How to check for undefined value of a double?

Question

I am trying to figure out a way to check for a undefined value of a slope in which case it would be vertical. I have tried using NULL but that doesn't seem to work.

double Point::Slope(Point &p2)
{
   double slop = 0;
   slop = (y - p2.y) / (x - p2.x);

   if (slop == NULL)
   {
      slop = 10e100;
   }

   return slop;   
}

This would help http://stackoverflow.com/questions/570669/checking-if-a-double-or-float-is-nan-in-c/570694#570694 — Hauleth, Nov 21 '11 at 23:09
You need to define what would be outside the range of acceptable values and then check for that. — r_ahlskog, Nov 21 '11 at 23:10
This question is not a duplicate of the indicated question. See this conversation on meta: http://meta.stackexchange.com/questions/109993/guidelines-for-closing-questions-as-exact-duplicate — John Dibling, Nov 22 '11 at 14:50

Walter · Accepted Answer · 2018-03-21T23:53:31.810

3

If you mean nan ('not a number') with "undefined", you should avoid computing one in the first place, i.e. by checking that the denominator of a '/' operation is not zero. Second, you can always check for nan by

#include <cmath>
bool std::isnan(x);   // since C++11
bool isnan(x);        // pre C++11, from the C math library, defined as macro

see the man pages, or cppreference.

edited Mar 21 '18 at 23:53

answered Nov 21 '11 at 23:11

Walter

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`isnan` is not a Standard function, AFAICT. – John Dibling Nov 22 '11 at 14:48
@JohnDibling It is since C++11. – Walter Mar 21 '18 at 23:54

score 1 · Answer 2 · answered Nov 21 '11 at 23:13

1

In C++, NULL == 0. This is not what you seek.

Maybe this may help you : http://www.gnu.org/s/hello/manual/libc/Infinity-and-NaN.html

Try the isnan(float) function.

answered Nov 21 '11 at 23:13

Clement Bellot

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score 0 · Answer 3 · answered Nov 21 '11 at 23:14

I'd recommend avoiding the divide-by-zero all together (by the way... why don't you call it slope instead of slop?):

double Point::Slope(Point&p2)
{
    double slope = 0;
    double xDelta = x - p2.x;
    double yDelta = y - p2.y;

    if (xDelta != 0)
    {
        slope = yDelta / xDelta;
    }

    return slope;   
}

How to check for undefined value of a double?

3 Answers3