diff --git a/ch17/ex17.11.12.13/main.cpp b/ch17/ex17.11.12.13/main.cpp
index 4255a60c..4e09cfad 100644
--- a/ch17/ex17.11.12.13/main.cpp
+++ b/ch17/ex17.11.12.13/main.cpp
@@ -1,19 +1,16 @@
 /***************************************************************************
- *  @file       main.cpp
- *  @author     gau fung
- *  @date       25  May 2015
- *  @remark     This code is for the exercises from C++ Primer 5th Edition
- *  @note
- ***************************************************************************/
+*  @file       main.cpp
+*  @author     gau fung
+*  @date       25  May 2015
+*  @remark     This code is for the exercises from C++ Primer 5th Edition
+*  @note
+***************************************************************************/
 //!
 //! Exercise 17.11:
 //! Define a data structure that contains an integral object to track responses
 //! to a true/false quiz containing 10 questions. What changes, if any, would
 //! you need to make in your data structure if the quiz had 100 questions?
 //!
-//      std::bitset<10> reponses;
-//      std::bitset<100> reponses2;
-//!
 //! Exercise 17.12:
 //! Using the data structure from the previous question, write a function that
 //! takes a question number and a value to indicate a true/false answer and
@@ -32,54 +29,69 @@
 #include <utility>
 #include <string>
 #include <iostream>
-//class type: quiz (for Data Stuture)
+
+//class Quiz
 template<std::size_t N>
-class Quiz{
+class Quiz
+{
 public:
-    //constructor
-	Quiz()=default;
-	Quiz(std::string& s):bitquiz(s){}
-	//generate the grade for this quiz
-	template<std::size_t M>
-	friend std::size_t grade(Quiz<M>&,Quiz<M>&);	
-	//output the bitset
-	template<std::size_t M>
-	friend std::ostream& operator<<(std::ostream&,Quiz<M>&);
+    //constructors
+    Quiz() = default;
+    Quiz(std::string& s) :bitquiz(s){}
+
+    //generate grade
+    template<std::size_t M>
+    friend std::size_t grade(Quiz<M> const&, Quiz<M> const&);
 
-	//update bitset
-	void update(std::pair<std::size_t,bool>);
+    //print
+    template<std::size_t M>
+    friend std::ostream& operator<<(std::ostream&, Quiz<M> const&);
+
+    //update bitset
+    void update(std::pair<std::size_t, bool>);
 private:
-    //the bitset
-	std::bitset<N> bitquiz;	
+    std::bitset<N> bitquiz;
 };
 #endif
+
 template<std::size_t N>
-void Quiz<N>::update(std::pair<std::size_t,bool> pair){
-	 bitquiz.set(pair.first,pair.second);//update the value
+void Quiz<N>::update(std::pair<std::size_t, bool> pair)
+{
+    bitquiz.set(pair.first, pair.second);
 }
+
 template<std::size_t M>
-std::ostream& operator<<(std::ostream& os,Quiz<M>& quiz){
-	os<<quiz.bitquiz;
-	return os;
+std::ostream& operator<<(std::ostream& os, Quiz<M> const& quiz)
+{
+    os << quiz.bitquiz;
+    return os;
 }
+
 template<std::size_t M>
-std::size_t grade(Quiz<M>& corAns,Quiz<M>& stuAns){
-	auto result = stuAns.bitquiz ^ corAns.bitquiz;
+std::size_t grade(Quiz<M> const& corAns, Quiz<M> const& stuAns)
+{
+    auto result = stuAns.bitquiz ^ corAns.bitquiz;
     result.flip();
     return result.count();
 }
-int main(){
-	//Ex17_11
-	std::string s="1010101";
-	Quiz<10> quiz(s);
-	std::cout<<quiz<<std::endl;
-	//EX17_12
-	quiz.update(std::make_pair(1,true));
-	std::cout<<quiz<<std::endl;
-	//Ex17_13
-	std::string answer="10011";
-	std::string stu_answer="11001";
-	Quiz<5> ans(answer),stu_ans(stu_answer);
-	std::cout<<grade(ans,stu_ans);
-	return 0;
+
+
+int main()
+{
+    //Ex17_11
+    std::string s = "1010101";
+    Quiz<10> quiz(s);
+    std::cout << quiz << std::endl;
+
+    //EX17_12
+    quiz.update(std::make_pair(1, true));
+    std::cout << quiz << std::endl;
+
+    //Ex17_13
+    std::string answer = "10011";
+    std::string stu_answer = "11001";
+    Quiz<5> ans(answer), stu_ans(stu_answer);
+    std::cout << grade(ans, stu_ans) << std::endl;
+
+    return 0;
 }