Vector access

Learn to access vector elements with the [] operator.

Access with the [] operator

The fast way to access vector elements is the square bracket operator: nums[0] gives the first element. Unlike at it does not check bounds, but it is faster; it is preferred inside loops where the index is known to be safe.

Indices are zero-based; on a vector of n elements valid indices range from 0 to n-1. Accessing nums[n] is undefined behavior. The [] operator both reads and writes: nums[0] = 99; changes the first element.

at vs []

at and [] do the same thing but at different safety levels. nums.at(i) checks bounds and throws std::out_of_range on an invalid index. nums[i] does not check, so an invalid index is undefined behavior.

A useful habit: use [] when the index is safe and speed matters; use at when the index is untrusted or uncertain. Loops usually use [] because the loop variable already stays in range.

Exercise Sum the elements with []

Declare std::vector<int> nums = {10, 20, 30, 40};. Sum all elements using the [] operator and print the result with std::cout.

Show example solution

#include <iostream>
#include <vector>

int main() {
    std::vector<int> nums = {10, 20, 30, 40};
    int sum = 0;
    for (int i = 0; i < nums.size(); i++) {
        sum += nums[i];
    }
    std::cout << sum << std::endl;
    return 0;
}

Test yourself

What does nums[0] = 99; do?
```
std::vector<int> nums = {10, 20, 30};
nums[0] = 99;
```
- Throws an error
- Makes the vector size 99
- Sets the first element to 99
- Accesses the 99th element
Show answer

Sets the first element to 99 — [] assigns, the first element becomes 99.
Change the first element
```
nums[0] ____ 99;
```
Show answer
```
nums[0] = 99;
```
Match the access forms

nums[0]

First element
nums[size-1]

Last element
nums.at(0)

Checked first element
nums[i] = 9

Change i-th element

Next lesson: Vector loop

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