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Author Topic: imprecise calculation  (Read 746 times)

Offline matze

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imprecise calculation
« on: August 27, 2019, 09:23:41 am »
Hello, I have written a function that does not work correctly, because the function is expected to be inaccurate, which is not due to the source code. It also calculates too imprecisely if I choose the floating point model precise.

I have developed a test program that calls this function:
Code: [Select]
#include <stdio.h>
#include <math.h>


static unsigned short int getDecimalPlaces(double value)
{
    unsigned short int result=0;
    double currentValue=value>=0.0?value:-value;
    currentValue=currentValue-floor(value);
    while(currentValue!=0.0)
    {
        result++;
        currentValue*=10;
        double floorValue=floor(currentValue);
        currentValue-=floorValue;
    }
    return result;
}

int main(int argc, char *argv[])
{
    double value1=99999.999999; //double value with 6 decimal places
    unsigned int decimalChars=getDecimalPlaces(value1);
    printf("%.6f has %u decimal places\n",value1,decimalChars);
    return 0;
}

When running the test program, the following appears in the console:
99999.999999 has 36 decimal places
???
Is there a way to configure Pelles C so that the function getDecimalPlaces works correctly?
« Last Edit: August 27, 2019, 09:31:35 am by matze »

Offline TimoVJL

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Re: imprecise calculation
« Reply #1 on: August 27, 2019, 11:23:08 am »
I don't think it's a Pelles C bug.
Test your example with other compilers and check actual and immediate values.
https://en.wikipedia.org/wiki/Double-precision_floating-point_format

May the source be with you