Looping Over Vectors

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Looping Over Vectors#

Often we will want to loop over the elements contained in a vector, and C++ provides a range-based loop for this purpose. For example to loop over all elements and output them to the terminal, we could do:

Listing 32 vector_loop.cpp#
#include <iostream>
#include <vector>

int main() {

    std::vector<double> container{1.0, 2.0, 3.0, 4.0};

    for (auto e : container) {
        std::cout << e << std::endl;
    }

}

Here we see a new keyword, auto. Each pass through this loop, e will take on the next value in our vector called container. The body of the loop is contained in the {...}.

C++ requires that we declare the data type of e. In this case, the compiler knows that it will have to be a double, since the vector is a vector of double, so we can use the auto keyword to have the compiler automatically deduce the data type.

List initializer#

If we don’t want to reuse the values that we are looping over, we can do a shortcut like:

Listing 33 vector_initialization_list.cpp#
#include <iostream>

int main() {

    for (auto e : {1.0, 2.0, 3.0, 4.0}) {
        std::cout << e << std::endl;
    }

}

Here, the values in the { } in the for loop are in what is called an initializer_list. We won’t worry about this distinction, but note that when we places the values directly in the loop, as here, we cannot update or access these values outside of the loop.

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