Program
{
static void Main(string[] args)
{
// initial number may be slightly low or high
double d1 = 137.889999;
double d2 = 137.890000;
double d3 = 137.890001;
Console.WriteLine("Rounded d1 = " + Round(d1).ToString());
Console.WriteLine("Rounded d2 = " + Round(d2).ToString());
Console.WriteLine("Rounded d3 = " + Round(d3).ToString());
}
static decimal Round(Double d)
{
double rd1 = d + 0.0001; // add 1/100th of a cent
double rd2 = rd1 * 100.0; // integral part of double is cents
int i = (int) rd2; // integer is cents
decimal dec1 = (decimal) i; // decimal is cents
decimal dec2 = dec1 / 100m; // decimal is dollars.cents
// no precision errors with decimal divide
return dec2;
}
}
}
I laid out the steps in Round() for clarity, you could easily write it in
fewer lines. The result is:
Rounded d1 = 137.89
Rounded d2 = 137.89
Rounded d3 = 137.89
Double has a precision of 64 bits - there's no way your number is going to
be half a cent out because of rounding.
David.
On Thu, 28 Aug 2008 11:39:42 -0400, Eddie Lascu <[EMAIL PROTECTED]>
wrote:
>David,
>
>It all depends on what method are you using: Math.Round or Math.Truncate.
>Round rounds the double to the nearest integer. So
>
>Math.Round( 13788.999 ) will return 13789 as opposed to Math.Truncate who
>will return 13788.
>
>So far so good. The problem will appear when the number is 13788.5. This
>will be rounded to the nearest even number so Math.Round will return 13788
>(incorrect). Hopefully this will never happen, because as some of you
>suggested, that means that the initial amount of $137.89 is stored in a
>double as 137.885000, off by 0.005, a very big precision error.
>
>My point is that adding anything will not help me because I can introduce
>the error. Just for the sake of argument, let's assume that a precision
>error of 0.005 can happen. If that, then an error of 0.0045 can happen
too.
>If my amount of $137.89 is actually stored as $137.8945 and I add 0.001 I
>get 137.8955 which will round up to 137.90 - wrong number. We have already
>established that Math.Truncate is not good so I am only left with
Math.Round
>and with the hope that a precision error of 0.005 is too big to actually
>happen.
>
>Thanks to everyone who chimed in and shared experience.
>Best regards,
>Eddie
>
>
>
>
>-----Original Message-----
>From: Discussion of advanced .NET topics.
>[mailto:[EMAIL PROTECTED] Behalf Of David Nicholson
>Sent: Wednesday, August 27, 2008 8:04 PM
>To: ADVANCED-DOTNET@DISCUSS.DEVELOP.COM
>Subject: Re: [ADVANCED-DOTNET] Converting doubles into integers without
>rounding errors
>
>
>Since there seems to be no clear solution so far, I've gone back to the
>beginning. I suggest you get more information by writing some code to
>display your numbers to higher precision. My expectation is that the
>137.89 is actually 137.889999 (probably more nine's). So your code below
>converts this to 13788.9999, then truncates to 137.88.
>
>So add a small value (e.g. .0001) first. Then the outcomes for numbers
>above and below the intended number look something like:
>
>137.889999 -> 137.890099 -> 13789.0099 -> 13789
>137.890000 -> 137.890100 -> 13789.0100 -> 13789
>137.890001 -> 137.890101 -> 13789.0101 -> 13789
>
>David.
>
>
>On Mon, 25 Aug 2008 12:27:52 -0400, Eddie Lascu <[EMAIL PROTECTED]>
>wrote:
>
>>Hello everyone,
>>
>>I have some objects that contain an amount field that is declared as
>double.
>>Since it contains amounts, it always has only two decimal digits that are
>>significant. During the process I need to convert that double into an
>>integer by removing the decimal point. For example, $78.59 should be
>>converted to integer 7859 and $101.53 to 10153. in my code I have
>>uint nIntAmount = (uint)(objMyObject.Amount * 100);
>>
>>The problem I am facing is that sometimes, very rarely, there is a
>rounding
>>error that is introduced and the integer obtained is off by a cent (plus
>or
>>minus). For example, this is a line that was traced in my log file:
>>
>>"Updating the batch with $137.89 as the amount in the transaction. This
>>amount was converted to 13788."
>>
>>Can either of you suggest a different way to convert the amounts in
>integers
>>without this nagging rounding error?
>>
>>Any help will be appreciated,
>>
>>Eddie
>>
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