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Floating Decimal Points

Posted on 2000-03-31
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Last Modified: 2010-04-02
int item number;
float price; float numsold;
float income; float discount;
float saleprice;

My program reads from an input file and output is #sold - price -discount - saleprice - income
my calculations for saleprice returns:
Ex.- 5.0575
setw(2) returns 5.06 which is correct.

When the program calculates income-
saleprice * numsold the program returns:
65.747498
setw(2) = 65.75 which is incorrect.
The correct answer is 65.78
RAM is using (65.747498 * 13)=65.747498
How do I correct this problem w/ floating point decimals?
Am I overlooking something simple?
I have tried everything I know of.
Help!!!!!!! Thanks    David A Davis
 Nash Community College Rocky Mt NC
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Question by:DavidAshley
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Accepted Solution

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nietod earned 50 total points
ID: 2673576
The quick fix is to round all the values to 2 decimal places (round to cents) after you do the calcualtions (also within the calculations, after and multiply or divide steps)..  at the moment you are rounding to 2 decimals only when you print, but not after each calculations.  So the rounding errors that are occuring in your calculations are "componding" and are becoming noticable.

continues
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Expert Comment

by:nietod
ID: 2673593
You can round a number to an integer value by adding .5 to and then using the floor function.  To round to 2 decimals, you multiply it by 100, then round to an intger, then divide by 100.  For example

double RoundToCents(double D)
{
   return floor( (100*D)+ 0.5) / 100;
};

So use this function to round numbers that you create as a result of a calculation before the number is used in a 2nd calculation.

Let me know if you have any questions.
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Expert Comment

by:ozo
ID: 2674387
It is often more dependable to work in exact integer cents instead of approximate floating dollars.
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Expert Comment

by:nietod
ID: 2674532
Yes, I had actually meant to mention this point, that is why I called it a "quick fix" then forgot to mention it at the end.

Floating point numbers are always approximate (although there are certian guarantees that you are given) and thus are really  not appropriate for accounting type calculations where you have to be accurate to the exact penny.  The rounding sugggestion I made will usually be enough to guarantee good results, but occasionally you might still be off a penny.  a better idea would be to work with a number that guarantees perfect results to a specified level of precission.  You can use integers to represent amounts expressed in cents.   i.e $12.34 would be specified as 1234.  Another option would be to use BCD numbers to represent the values.  The advantage of BCD is that it can have a much greater range than integers and it can represent decimal values too.  However intergers are smaller and more efficient.
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Author Comment

by:DavidAshley
ID: 2678559
Thanks a bunch.  You have saved me lots of time, blood, sweat and tears.  Plus I will get an A on this program. David
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