vector::at() in C++ STL (original) (raw)

Last Updated : 19 Nov, 2024

In C++, vector at() is a built-in method used to access an element in a vector using index. It is the only access method that performs bound checking before accessing the element to confirm whether the given index lies is within the vector.

Let’s take a quick look at a simple example that uses vector at() method:

C++ `

#include <bits/stdc++.h> using namespace std;

int main() { vector v = {1, 3, 4, 9};

// Accessing the element of second index
cout << v.at(2);
return 0;

}

`

This article covers the syntax, usage, and common queries of vector at() method in C++:

Table of Content

• Syntax of vector at()
• Examples of vector at()
  • Modifying Elements Using vector at()
  • Catching Out-of-Range Exceptions with vector at()
• vector at() in C++ - FAQs

Syntax of vector at()

The vector at() is a member method of std::vector class defined inside ****** header file.

v.**at(i);

**Parameters

• **i: 0-based position of the element in the vector.

**Return Value

• Returns the reference to the element at given index.
• If the index is out of bounds, an **std::out_of_range exception is thrown.

Examples of vector at()

The below examples illustrate the common uses of vector at() method:

Modifying Elements Using vector at()

C++ `

#include <bits/stdc++.h> using namespace std;

int main() { vector v = {1, 3, 4, 9};

// Modify the element at 2nd index
v.at(2) = 7;
cout << v.at(2);

return 0;

}

`

Catching Out-of-Range Exceptions with vector at()

C++ `

#include <bits/stdc++.h> using namespace std;

int main() { vector v = {1, 3, 4, 9};

try {
  
    // Attempting to access out of range index
    cout << v.at(5) << endl;
  
} catch (const out_of_range& e) {
    cout << "Exception: " << e.what() << endl;
}

return 0;

}

`

Output

Exception: vector::_M_range_check: __n (which is 5) >= this->size() (which is 4)