Strange results with floating-point comparison

Question

I have this simple test:

double h;
...
// code that assigns h its initial value, used below
...
if ((h>0) && (h<1)){
 //branch 1 -some computations
}
else{
 //branch 2- no computations
}

I listed my values as I got some really strange results and for example if: h=1 then the first branch is reached and I do not understand why since if h=1 I want branch2 to be computed.
Am I getting confused by something so obvious?

---

Edit:

This is how I compute and then use h:

double* QSweep::findIntersection(edge_t edge1,edge_t edge2) {  
point_t p1=myPoints_[edge1[0]];
point_t p2=myPoints_[edge1[1]];
point_t p3=myPoints_[edge2[0]];
point_t p4=myPoints_[edge2[1]];

double xD1,yD1,xD2,yD2,xD3,yD3,xP,yP,h,denom;
double* pt=new double[3];
        
// calculate differences  
xD1=p2[0]-p1[0];  
xD2=p4[0]-p3[0];  
yD1=p2[1]-p1[1];  
yD2=p4[1]-p3[1];  
xD3=p1[0]-p3[0];  
yD3=p1[1]-p3[1];    

xP=-yD1;
yP=xD1;
denom=xD2*(-yD1)+yD2*xD1;
if (denom==0) {
    return NULL;
}
else{
h=(xD3*(-yD1)+yD3*xD1)/denom;
}
std::cout<<"h is"<<h<<endl;
if (h < 1) std::cout<<"no"<<endl;
else std::cout<<"yes"<<endl;
if (h==1) {
    return NULL;
}
else{
if ((h>0)&&(h<1)){
    pt[0]=p3[0]+xD2*h;  
    pt[1]=p3[1]+yD2*h;
    pt[2]=0.00;
}
else{
    return NULL;
}
}


return pt;  

}

---

Edit:

Okay, so it is clear how I should reformulate the condition.

From:

double h;
if (h==1){
   //computations here
}

To:

double h;
if (abs(h-1)<tolerance){
  //computations here
}

When I use double numbers.

But how do I reformulate this?

double h;
if (h<1){
   //computations here
}

Answer 1

Since h is a double, it may have been close enough to 1 to print as 1, but it is actually a bit less than 1 so the comparison succeeds. Floating-point numbers do that a lot.

Answer 2

Check the actual value of h by printing it out with maximum precision. You'll probably find that it is actually slightly less than 1.0.

I ran the following code as a test

#include <iostream>

int main()
{
    double h = 1.0;
    if((h>0) && (h<1))
    {
        std::cout << "first branch" << std::endl;
    }
    else
    {
        std::cout << "second branch" << std::endl;
    }
}

and the output was "first branch" (using g++ 4.3.2 on Ubuntu 8.10), but Indeera mentioned in a comment that the same code running on Windows XP compiled with VS2005 gives the output "second branch" (thanks, Indeera).

You might change your code to compare the differences between h and 0.0 and h and 1.0 to some small delta value.

double allowedDelta = 0.000001;

if( ((h - 0.0) > allowedDelta) && ((1.0 - h) > allowedDelta) )
... // h is between 0.000001 and 0.9999990

Note that "(h - 0.0)" can be replaced with "h" in this special case. I'm leaving it the way it is for illustrative value.

Also note that if you were only making one comparison you'd need to compare the delta to the absolute value of the difference between h and some constant. Since you're checking a range here, the two comparisons ANDed together make a special case where you can bypass the use of abs. If h is a negative value or some positive value greater than 1.0 it will be out of range and fail one of the two tests above.