fixed issue [LWG2192](https://cplusplus.github.io/LWG/issue2192) for std::abs on MacOS

kvedala · kvedala · commit 4025aa92e5d1 · 2020-05-30T21:36:17.000-04:00
diff --git a/machine_learning/adaline_learning.cpp b/machine_learning/adaline_learning.cpp
@@ -22,6 +22,7 @@
 
 #include <cassert>
 #include <climits>
+#include <cstdlib>
 #include <ctime>
 #include <iostream>
 #include <vector>
