precision problem

[RexLee]

I have a contest coming up and I am doing problems for it.

And i have a problem where you enter x, y coordinates for 3 points.

Than calculate the area.

area = a * b * sinw / 2

cosw = (a * a + b * b - c * c) / 2 * a * b

a, b, c are the 3 sides connected using the 3 points, w is the degree

This is my code: (c++)

#include <iostream>#include <cmath>int main(){    long double area;    long double a, b, c;    long double cosw, sinw;    long double x[3], y[3];    while (std::cin >> x[0] >> y[0] >> x[1] >> y[1] >> x[2] >> y[2])    {          if (x[0] == 0 && y[0] == 0 && x[1] == 0 && y[1] == 0 && x[2] == 0 && y[2] == 0) break;                a = sqrt(abs((x[0] - x[1]) * (x[0] - x[1]) + (y[0] - y[1]) * (y[0] - y[1])));          b = sqrt(abs((x[1] - x[2]) * (x[1] - x[2]) + (y[1] - y[2]) * (y[1] - y[2])));          c = sqrt(abs((x[2] - x[0]) * (x[2] - x[0]) + (y[2] - y[0]) * (y[2] - y[0])));          cosw = (a * a + b * b - c * c) / (2 * a * b);          sinw = sqrt(1 - cosw * cosw);          area = (a * b * sinw) / 2;          std::cout << area << std::endl;    }    return 0;}

It works for some of the test values.

The diff between answer has to be within 1.

Input:

3.1415 2.7777 -3.9123 0.2133 0.4324 -11.111 

-8.675309 1.41421 9.999 0.0001 9.999 1.41421 

0.7071 7.732 2.718 -1.005 -6.931 0.866025 

0.6125 0.03125 99.999 0.9125 99.999 -0.56875 

Output:

45.5104 

13.2038 

40.2704 

73.6081 

My code is within difference except for the second one and the last one.

Is this because of precision?

 

 

[Rorgash]

too easy, answer is 2

[Guest]

Answer is 2 1/2

[Ciccioo]

what is the output supposed to be?

 

doubles shouldn't give you errors in that order of magnitude, i really don't think that's the case

[RexLee]

The output is the area of the triangle.

[RexLee]

The problem is from here: http://hpcodewars.org/past/cw16/problems/2013ProblemSetFinalPost.pdf

(problem 6)

[Ciccioo]

and what's your problem? your program gives the exact results for the second and fourth cases too

[RexLee]

Really?

What compiler are you using.

I am getting 9.xxx for the second one, and 70.xxx for the last one.

[Ciccioo]

Really?

What compiler are you using.

I am getting 9.xxx for the second one, and 70.xxx for the last one.

are you sure you're writing in the right numbers?

i'm using GCC, but this really shouldn't be about it

[Ciccioo]

alright i see, i had to remove the abs() part because the compiler couldn't find it

abs returns an int, not a floating point

also, abs() is not required because

a^2 + b^2 >= 0

for every a, b

 

so yeah, just take away those abs() and you're good

[RexLee]

  Oh thanks. It works now.