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diff --git a/CM2005 Object Oriented Programming/Topic 2/2.3.13/main.cpp b/CM2005 Object Oriented Programming/Topic 2/2.3.13/main.cpp
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+++ b/CM2005 Object Oriented Programming/Topic 2/2.3.13/main.cpp	
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+#include <iostream>
+#include <string>
+#include <vector>
+
+enum class OrderBookType
+{
+  bid,
+  ask
+};
+
+class OrderBookEntry
+{
+  public:
+    OrderBookEntry(
+      double price,
+      double amount,
+      std::string timestamp,
+      std::string product,
+      OrderBookType orderType);
+    double price;
+    double amount;
+    std::string timestamp;
+    std::string product;
+    OrderBookType orderType;
+};
+
+//constructor declaration. It needs to be placed after the class declaration
+OrderBookEntry::OrderBookEntry(
+  double price,
+  double amount,
+  std::string timestamp,
+  std::string product,
+  OrderBookType orderType
+)
+: price{price}, amount{amount}, timestamp{timestamp}, product{product}, orderType{orderType}
+{
+
+}
+
+double computeAveragePrice(std::vector<OrderBookEntry>& entries)
+{
+  double avg = 0;
+  for(unsigned int i=0; i<entries.size(); ++i) //thid doesn't create a copy, ++i increments on the same variable
+  {
+    //std::cout << "The price is " << entries[i].price << std::endl;
+    avg += entries[i].price;
+  }
+  return avg/entries.size();
+}
+
+int main()
+{
+
+  std::vector<OrderBookEntry> orders;
+
+  orders.push_back( OrderBookEntry{
+                                    10000,
+                                    0.002,
+                                    "2020/03/17 17:01:24.884492",
+                                    "BTC/USDT",
+                                    OrderBookType::bid});
+
+  orders.push_back( OrderBookEntry{
+                                    50000,
+                                    0.002,
+                                    "2020/03/17 17:01:24.884492",
+                                    "BTC/USDT",
+                                   OrderBookType::bid});
+
+  // Version 1
+  std::cout << "Version 1" << std::endl;
+  for(OrderBookEntry order : orders) //this create a copy of the members of orders and assigns it to 'order'
+  {
+    std::cout << "The price is " << order.price << std::endl;
+  }
+
+  std::cout << "---new line---" << std::endl;
+
+  for(OrderBookEntry &order : orders) //this doesn't crate a copy
+  {
+    std::cout << "The price is " << order.price << std::endl;
+  }
+
+  // Version 2
+  std::cout << "Version 2" << std::endl;
+  for(unsigned int i=0; i<orders.size(); ++i) //thid doesn't create a copy, ++i increments on the same variable
+  {
+    std::cout << "The price is " << orders[i].price << std::endl;
+  }
+
+  //version 3
+  std::cout << "Version 3" << std::endl;
+  for(unsigned int i=0; i<orders.size(); ++i) //thid doesn't create a copy, ++i increments on the same variable
+  {
+    std::cout << "The price is " << orders.at(i).price << std::endl;
+  }
+
+  std::cout << "Average price: " << computeAveragePrice(orders) << std::endl;
+}
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