Wrong calculation for decimal amount in C language

Question

I'm trying to calculate the decimal amount in float but it won't calculate if the input is "0.01". However, it will calculate if the input is "0.02" but with wrong calculation. Here is the code:

#include <stdio.h>
#include <cs50.h>

float MCounting = 0.00;
int MAmountCoin = 0;
float MAmountUsed = 0.00;
int MCoinCount = 0;
float MRemainAmount = 0;
int MCoinOut = 0;
int MTotCoinOut = 0;

int main(void)
{
float Amount;
float MRemainAmount;
do 
 {
    printf("Specify the amount you want in change:  ");
    Amount = GetFloat();
    MRemainAmount = Amount;
  }
  while (Amount < 0 );

    if (MRemainAmount > 0 || MRemainAmount < .05 )
    printf ("\n\n ***** Calculatin for 0.01 *****\n");
        {
         printf ("MRemainAmount Before calculation: %.2f\n",MRemainAmount);
         MCoinOut = MRemainAmount / .01;
         printf ("MCoinOut = %i...MTotCoinOut = %i\n",MCoinOut,MTotCoinOut);
         MRemainAmount = MRemainAmount - (MCoinOut * .01);
         printf ("MRemainAmount = %.2f\n",MRemainAmount);
         MTotCoinOut = MCoinOut + MTotCoinOut;
         printf ("MTotCoinOut = %i\n",MTotCoinOut);
        }
    { printf("Total Coin Out%i\n",MTotCoinOut); } 

}

What's going wrong and how can I fix it?

Answer 1

You are hitting your epsilon limit. Since you are using floats you are limited in representation by FLT_EPSILON; if you were using a double, you would see improved resolution of DBL_EPSILON. (These values are from <float.h>)

#define DBL_EPSILON     2.2204460492503131e-016 /* smallest such that 1.0+DBL_EPSILON != 1.0 */
#define FLT_EPSILON     1.192092896e-07F        /* smallest such that 1.0+FLT_EPSILON != 1.0 */

Thus if you are using a value like 10000, roughly, you're smallest change in value is something in the vicinity of 10000 * FLT_EPSILON, which would be about .012. If you want to represent with better precision, use doubles.

Answer 2

It is due to the imprecise representation of floating point numbers in the computers memory.

Read up on http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html