Vectors

In C++, normal arrays have a fixed size, meaning you can’t change how many elements they hold once they’re created. This can be a problem if you don’t know in advance how many items you’ll need. To solve this, C++ provides std::vector. A vector works like an array, but it can grow or shrink when you add or remove elements.

Defining Vectors

Vectors are part of a library called <vector> and is defined as follows.

#include <vector>
using namespace std;
vector<type> name;

• name is the name of the vector.
• type is the data type stored in the vector. It can be any valid C++ type (int, double, string, etc).

After defining a vector, the vector is empty (like an array of size 0). To give a vector an initial size, use the following syntax.

#include <vector>
using namespace std;
vector<type> name(size, value);

• size is the number of elements in the vector.
• value is the initial value given to all elements in the vector.

Operations on Vectors

Vectors behave similarly to arrays. However, vectors have additional functionalities. Let’s say that we defined a vector named v.

#include <vector>
using namespace std;
vector<int> v;

Since v can change in size we use v.size() to get the number of elements in v.

The two main functionalities that make vectors special are v.push_back(x) and v.pop_back().

• v.push_back(x) adds the value x to the end of the vector.
• v.pop_back() removes the last element in the vector.

Here is a table of common operations that can be performed on vectors.

Function	Description
v.size()	Returns the current number of elements in the vector.
v.push_back(x)	Appends element x to the end of the vector.
v.pop_back()	Removes the last element from the vector.
v.resize(n)	Changes the number of elements in the vector to n. If n is larger than the current size of the vector, it adds elements; if it is smaller, it removes excess elements from the end.
v.clear()	Removes all elements from the vector.

Example

Let’s take a look at a code using vectors.

#include <iostream>
#include <vector>
using namespace std;

int main() {
    
    vector<int> v(3, 5);         // defines a vector of size 3 where all elements are 5

    v.pop_back();                // removes the last element so v will be {5, 5}

    v.push_back(2);              // adds an element 2 to the end so v be {5, 5, 2}
    v.push_back(1);              // adds an element 1 to the end so v be {5, 5, 2, 1}

    cout << v.size() << endl;    // prints the number of elements in v

    for (int i = 0; i < v.size(); i++) {
        cout << v[i] << ' ';     // prints element at index i
    }
}

This program will output

4
5 5 2 1

This demonstrates basic vector operations — adding and removing elements — and their effect on both content and size.