minor refactor expected improvement

Minor increase in computational efficiency by only computing the value once instead of twice

Author: ben-arnao

bayes_opt/util.py

@@ -113,9 +113,10 @@ def _ei(x, gp, y_max, xi):
         with warnings.catch_warnings():
             warnings.simplefilter("ignore")
             mean, std = gp.predict(x, return_std=True)
-
-        z = (mean - y_max - xi)/std
-        return (mean - y_max - xi) * norm.cdf(z) + std * norm.pdf(z)
+  
+        a = (mean - y_max - xi)
+        z = a / std
+        return a * norm.cdf(z) + std * norm.pdf(z)
 
     @staticmethod
     def _poi(x, gp, y_max, xi):