How do I add elements to an empty vector in a loop?

I am trying to create an empty vector inside a loop, and want to add an element to the vector each time something is read in to that loop.

#include <iostream> #include <vector> using namespace std; int main() { std::vector<float> myVector(); float x; while(cin >> x) myVector.insert(x); return 0; } 

But this is giving me error messages.

5 Answers

You need to use std::vector::push_back() instead:

while(cin >> x) myVector.push_back(x); // ^^^^^^^^^ 

and not std::vector::insert(), which, as you can see in the link, needs an iterator to indicate the position where you want to insert the element.

Also, as what @Joel has commented, you should remove the parentheses in your vector variable's definition.

std::vector<float> myVector; 

and not

std::vector<float> myVector(); 

By doing the latter, you run into C++'s Most Vexing Parse problem.

Use push_back:

while(cin >> x) myVector.push_back(x); 

The insert function takes an iterator as the first argument, indicating the position to insert.

Also, you need to get rid of the parentheses in the declaration of myVector:

std::vector<float> myVector; 

If you want to use myVector.insert(), use it like myVector.insert(myVector.end(), x). This will append x at the end of myVector. You can insert x in the beginning by myVector.insert(myVector.begin(), x).

Another option is to use std::vector::emplace_back() instead of std::vector::push_back(). The makes some optimizations and doesn't take an argument of type vector::value_type, it takes variadic arguments that are forwarded to the constructor of the appended item, while push_back can make unnecessary copies or movements.

This is demonstrated in the std::vector::emplace_back documentation and here is a related question.

Usage example:

std::vector<int> myVector; while (cin >> x) { myVector.emplace_back(x); } 

The code below may answer your question and also brings some other examples regarding how to insert new elements in different position or index.

#include <iostream> #include <vector> using namespace std; int main() { vector<int> vector_of_integers{}; vector_of_integers.push_back(1); // O(1) vector_of_integers.push_back(3); // O(1) vector_of_integers.push_back(5); // O(1) vector_of_integers.push_back(7); // O(1) for (int i = 8; i <= 10; i++) vector_of_integers.push_back(i); // Printing out all the elements of vector of integers - Method 1 copy(vector_of_integers.begin(), vector_of_integers.end(), ostream_iterator<int>(cout, " ")); // 1 3 5 7 8 9 10 cout << endl << endl; // Inserting '2' at index 1 vector<int>::iterator it{ vector_of_integers.begin() }; advance(it, 1); vector_of_integers.insert(it, 2); // O(N+M) => M is size of elements to be inserted // Printing out all the elements of vector of integers - Method 2 for (auto const& element : vector_of_integers) std::cout << element << " "; // 1 2 3 5 7 8 9 10 cout << endl << endl; // "it" no longer valid, get a new one it = vector_of_integers.begin(); vector_of_integers.insert(it + 4, 6); // O(N+M) => M is size of elements to be inserted // Printing out all the elements of vector of integers - Method 3 for (it = vector_of_integers.begin(); it != vector_of_integers.end(); it++) std::cout << *it << ' '; // 1 2 3 5 6 7 8 9 10 cout << endl << endl; // insert '4' 7 times at index 3 vector<int> new_vector_to_be_inserted(7, 4); vector_of_integers.insert(vector_of_integers.begin() + 3, new_vector_to_be_inserted.begin(), new_vector_to_be_inserted.end()); // O(N+M) => M is size of elements to be inserted // Printing out all the elements of vector of integers - Method 4 for (int i = 0; i < vector_of_integers.size(); i++) cout << vector_of_integers.at(i) << ' '; // 1 2 3 4 4 4 4 4 4 4 5 6 7 8 9 10 cout << endl << endl; return 0; }