[c++] Added Lambda Expressions and Range for

c++.html.markdown

@@ -801,6 +801,106 @@ void doSomethingWithAFile(const std::string& filename)
 //   all automatically destroy their contents when they fall out of scope.
 // - Mutexes using lock_guard and unique_lock
 
+///////////////////////////////////////
+// Lambda Expressions (C++11 and above)
+///////////////////////////////////////
+
+// lambdas are a convenient way of defining an anonymous function
+// object right at the location where it is invoked or passed as 
+// an argument to a function.
+
+// Example consider sorting a vector of pairs using the second 
+// value of the pair
+
+vector<pair<int, int> > tester;
+tester.push_back(make_pair(3, 6));
+tester.push_back(make_pair(1, 9));
+tester.push_back(make_pair(5, 0));
+
+// Pass a lambda expression as third argument to the sort function
+// sort is from the <algorithm> header
+
+sort(tester.begin(), tester.end(), [](const pair<int, int> &lhs, const pair<int, int> &rhs) {
+		return lhs.second < rhs.second;
+	});
+
+// Notice the syntax of the lambda expression,
+// [] in the lambda is used to "capture" variables.
+// For Example:
+
+vector<int> dog_ids;
+// number_of_dogs = 3;
+for(int i = 0; i < 3; i++){
+	dog_ids.push_back(i);  
+}
+
+int weight[3];
+weight[0] = 30, weight[1] = 50, weight[2] = 10;
+
+// Say you want to sort dog_ids according to the dogs' weights
+// So dog_ids should in the end become: [2, 0, 1]
+
+// Here's where lambda expressions come in handy
+
+sort(dog_ids.begin(), dog_ids.end(), [weight](const int &lhs, const int &rhs) {
+		return weight[lhs] < weight[rhs];
+	});
+// Note we captured "weight" in the above example.
+
+// lambda are really useful for the case of structs
+// You can use lambda expressions instead of overloading
+// the "<" operator
+
+struct dog{
+	int weight, age;
+}dogs[3];
+
+dogs[0].weight = 30, dogs[0].age = 4;
+dogs[1].weight = 40, dogs[1].age = 10;
+dogs[2].weight = 20, dogs[2].age = 9;
+
+// Say I want to sort the dogs array by the dogs' weights
+
+sort(dogs, dogs+3, [](const dog &lhs, const dog &rhs) {
+		return lhs.weight < rhs.weight;
+	});
+// dogs is now sorted according to their weight
+
+// Do something with the dogs
+
+// Now I want to sort the dogs by in descending order of their age
+
+sort(dogs, dogs+3, [](const dog &lhs, const dog &rhs) {
+		return lhs.age > rhs.age;
+	});
+// dogs is now sorted in descending order of their age
+
+
+///////////////////////////////
+// Range For (C++11 and above)
+///////////////////////////////
+
+// You can use a range for loop to iterate over a container
+int arr[] = {1, 10, 3};
+
+for(int elem: arr){
+	cout<<elem<<'\n';  
+}
+
+// You can use "auto" and not worry about the type of the elements of the container
+// Caveat: Don't assign inside the range for loop
+// For example:
+
+for(auto elem: arr) {
+	elem = -1;  
+}
+
+// "arr" remains unchanged
+
+// Why doesn't it change?
+// What actually is happening is that the value of "arr[i]" is stored in 
+// the variable "elem" in every iteration. So assigning "elem" doesn't assign "arr[i]".
+// For more, checkout: http://en.cppreference.com/w/cpp/language/range-for
 
 /////////////////////
 // Fun stuff