Re: Rounding error?

It all comes down to the inexactitude in the way computers represent floating point values. By printing the values out with as many digits of precision as you can, you'll see that one value rounds down, and the other rounds up.

printf "%.17f\n", 40.88050;
40.88049999999999800

printf "%.17f\n", 41.78050;
41.78050000000000400
[download]

The (probable) reason that you see a different result from C, is that you are (probably) using single precision (floats) in your C code, whilst Perl uses double precision. (Probably:)

See Re: Re: Re: Bug? 1+1 != 2 for more information.

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Comment on Re: Rounding error? Download Code

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Re^2: Rounding error? by pg (Canon) on Oct 19, 2004 at 03:27 UTC
"The (probable) reason that you see a different result from C, is that you are (probably) using single precision (floats) in your C code, whilst Perl uses double precision. (Probably:)" This seems to be a good guess: `#include <stdio.h> #include <stdlib.h> int main(int argc, char * * argv) { { double a = 40.88050; printf("%.3f\n",a); printf("%.17f\n",a); a = 41.78050; printf("%.3f\n",a); printf("%.17f\n",a); } { float a = 40.88050; printf("%.3f\n",a); printf("%.17f\n",a); a = 41.78050; printf("%.3f\n",a); printf("%.17f\n",a); } return 0; }` [download] Prints: `40.880 40.88049999999999784 41.781 41.78050000000000352 40.881 40.88050079345703125 41.780 41.78049850463867188` [download]	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re^2: Rounding error?
by pg (Canon) on Oct 19, 2004 at 03:27 UTC

"The (probable) reason that you see a different result from C, is that you are (probably) using single precision (floats) in your C code, whilst Perl uses double precision. (Probably:)"

This seems to be a good guess:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char * * argv) {
    {
        double a = 40.88050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
        a = 41.78050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
    }
    {
        float a = 40.88050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
        a = 41.78050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
    }
    return 0;    
}
[download]

Prints:

40.880
40.88049999999999784
41.781
41.78050000000000352
40.881
40.88050079345703125
41.780
41.78049850463867188
[download]

[reply]
[d/l]
[select]