453 - Intersecting Circles

[brianfry713]

One circle is inside the other.

[Shahidul.CSE]

Getting WA. But why? My code passes all the input on this forum.
Here is my code:

Code: Select all

#include <stdio.h>
#include <math.h>
#define eps 1e-4
int main()
{
    double a, b, c, d, c1, c2, X1, X2, Y1, Y2, e, f, p, k, r, s;
    while(scanf("%lf %lf %lf", &a, &b, &r)!=EOF)
    {
        scanf("%lf %lf %lf", &c, &d, &s);


        e = c - a;
        f = d - b;
        p = sqrt(e*e + f*f);
        k = (p*p + r*r - s*s)/(2.0*p);

        if(fabs(a-c)<eps && fabs(b-d)<eps)
        {
            printf("(%.3lf,%.3lf)\n", a,b);
            continue;
        }

        if(fabs(a-c)<eps && fabs(b-d)<eps && fabs(r-s)<eps)
        {
            printf("THE CIRCLES ARE THE SAME\n");
            continue;
        }

        if(p>(r+s)+eps || (p+s)+eps<r || (p+r)+eps<s)
        {
            printf("NO INTERSECTION\n");
            continue;
        }

        X1 = X2 = a + (e*k)/p;
        Y1 = Y2 = b + (f*k)/p;
        if(fabs(r*r - k*k)<1e-6);
        else
        {
            X1 -= (f/p)*sqrt(r*r - k*k);
            Y1 += (e/p)*sqrt(r*r - k*k);
            X2 += (f/p)*sqrt(r*r - k*k);
            Y2 -= (e/p)*sqrt(r*r - k*k);
        }
        if(X1<eps && X1>-eps)
            X1=0.0;
        if(X2<eps && X2>-eps)
            X2=0.0;
        if(Y1<eps && Y1>-eps)
            Y1=0.0;
        if(Y2<eps && Y2>-eps)
            Y2=0.0;

        if(X1+eps<X2)
        {
            printf("(%.3lf,%.3lf)", X1, Y1);
            if(fabs(X1-X2)<eps && fabs(Y1-Y2)<eps);
            else
                printf("(%.3lf,%.3lf)", X2, Y2);
        }
        else
        {
            printf("(%.3lf,%.3lf)", X2, Y2);
            if(fabs(X1-X2)<eps && fabs(Y1-Y2)<eps);
            else
                printf("(%.3lf,%.3lf)", X1, Y1);
        }
        printf("\n");
    }
    return 0;
}

[brianfry713]

I got AC by adding 5e-5 while printing the points, and 1e-6 for EPS everywhere else - including when determining if there are one or two intersection points.

[Shahidul.CSE]

I got AC by adding 5e-5 while printing the points, and 1e-6 for EPS everywhere else - including when determining if there are one or two intersection points.

Sorry, I couldn't understand by your answer that where to use 1e-6 and where to use 5e-5. Please, say it clearly, with code.

And how to determine when to use which value for EPS? Because it is so sensitive in programming contest.

[brianfry713]

#define EPS1 1e-6
#define EPS2 5e-5
printf("(%.3lf,%.3lf)", X2 + EPS2, Y2 + EPS2);

Everywhere else use EPS1

On most problems a small EPS like 1e-8 works, but this problem is tricky
http://floating-point-gui.de/

[Shahidul.CSE]

#define EPS1 1e-6
#define EPS2 5e-5
printf("(%.3lf,%.3lf)", X2 + EPS2, Y2 + EPS2);

Everywhere else use EPS1

On most problems a small EPS like 1e-8 works, but this problem is tricky
http://floating-point-gui.de/

Even getting WA!
Here is my changed code:

Code: Select all

#include <stdio.h>
#include <math.h>
#define eps 1e-6
#define EPS 5e-5
int main()
{
    double a, b, c, d, c1, c2, X1, X2, Y1, Y2, e, f, p, k, r, s;
    while(scanf("%lf %lf %lf", &a, &b, &r)!=EOF)
    {
        scanf("%lf %lf %lf", &c, &d, &s);

        e = c - a;
        f = d - b;
        p = sqrt(e*e + f*f);
        k = (p*p + r*r - s*s)/(2.0*p);

        if(fabs(a-c)<eps && fabs(b-d)<eps)
        {
            printf("(%.3lf,%.3lf)\n", a+EPS,b+EPS);
            continue;
        }

        if(fabs(a-c)<eps && fabs(b-d)<eps && fabs(r-s)<eps)
        {
            printf("THE CIRCLES ARE THE SAME\n");
            continue;
        }

        if(p>(r+s)+eps || (p+s)+eps<r || (p+r)+eps<s)
        {
            printf("NO INTERSECTION\n");
            continue;
        }

        X1 = X2 = a + (e*k)/p;
        Y1 = Y2 = b + (f*k)/p;
        if(fabs(r*r - k*k)<eps);
        else
        {
            X1 -= (f/p)*sqrt(r*r - k*k);
            Y1 += (e/p)*sqrt(r*r - k*k);
            X2 += (f/p)*sqrt(r*r - k*k);
            Y2 -= (e/p)*sqrt(r*r - k*k);
        }
        if(X1<eps && X1>-eps)
            X1=0.0;
        if(X2<eps && X2>-eps)
            X2=0.0;
        if(Y1<eps && Y1>-eps)
            Y1=0.0;
        if(Y2<eps && Y2>-eps)
            Y2=0.0;

        if(X1+eps<X2)
        {
            printf("(%.3lf,%.3lf)", X1+EPS, Y1+EPS);
            if(fabs(X1-X2)<eps && fabs(Y1-Y2)<eps);
            else
                printf("(%.3lf,%.3lf)", X2+EPS, Y2+EPS);
        }
        else
        {
            printf("(%.3lf,%.3lf)", X2+EPS, Y2+EPS);
            if(fabs(X1-X2)<eps && fabs(Y1-Y2)<eps);
            else
                printf("(%.3lf,%.3lf)", X1+EPS, Y1+EPS);
        }
        printf("\n");
    }
    return 0;
}

And with previous code, I got correct outputs for all the test cases posted on this forum. But with the above code, I got wrong outputs for several test cases. Such as, I got output 0.087 instead of 0.086
So, what to do?

[brianfry713]

Input:

Code: Select all

0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 1.0
0.0 0.0 1.0

AC output:

Code: Select all

(0.000,0.000)
THE CIRCLES ARE THE SAME

[metaphysis]

Another problem tricky float point number involved.

[anacharsis]

I think the test cases for this problem actually uncover literally every precision problem that you can have with floating point numbers.
NaNs from subtracting two non-NaN floats, negative 0's...this was quite a party!

Also, remember that a point is a degenerate circle.
So, you have to cover the case where you have one zero radius circle that sits on the other circle's border.
And, you have to cover the case where two 0 radius circles have the same "center" - this case still has a single intersection.

I can say my geo libraries are much improved after this one!

[NAbdulla]

gootsa,please check this testcase..

Code: Select all

input
1.0 1.0 5.0
1.0 1.0 10.0

output must be
(1.000,1.000)

please ,send your new code to me,if you dont mind....
usuttra@yahoo.com

Why? are they intersect? or this is a special case?