E-Olimp/2130 - The angle between the vectors/2130.cpp

   1 using namespace std;
   2 #include <algorithm>
   3 #include <iostream>
   4 #include <iterator>
   5 #include <numeric>
   6 #include <sstream>
   7 #include <fstream>
   8 #include <cassert>
   9 #include <climits>
  10 #include <cstdlib>
  11 #include <cstring>
  12 #include <string>
  13 #include <cstdio>
  14 #include <vector>
  15 #include <cmath>
  16 #include <queue>
  17 #include <deque>
  18 #include <stack>
  19 #include <list>
  20 #include <map>
  21 #include <set>
  22
  23 #define foreach(x, v) for (typeof (v).begin() x=(v).begin(); x !=(v).end(); ++x)
  24 #define For(i, a, b) for (int i=(a); i<(b); ++i)
  25 #define D(x) cout << #x " is " << x << endl
  26
  27 const double pi = 2 * acos(0.0);
  28
  29 int main(){
  30     // for (int i = 0; i < 1000; ++i){
  31     //     int n = 10;
  32     //     while (true) {
  33     //         int a = rand() % n, b = rand() % n, c = rand() % n, d = rand() % n;
  34     //         if (rand() % 2) a = -a;
  35     //         if (rand() % 2) b = -b;
  36     //         if (rand() % 2) c = -c;
  37     //         if (rand() % 2) d = -d;
  38     //
  39     //         if (a == 0 and b == 0 or c == 0 and d == 0) continue;
  40     //         printf("%d %d %d %d\n", a, b, c, d);
  41     //         break;
  42     //     }
  43     // }
  44
  45     double ax, ay, bx, by;
  46     while (cin >> ax >> ay >> bx >> by) {
  47         double k = (ax * bx + ay * by) / (hypot(ax, ay) * hypot(bx, by) );
  48         if (k < -1.0) k = -1.0;
  49         if (k > 1.0) k = 1.0;
  50         double ans = acos( k );
  51         //ans = floor(100000.0 * ans + 0.5) / 100000.0;
  52         //cout << ax << " " << ay << " " << bx << " " << by << endl;
  53         //assert(ans >= 0);
  54         assert(ans == ans);
  55         printf("%.5lf\n", fabs(ans));
  56     }
  57     return 0;
  58 }