size and at
- C++ course
- Beginner
- Lesson 61/116
- 5 min
Learn vector management with .size() and .at().
size() and at()
Two core vector methods: size() returns the element count, at(i) accesses the element at index i. nums.size() tells how many elements there are, while nums.at(0) gives the first element.
The key difference between at and [] is that at checks bounds. An invalid index throws std::out_of_range; [] does no check, so an invalid index is undefined behavior. Prefer at for safe access.
Why is bounds checking important?
On a three-element vector, nums.at(3) throws because valid indices are 0, 1 and 2. This lets the program report a meaningful error before crashing.
The same access with nums[3] would not check; the program would read a random memory cell, producing bugs that are hard to trace later. That is why at is safer with user-supplied indices.
Practice Print three elements with at()
Declare std::vector<int> nums = {10, 20, 30};. Print the values of at(0), at(1) and at(2) on a single line separated by spaces, using std::cout.
Starter code
#include <iostream>
#include <vector>
int main() {
std::vector<int> nums = {10, 20, 30};
// Print three elements with at()
return 0;
}Sample solution
#include <iostream>
#include <vector>
int main() {
std::vector<int> nums = {10, 20, 30};
std::cout << nums.at(0) << " " << nums.at(1) << " " << nums.at(2) << std::endl;
return 0;
}Expected output
10 20 30
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match size() and at()
| Term | Meaning |
|---|---|
size() |
Element count |
at(0) |
First element |
at(size-1) |
Last element |
out_of_range |
Out-of-bounds error |
2 On a 3-element vector, what does nums.at(5) do?
std::vector<int> nums = {10, 20, 30};
nums.at(5);- Undefined behavior
- Throws std::out_of_range Correct answer
- Returns 5
- Returns 30
Explanation: at checks bounds and throws std::out_of_range on an invalid index.
3 Access the second element with at()
std::cout << nums.____(1) << std::endl;std::cout << nums.at(1) << std::endl;Explanation: at(1) accesses the second element.