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Floating Point Problem

Posted on 1998-12-30
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Last Modified: 2012-05-05
I have a C program that is running into problems rounding a float.  The calculation is as follows

//faceamt = 5000000 and rate = 205.07
premium = (faceamt / 1000) * rate;  
Round2Decimals(&premium);

//*******************************
void Round2Decimals(float *fn)
//********************************
{
 long lrd;
 double dg;
       dg = ceil (*fn * 100);
       if (dg - (double) *fn * 100 >= 0.51)
       dg -= 1.0;
       lrd = (long) dg;
       *fn = (float) lrd / 100;
}    

In the debugger, premium is calculated as 1.02535+e6 which appears to be correct.  This value is then passed to a function to round it to two decimals...
The problem is that dg ends up with 102535007 which is incorrect.  Where did the last two digits (07) come from and how can I avoid this without having to declare all variables as Doubles .  
Thanks.

Upon further investigation...
I think the problem is typecasting a float to a double.  
Example
float fltnum;
double dblnum;
fltnum = 205.07
dblnum = (double)fltnum;

//dblnum is now 205.07000732422.  This is what is causing the problems...
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Question by:mtakasaki
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22 Comments
 

Author Comment

by:mtakasaki
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Edited text of question
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Expert Comment

by:ozo
Comment Utility
keep all amounts in cents instead of dollars
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by:marcjb
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Be careful that the multiplication by 100 doesn't overflow the lrd value.  1.02535e8 would cause a problem.
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by:marcjb
Comment Utility
One other thing, is premium defined as a pointer or a float?
If it is a float, then the line
    Round2Decimals(premium);
is incorrect.  You would need
    Round2Decimals(&premium);

If it is a pointer, the line
    premium = (faceamt / 1000) * rate;  
is incorrect.  You would need
    *premium = (faceamt / 1000) * rate;  

Hope this helps

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Author Comment

by:mtakasaki
Comment Utility
Edited text of question
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Author Comment

by:mtakasaki
Comment Utility
Edited text of question
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by:ginde
Comment Utility
If Possible Convert Float to Double.

This program works

#include <stdio.h>

#include <math.h>

    //*******************************
    void Round2Decimals(float *fn)
    //********************************
    {
     long lrd;
     double dg;
      dg = ceil (*fn * 100);
      if (dg - (double) *fn * 100 >= 0.51)
           dg -= 1.0;
      lrd = (long) dg;
      *fn = (float) lrd / 100;
    }    

void main()
{
float faceamt;
float rate;
float premium;

faceamt = (float)5000000;
rate = (float)205.07;
    premium = (faceamt / 1000) * rate;  
    Round2Decimals(&premium);
}
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by:ginde
Comment Utility
As per my knowledge Float keeps only 7 digits intact, where as double keeps upto 15 digits. So you would be better off using double instead of float.

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Author Comment

by:mtakasaki
Comment Utility
Sorry but this did not work... in the debugger, premium has the value 1.02535+006 (which looks to be correct).  However, display this using a printf and the value is actually 1025350.0625.  This value is used when it is cast to a double (within the Round2Decimals() ).  
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by:ginde
Comment Utility
You need to convert all float to Double to get it working. Please try this ( after conversion)

I think, Float is not reliable after 7 digits.


#include <stdio.h>

#include <math.h>

    //*******************************
    void Round2Decimals(double *fn)
    //********************************
    {
     long lrd;
     double dg;
      dg = ceil (*fn * 100);
      if (dg - (double) *fn * 100 >= 0.51)
           dg -= 1.0;
      lrd = (long) dg;
      *fn = (float) lrd / 100;
    }    

void main()
{
double faceamt;
double rate;
double premium;

faceamt = (double)5000000.00;
rate = (double)205.07;
    premium = (faceamt / 1000) * rate;  
    Round2Decimals(&premium);
}



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Expert Comment

by:rbr
Comment Utility
YOU HAVE TO USE DOUBLE SINCE YOU NEED MORE THAN 7 DIGITS.
Have you checked that your debugger display the full result of your variable. It looks to me that it uses a exponential format and displays only 6 digits.
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Expert Comment

by:rbr
Comment Utility
Your code will be even faster if you use double, since all functions in math.h are are double functions and you don't need to convert float to double all the time.
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Author Comment

by:mtakasaki
Comment Utility
Thanks for the suggestion to convert variables to doubles but I already knew that would help.  I'm looking for a way to deal with this problem without having to convert to doubles (i.e. possibly a compiler option???).
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by:a111a111a111
Comment Utility
More points needed.
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by:scrapdog
Comment Utility
I'll agree with ozo...it is best store all values as integers, then use a float for the result of the final calculation.
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by:jos010697
Comment Utility
May I suggest the excellent article 'what every computer scientist should know about
floating point arithmetic', by David Goldberg (March 1991 Computing Surveys).

It carefully explains (among other useful stuff) what's happening when rounding,
converting and chopping floating point mantissas (significands).

kind regards,

Jos aka jos@and.nl
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by:rbr
Comment Utility
Why don't you want to use double?
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by:a111a111a111
Comment Utility
Any way. 50 points are not sufficient to my answer.
Sorry.

shayplace@hotmail.com
0
 
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by:scrapdog
Comment Utility
Instead of saying what is not sufficient, why don't you name your price?  I presume that by making this statement, you have a correct and superior answer to all previous ones posted.
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Expert Comment

by:ahoffmann
Comment Utility
why do you expect exact values (7 digits) when casting incompatible types (long, float, double) ??
See jos' comment.
Also keep in mind that casting depends on the compiler, includes and  processor type.

BTW,
    fltnum = 205.07;
    dblnum = (double)fltnum;
check fltnum, and you will see that it is 205.07000732422 too.

If you want to know what really happens, you need to print the value bit by bit, not using
any libc routine which reads a bunch of bits/bytes.
Also make shure that the variables are set to zero before using.
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Accepted Solution

by:
jim_pettinato earned 50 total points
Comment Utility
Try this:

float RoundToNearestCent(float num)
{
  char buf[20];
  sprintf("%12.2f", num);
  return atof(buf);
}
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Expert Comment

by:jim_pettinato
Comment Utility
Oops.. error in my answer's sprintf statement.

should read sprintf(buf, "%12.2f", num);
                  * ___ *
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