Vectors in C++ is an interesting data structure which is bit similar to arrays. Not having a fixed size is the main difference though. The vector class is in C++ std namespace.

Vectors are associated with some useful methods which are handy in controlling vectors. Refer to the guide for more info.

Method *push_back( )* to be used to add elements to a vector. Accessing an element can be done through accessing the relevant index as in arrays. *size( )* gives the number of elements in a vector.

## Defining a Vector

Creating an instance of the vector class.

#include <iostream> #include <vector> using namespace std; int main() { vector<string> names; cout << "Size of the vector : " << names.size() << endl; return 0; }

## Assign Values to a Vector

#include <iostream> #include <vector> using namespace std; int main() { vector<string> names; names.push_back("Alex"); names.push_back("Bob"); cout << "Size of the vector :" << names.size() << endl; cout << "First Name :" << names[0] << endl; names[0] = "Alex the great"; cout << "First Name :" << names[0] << endl; return 0; }

## Example

Here is an example on how vectors can be used to solve a problem.

Write a function that accepts an upper bound positive integer and then calculate all the prime numbers within the given integer (excluding). Prime number set should starts with 2 in the index = 0.

#include <iostream> #include <vector> using namespace std; vector<int> get_primes(int num); bool is_prime(int num); int main() { vector<int> output; output = get_primes(20); for(int j=0; j < output.size(); j++){ cout << "Prime No at index - " << j << " = " << output[j] << endl; } return 0; } vector<int> get_primes(int limit){ vector<int> primes; for(int i = 2; i < limit; i++){ if(is_prime(i)){ primes.push_back(i); } } return primes; } bool is_prime(int num){ for(int i = 2; i<num; i++){ int r = num % i; if(r ==0){ return false; } } return true; }

**Output**

Prime No at index - 0 = 2 Prime No at index - 1 = 3 Prime No at index - 2 = 5 Prime No at index - 3 = 7 Prime No at index - 4 = 11 Prime No at index - 5 = 13 Prime No at index - 6 = 17 Prime No at index - 7 = 19

The code is self-explanatory. Notice how vectors are used in methods. Assigning values and accessing the indexes.