Question

Decimal rounding occuring when not expected to. Why?

Asked by: sdamiano

Hi,

we have some simple C# code that accesses a data table containing weights for a portfolio. Each weight is expressed in decimal with 4 digits, with the restriction that they must add up to 100(don't need to be positive, although in this case and most practile applications they are). For example, a set of weights for a portfolio containing 5 assets is the following:

0.1469
0.1594
0.1525
0.3887
0.1525
=====
1.0000

The weights are input via an Infragistics table control, linked to a .NET data table, the code below goes over each row (portfolio asset), extracts the weight and computes a running total:


// dtPoints is System.Data.DataTable object

double dblSum = 0.0;
foreach(DataRow dRow in dtPoints.Rows)
{
  if ( dRow[PortfolioPointInfo.ColumnNames.Weight]  == DBNull.Value ) continue;
  double dblWeight = Convert.ToDouble( dRow[PortfolioPointInfo.ColumnNames.Weight] );
  dblSum = dblSum + dblWeight;
}

The first 3 iterations are ok, but on the 4-th one the running total gets rounded:

Iteration #1: Weight = 0.1469, Running Total = 0.1469
Iteration #2: Weight = 0.1594, Running Total = 0.3063
Iteration #3: Weight = 0.1525, Running Total = 0.4588,
Iteration #4: Weight = 0.3887, Running Total = 0.84749999999999992

Why the rounding? Cannot understand why it is happening. All numbers use 4 decimals, so I expect the total to have 4 decimals as well.

Any suggestion to help understand this will be much appreciated!

Best,
Stefano

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Asked On
2006-10-16 at 17:19:24ID22026656
Topic

C# Programming Language

Participating Experts
2
Points
500
Comments
4

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Answers

 

by: ArcaArtemPosted on 2006-10-16 at 23:43:28ID: 17745210

Hi,

Floating point variables cannot hold an exact value but an approximation. For example double d = 1.2 will not cause d to have 1.2 but 1.19999999...

It's best to use "decimal" and I believe it fits perfectly to your requirement.

HTH,
Arca

 

by: rameedevPosted on 2006-10-16 at 23:52:53ID: 17745247

i tried the above code with Decimal datatype and it works fine....is it possible for you to chnage the datatype from double to decimal

as for why it is happening...i am not able to figure it out...still looking...

 

by: ArcaArtemPosted on 2006-10-17 at 00:03:27ID: 17745295

Yes, I believe it is possible to change it by using Convert class.

As for why it is happening... Well, its to do with the way the computer represents real numbers in binary. You'll find more information on:

http://en.wikipedia.org/wiki/Floating_point#Problems_with_floating-point

But basicly it's saying that, floating-point numbers cannot represent real numbers exactly but a very closely approximates.

HTH,
Arca

 

by: sdamianoPosted on 2006-10-17 at 06:53:15ID: 17747594

Thanks, very helpful answers!

Stefano

20120131-EE-VQP-002

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