1 #ifndef STAN_MATH_FWD_SCAL_FUN_FABS_HPP
2 #define STAN_MATH_FWD_SCAL_FUN_FABS_HPP
3 
4 #include <stan/math/fwd/core.hpp>
5 #include <stan/math/prim/scal/fun/constants.hpp>
6 #include <stan/math/prim/scal/fun/is_nan.hpp>
7 #include <stan/math/prim/scal/fun/value_of.hpp>
8 #include <stan/math/prim/scal/meta/likely.hpp>
9 #include <cmath>
10 
11 namespace stan {
12 namespace math {
13 
14 template <typename T>
fabs(const fvar<T> & x)15 inline fvar<T> fabs(const fvar<T>& x) {
16   using std::fabs;
17 
18   if (unlikely(is_nan(value_of(x.val_))))
19     return fvar<T>(fabs(x.val_), NOT_A_NUMBER);
20   else if (x.val_ > 0.0)
21     return x;
22   else if (x.val_ < 0.0)
23     return fvar<T>(-x.val_, -x.d_);
24   else
25     return fvar<T>(0, 0);
26 }
27 
28 }  // namespace math
29 }  // namespace stan
30 #endif
31