Re: Weirdness: (-0.666667 + 0.333333)

From: Corne' Cornelius (cornec_at_nospam-for-noreason.uy)
Date: 05/06/04

  • Next message: Gert Van den Eynde: "Re: Weirdness: (-0.666667 + 0.333333)" 
    Date: Thu, 06 May 2004 09:17:40 +0200

    i should've made it more clear what i actually want to do.

    I'm not really trying to compare 2 double which could be in accurate
    becuase of rounding errors.

    I'm writing a program that draws graphs on a cartesian plane, so in
    order to go on the x-axis from say -2, to +2, in 20 intervals, i need
    one of these point to be zero or very close to it.

    how could i make sure i don't get rounding problems ?

    Corne' Cornelius wrote:
    > Hi,
    >
    > I'm experiencing some weirdness in a program, when subtracting 2
    > (double)'s which should result in 0, but instead it returns
    > -1.11022e-16. It looks to me that changing the double x_step decleration
    > to unsigned type, might help, but the compiler complains when i try that.
    >
    > Any ideas ?
    >
    > #include <iostream>
    > using namespace std;
    >
    > int main(int argc, char *argv[]) {
    >
    > double x1 = -1;
    > double x2 = 1;
    > double nx = 6;
    > int pos = 0;
    > double x = 0.0;
    > double x_step = 0.0;
    >
    > x_step = (x2 - x1) / nx;
    >
    > x = x1;
    > for (pos = 0; pos < nx; pos++) {
    > cout << x << " +\t" << x_step << " =\t " << (x +
    > x_step) << endl;
    > x = x + x_step;
    > }
    >
    > return 0;
    > }
    >
    >
    > Output:
    >
    > -1 + 0.333333 = -0.666667
    > -0.666667 + 0.333333 = -0.333333
    > -0.333333 + 0.333333 = -1.11022e-16
    > -1.11022e-16 + 0.333333 = 0.333333
    > 0.333333 + 0.333333 = 0.666667
    > 0.666667 + 0.333333 = 1
    >

  • Next message: Gert Van den Eynde: "Re: Weirdness: (-0.666667 + 0.333333)"

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