C++ Print Vector

What is a Vector?

In C++, a vector refers to a variable used to store multiple values of the same type. Think of a vector as an implementation of an array.

However, unlike an array, a vector is dynamic and can grow or shrink in size as per our requirements.

C++ Create Vector

In C++, vectors are defined in the vector header file included as part of the C++ Standard Library.

To use vectors in C++, we need to include the header file as shown below:

#include <vector>

Once we include the header file, we can declare a vector using the syntax shown below:

std::vector<T> vector_name

The T parameter in the syntax above refers to any primitive C++ data type.

For example, to declare an int vector, we can do:

#include <iostream>
#include <vector>

int main() {
  std::vector<int> my_vector = {1,2,3,4,5};
}

In the example above, we declare a vector called my_vector of type int. It holds the elements defined inside the curly braces.

C++ Accessing Vector Elements

In C++, we can access the elements of a vector using the at() method. The function allows us to specify an index and retrieve the value at that index.

Consider the example below:

#include <iostream>
#include <vector>

int main() {
  std::vector<int> my_vector = {1,2,3,4,5};
  std::cout << my_vector.at(1) << std::endl;
}

In the example code above, we use the at() function to access the element at index 1 of the my_vector vector.

If we compile and run the code above, we should see an output as shown:

$ g++ vect.cpp
$ ./a.out
2

C++ Print Vector – For Loop

We can iterate over each vector's index using the at() function and a for loop and return the corresponding element.

Take the illustration shown below:

#include <iostream>
#include <vector>

int main() {
  std::vector<int> my_vector = {1,2,3,4,5};
  for (int i = 0; i < my_vector.size(); i++) {
    std::cout << my_vector.at(i) << std::endl;
  }
}

In the example above, we use the size() function to get the size of the vector. This prevents us from manually guessing the size of the vector and accessing out-of-bounds elements.

We then use each index of the vector in the at() function to get the corresponding element.

The code above should return:

$ g++ vect.cpp
$ ./a.out
1
2
3
4
5

Instead of the at() function, you can use vector indexing as shown:

#include <iostream>
#include <vector>

int main() {
  std::vector<int> my_vector = {1,2,3,4,5};
  for (int i = 0; i < my_vector.size(); i++) {
    std::cout << my_vector[i] << std::endl;
  }
}

The example above uses the indexing notation [] to access the element at the specified index.

C++ Print Vector – For_each

Another standard method of printing a vector is to use an iterator provided in the for_each method. This is provided in the algorithm library.

An example code is shown below:

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
  std::vector<int> my_vector = {1,2,3,4,5};
  for_each(my_vector.begin(), my_vector.end(), [](const int& n) {
    std::cout << n << std::endl;
  });
}

The for_each method allows us to apply a function to each element in the vector for a specific range. In this case, from the beginning to the end of the vector.

C++ Print Vector - std::copy()

We can also use the std::copy() method from the algorithm library to copy the vector elements to stdout.

An illustration is shown below:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

int main() {
  std::vector<int> my_vector = {1,2,3,4,5};
    copy(my_vector.begin(), my_vector.end(),
    std::ostream_iterator<int(std::cout, " "));
}

C++ Print Vector - Overload std::ostream::operator<<

We can also use the insertion operator to print a vector. This works by overloading the insertion operator to recognize an ostream object and a vector.

Take the example illustration below:

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>

std::ostream& operator<< (std::ostream& out, const std::vector<int>& vec) {
    for (size_t i = 0; i < vec.size(); i++) {
        out << vec[i];
        if (i < vec.size() - 1) {
            out << " ";
        }
    }
    return out;
}

int main() {
    std::vector<int> my_vector = {1, 2, 3, 4, 5};
    std::cout << my_vector << std::endl;
}

If we compile and run the code above, it should return the elements of the array as shown:

$ g++ vect.cpp
$ ./a.out
1 2 3 4 5

Conclusion

In this article, we discussed several methods of printing a vector in C++.