Merge pull request #1894 from andrjohns/feature/Feature-1

Bugfix: lognormal - wrong lpdf with integer sigma

Author: t4c1

stan/math/prim/prob/lognormal_lpdf.hpp

@@ -4,6 +4,7 @@
 #include <stan/math/prim/meta.hpp>
 #include <stan/math/prim/err.hpp>
 #include <stan/math/prim/fun/constants.hpp>
+#include <stan/math/prim/fun/inv.hpp>
 #include <stan/math/prim/fun/log.hpp>
 #include <stan/math/prim/fun/max_size.hpp>
 #include <stan/math/prim/fun/size.hpp>
@@ -63,7 +64,7 @@ return_type_t<T_y, T_loc, T_scale> lognormal_lpdf(const T_y& y, const T_loc& mu,
       inv_sigma_sq(size(sigma));
   if (include_summand<propto, T_y, T_loc, T_scale>::value) {
     for (size_t n = 0; n < stan::math::size(sigma); n++) {
-      inv_sigma[n] = 1 / value_of(sigma_vec[n]);
+      inv_sigma[n] = inv(value_of(sigma_vec[n]));
     }
   }
   if (include_summand<propto, T_y, T_loc, T_scale>::value) {
@@ -85,7 +86,7 @@ return_type_t<T_y, T_loc, T_scale> lognormal_lpdf(const T_y& y, const T_loc& mu,
       stan::math::size(y));
   if (!is_constant_all<T_y>::value) {
     for (size_t n = 0; n < stan::math::size(y); n++) {
-      inv_y[n] = 1 / value_of(y_vec[n]);
+      inv_y[n] = inv(value_of(y_vec[n]));
     }
   }
 

test/unit/math/prim/prob/lognormal_log_test.cpp

@@ -21,4 +21,7 @@ TEST(ProbLognormal, log_matches_lpdf) {
   EXPECT_FLOAT_EQ(
       (stan::math::lognormal_lpdf<double, double, double>(y, mu, sigma)),
       (stan::math::lognormal_log<double, double, double>(y, mu, sigma)));
+
+  EXPECT_FLOAT_EQ((stan::math::lognormal_lpdf(0.8, 2.0, 2.0)),
+                  (stan::math::lognormal_log(0.8, 2, 2)));
 }