Returning a value from the Lambda expression

Our two preceding samples of the Lambda expression are just for the purpose to print on console. It means the function does not need to return any value. However, we can ask the Lambda expression to return a value for an instance if we do the calculation inside the function and return the calculation result. Let's take a look at the following code to examine the use of this Lambda:

    /* lambda_returning_value.cpp */
    #include <vector>
    #include <algorithm>
    #include <iostream>

    using namespace std;

    auto main() -> int
    {
      cout << "[lambda_returning_value.cpp]" << endl;

      // Initializing a vector containing integer element
      vector<int> vect;
      for (int i = 0; i < 10; ++i)
        vect.push_back(i);

      // Displaying the elements of vect
      cout << "Original Data:" << endl;
      for_each(
             begin(vect),
             end(vect),
             [](int n){
                cout << n << " ";
            });
      cout << endl;

      // Creating another vect2 vector
      vector<int> vect2;
      // Resize the size of vect2 exactly same with vect
      vect2.resize(vect.size());
      // Doubling the elements of vect and store to vect2
      transform(
              begin(vect),
              end(vect),
              begin(vect2),
              [](int n) {
                return n * n;
            });

      // Displaying the elements of vect2
      cout << "Squared Data:" << endl;
      for_each(
             begin(vect2),
             end(vect2),
             [](int n) {
                cout << n << " ";
            });
      cout << endl;

      // Creating another vect3 vector
      vector<double> vect3;
      // Resize the size of vect3 exactly same with vect
      vect3.resize(vect.size());
      // Finding the average of the elements of vect
      // and store to vect2
      transform(
              begin(vect2),
              end(vect2),
              begin(vect3),
              [](int n) -> double {
                return n / 2.0;
            });

      // Displaying the elements of vect3
      cout << "Average Data:" << endl;
      for_each(
             begin(vect3),
             end(vect3),
             [](double d) {
                cout << d << " ";
            });
      cout << endl;

      return 0;
     }

When we use the transform() method in the preceding code, we have a Lambda expression that returns a value from the calculation of n * n. However, there's no return type stated in the expression. This is because we can omit the statement of the return type since the compiler has understood that the expression will return an integer value. So, after we have another vector, vect2, which has the same size as vect, we can invoke the transform() method along with the Lambda expression, and the value of vect will be doubled and stored in vect2.

We can, if we want to, specify the return type to the Lambda expression. As we can see in the preceding code, we transformed the vect3 vector based on all values of the vect vector, but now we specify the return type to double using the arrow symbol (->). The result of the preceding code should be like the following screenshot:

As we can see from the preceding screenshot, we have successfully found the doubled and average result using the Lambda expression.