CODEWITHSUNDEEP
c++doubleangletrigonometry
John Smith
John Smith

Reputation: 11

Why I get an incorrect answer when I substract 180 from the sum of the two angles when the sum itself shows as 180?

The program gets 4 points(their coords) that form a convex figure and should give me the angle A2+angle A4 - 180 degrees. For the example :

1 3
0 2
1 1
2 2

I get uA2=90 and uA4=90, but when I do uA2+uA4-180 I get 2.0568e-007, instead of 0. But when I do just uA2+uA4 I get 180. I am sorry if a similar question was asked, but I didn't know how to search for it. Can anyone tell me why it happens and how can I repair it to show the right answer? Tho code:

#include <iostream>
#include <fstream>
#include <cmath>

#define PI 3.14159265

using namespace std;

ifstream f("date.in");

struct punct
{
    double x,y;
}A1,A2,A3,A4;

int main()
{
    double uA2,uA4;
    f>>A1.x>>A1.y>>A2.x>>A2.y>>A3.x>>A3.y>>A4.x>>A4.y;
//calculate cos of both angles
    uA2=((A1.x-A2.x)*(A3.x-A2.x)+(A1.y-A2.y)*(A3.y-A2.y))/
            (sqrt((A1.x-A2.x)*(A1.x-A2.x)+(A1.y-A2.y)*(A1.y-A2.y))*sqrt((A3.x-A2.x)*(A3.x-A2.x)+(A3.y-A2.y)*(A3.y-A2.y)));
    uA4=((A1.x-A4.x)*(A3.x-A4.x)+(A1.y-A4.y)*(A3.y-A4.y))/
            (sqrt((A1.x-A4.x)*(A1.x-A4.x)+(A1.y-A4.y)*(A1.y-A4.y))*sqrt((A3.x-A4.x)*(A3.x-A4.x)+(A3.y-A4.y)*(A3.y-A4.y)));
//calculate angles
    uA2=acos(uA2)*180.0/PI;
    uA4=acos(uA4)*180.0/PI;
//the part that gives me an incorrect answer
    cout<<uA2+uA4-180<<endl;
}

Upvotes: 0

Views: 56

Answers (1)

abdullah
abdullah

Reputation: 662

For floating point computation, we can use some very small offset value for marginal error, for example 1e-6.

    double diff = uA2 + uA4 - 180, threshold = 1e-6;
    if (diff < threshold && diff > -threshold) cout << "0" << endl;
    else cout << diff << endl;
   // cout<<uA2+uA4-180<<endl;

Upvotes: 1

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