Accessing Elements

Part of the Logic & Flow section of Coddy's C++ journey — lesson 12 of 56.

Now that you can create vectors and add elements to them, you need to know how to retrieve specific elements from your vector. C++ provides two main ways to access individual elements: the square bracket operator and the .at() method.

The square bracket operator [] works just like with regular arrays. You specify the index of the element you want to access:

std::vector<int> numbers = {10, 20, 30, 40};
int first = numbers[0];    // Gets 10
int third = numbers[2];    // Gets 30

The .at() method provides an alternative way to access elements with an important difference - it includes bounds-checking:

int first = numbers.at(0);    // Gets 10
int third = numbers.at(2);    // Gets 30

The key difference is safety: if you try to access an index that doesn't exist, [] can cause unpredictable behavior, while .at() will throw an exception to alert you of the error. For learning purposes, both methods work well, but .at() provides extra protection against accessing invalid positions in your vector.

Challenge

Easy

Create a program that demonstrates vector element access by retrieving specific elements from a vector using both the square bracket operator and the .at() method.

The following inputs will be provided:

An integer n representing the number of elements in the vector
Then n integers to populate the vector
An integer index1 representing the first index to access
An integer index2 representing the second index to access

Your program should:

Create a std::vector<int> named data
Use push_back() to add each of the n input numbers to the vector
Access the element at index1 using the square bracket [] operator and store it in a variable named value1
Access the element at index2 using the .at() method and store it in a variable named value2
Print the accessed values and their corresponding indices
Print the first and last elements of the vector using index-based access

Use the following exact output format:

Element at index [index1]: [value1]
Element at index [index2]: [value2]
First element: [first value]
Last element: [last value]

For accessing the last element, use data.size() - 1 as the index with the square bracket operator.

Cheat sheet

C++ vectors provide two main ways to access individual elements:

Square bracket operator []:

std::vector<int> numbers = {10, 20, 30, 40};
int first = numbers[0];    // Gets 10
int third = numbers[2];    // Gets 30

The .at() method:

int first = numbers.at(0);    // Gets 10
int third = numbers.at(2);    // Gets 30

The key difference is safety: [] can cause unpredictable behavior if accessing invalid indices, while .at() throws an exception for bounds-checking protection.

To access the last element, use data.size() - 1 as the index.

Try it yourself

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

int main() {
    // Read input
    int n;
    cin >> n;
    
    vector<int> data;
    
    // Read n integers and add them to the vector
    for (int i = 0; i < n; i++) {
        int num;
        cin >> num;
        data.push_back(num);
    }
    
    int index1, index2;
    cin >> index1;
    cin >> index2;
    
    // TODO: Write your code below
    // Access elements using [] operator and .at() method
    // Store the accessed values in value1 and value2
    
    // Output the results
    cout << "Element at index " << index1 << ": " << value1 << endl;
    cout << "Element at index " << index2 << ": " << value2 << endl;
    cout << "First element: " << /* access first element */ << endl;
    cout << "Last element: " << /* access last element using data.size() - 1 */ << endl;
    
    return 0;
}