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The C++ vector::operator[] function is used to reference the element at the position specified inside the operator . The main distinction between operator[] function and the at() function is that the at() method throws an out-of-range exception when the location is outside the size of the vector, whereas this operator results in undefinable behavior. The time complexity of the operator[] function is constant.

Syntax

Following is the syntax for C++ vector::operator[] Function −

reference operator[] (size_type n);const_reference operator[] (size_type n) const;

Parameters

n − It indicates the posiion of an element present in the container.

Example 1

Let's consider the following example, where we are going to use the opertor=() function.

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

int main(void) {   
   vector<int> myvector = {1, 2, 3, 4, 5};
   for (int x = 0; x < myvector.size(); ++x)
      cout << myvector[x] << endl;
   return 0;
}

Output

When we compile and run the above program, this will produce the following result −

1
2
3
4
5

Example 2

Considering the another scenario, where we are going to take the string type.

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

int main(){
   vector<string> myvector{"BMW","AUDI","RS7","DUCATI"};
   for(int x=0; x<myvector.size(); x++)
      cout<<myvector.operator[](x)<<" ";
   return 0;
}

Output

On running the above program, it will produce the following result −

BMW AUDI RS7 DUCATI 

Example 3

In the following example, we are going to use the push_back() function to insert the elements and applying the operator[].

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

int main(){
   vector<string> cars;
   cars.push_back("RS7");
   cars.push_back("DUCATI");
   cars.push_back("VERNA");
   cars.push_back("CRUZE LT");
   cout <<"Result : " << cars[2];
   return 0;
}

Output

When we execute the above program, it will produce the following result −

Result : VERNA

Example 4

Following is the example, where we are going to print the position of the element that is not divisible by 2.

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

int main(){
   vector<int> mytutorial;
   mytutorial.push_back(1);
   mytutorial.push_back(2);
   mytutorial.push_back(3);
   mytutorial.push_back(4);
   mytutorial.push_back(5);
   mytutorial.push_back(6);
   mytutorial.push_back(7);
   for (int i = 0; i < mytutorial.size(); ++i) {
      if (i % 2 != 0) {
         cout << mytutorial[i];
         cout << " ";
      }
   }
   return 0;
}

Output

On running the above program, it will produce the following result −

2 4 6